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The Boy Electrician
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        :PG.Title: The Boy Electrician
        :PG.Id: 63207
        :PG.Rights: Public Domain
        :PG.Producer: James Simmons
        :PG.Credits: This file was produced from page images at Google Books.
        :DC.Creator: Alfred Powell Morgan
        :DC.Title: The Boy Electrician
        :DC.Language: en
        :DC.Created: 1913
        :PG.Released: 2020-09-15
        :coverpage: images/CoverImage.jpg

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.. topic:: Transcriber's Note

    This book was transcribed from scans of the original found at Google Books. 
    I have rotated some images. The more complex tables are treated as images.

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   :alt: A Boy’s Wireless Outfit made up of the Apparatus described in Chapter XIV.
   
   A BOY’S WIRELESS OUTFIT MADE UP OF THE APPARATUS DESCRIBED IN CHAPTER XIV. THE JUNIOR DYNAMO AND A COHERER OUTFIT CAN BE SEEN ON THE LOWER PART OF THE TABLE. 

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       The
       
       Boy 
       
       Electrician 

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       *Practical Plans for Electrical*
       
       *Apparatus for work and play, with an explanation*
       
       *Of the principles of every-day electricity.*

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       By
 
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       ALFRED P MORGAN 

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       *With illustrations by the author*

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       BOSTON 

       LOTHROP, LEE & SHEPARD CO.

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    Copyright, 1913, by Lothrop, Lee & Shepard Co. 

    Entered at Stationers’ Hall, London 

    Published July, 1914

    *All rights reserved*

    THE BOY ELECTRICIAN

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    NORWOOD PRESS
       
    Berwick & Smith Co.
       
    Norwood, Mass. U.S.A.    

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   TO THE SELF-RELIANT 

   **BOYS OF AMERICA,**

   OUR FUTURE ENGINEERS AND SCIENTISTS, THAN WHOM 

   NONE IN THE WHOLE WORLD ARE BETTER ABLE 

   TO WORK OUT AND SOLVE THE PROBLEMS 

   THAT EVER CONFRONT YOUNG 

   MANHOOD, THIS BOOK 

   IS CORDIALLY 

   DEDICATED. 

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THE BOY ELECTRICIAN
-------------------

INTRODUCTION 
````````````

Once upon a time, and this is a true tale, a boy had a whole 
railroad system for a toy. The train ran automatically, propelled by 
tiny electric motors, the signals went up and down, the station was 
reached, a bell rang, the train moved on again and was off on its 
journey around many feet of track to come back over the old route. 

The boy viewed his gift with raptured eyes, and then his face 
changed and he cried out in the bitterness of his disappointment: 
"But what do I do?" The toy was so elaborate that the boy was 
left entirely out of the play. Of course he did not like it. His cry 
tells a long story. 

The prime instinct of almost any boy at play is to *make* and to 
*create*. He will *make* things of such materials as he has at hand, and 
use the whole force of dream and fancy to create something out of 
nothing. The five-year-old will lay half a dozen wooden blocks 
together with a spool on one end and tell you it is a steam train. And it 
is. He has both made and created an engine, which he sees but which 
you don’t, for the blocks and spool are only a symbol of his creation. 
Give his older brother a telephone receiver, some wire and bits of 
brass, and he will make a wireless telegraph outfit and listen to a 
steamship hundreds of miles away spell out its message to the shore. 

The wireless outfit is not a symbol, but something that you can 
both hear and see in operation even though you may not understand 
the whispering of the dots and dashes. And as soon as the mystery 
of this modern wonder more firmly grips your imagination, you 
perhaps may come to realize that we are living more and more in the 
age of electricity and mechanism. Electricity propels our trains, lights 
our houses and streets, makes our clothes, cures our ills, warms us, 
cooks for us and performs an innumerable number of other tasks at 
the turning of a little switch. A mere list is impossible. 

Almost every boy experiments at one time or another with 
electricity and electrical apparatus. It is my purpose in writing this 
book to open this wonderland of science and present it in a manner 
which can be readily understood, and wherein a boy may "do 
something." Of course there are other books with the same purport, but 
they do not accomplish their end. They are not practical. I can 
say this because as a boy I have read and studied them and they 
have fallen far short of teaching me or my companions the things 
that we were seeking to learn. If they have failed in this respect, 
they have done so perhaps not through any inability of the author, 
but from the fact that they have not been written from the *boy’s 
standpoint*. They tell what the author *thought* a boy ought to know 
but not what he really does want to know. The apparatus described 
therein is for the most part imaginary. The author thought it might 
be possible for a boy to build motors, telegraph instruments, etc., 
out of old bolts and tin cans, but *he never tried to do so himself*. 

The apparatus and experiments that I have described have been 
constructed and carried out by *boys*. Their problems and their 
questions have been studied and remedied. I have tried to present 
practical matter considered wholly from a boy’s standpoint, and to 
show the young experimenter just what he can do with the tools 
and materials in his possession or not hard to obtain. 

To the boy interested in science, a wide field is open. There is no 
better education for any boy than to begin at the bottom of the 
ladder and climb the rungs of scientific knowledge, step by step. It 
equips him with information which may prove of inestimable worth 
in an opportune moment. 

There is an apt illustration in the boy who watched his mother
empty a jug of molasses into a bowl and replace the cork. His mother
told him not to disturb the jug, as it was empty. He persisted,
however, and turned the jug upside down. No more molasses came, but
*out crawled a fly*. New developments in science will never cease. Invention will 
follow invention. The unexpected is often a valuable clue. The Edisons
and Teslas of to-day have not discovered everything. *There is a fly
in the molasses*, to be had by persistence. Inspiration is but a
starting-point. Success means work, days, nights, weeks, and years.

There can be no boy who will follow exactly any directions given 
to him, or do exactly as he is told, of his own free will. He will 
"bolt" at the first opportunity. If forced or obliged to do as he is 
directed, his action will be accompanied by many a "why?" 
Therefore in presenting the following chapters I have not only told how to 
*make* the various motors, telegraphs, telephones, radio receivers, etc. 
but have also explained the principles of electricity upon which they 
depend for their operation, and how the same thing is accomplished 
in the every-day world. In giving directions or offering cautions, 
I have usually stated the reason for so doing, in the hope that this 
information may be a stimulant to the imagination of the young 
experimenter and a useful guide in enabling him to proceed along some 
of the strange roads on which he will surely go. 

ALFRED P. MORGAN

UPPER MONTCLAIR, N. J. 

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   :alt: Contents
   
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   :alt: THE BOY ELECTRICIAN 
   
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   :alt: MAGNETS AND MAGNETISM 
   
CHAPTER I MAGNETS AND MAGNETISM
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Over two thousand years ago, in far-away Asia Minor, 
a shepherd guarding his flocks on the slope of Mount Ida 
suddenly found the iron-shod end of his staff adhering to 
a stone. Upon looking further around about him he found 
many other pieces of this peculiar hard black mineral, the 
smaller bits of which tended to cling to the nails and studs 
in the soles of his sandals. 

This mineral, which was an ore of iron, consisting of iron 
and oxygen, was found in a district known as Magnesia, 
and in this way soon became widely known as the 
"Magnesstone," or magnet. 

This is the story of the discovery of the magnet. It 
exists in legends in various forms. As more masses of this 
magnetic ore were discovered in various parts of the world, 
the stories of its attractive power became greatly 
exaggerated, especially during the Middle Ages. In fact, 
magnetic mountains which would pull the iron nails out of ships, 
or, later, move the compass needle far astray, did not lose 
their place among the terrors of the sea until nearly the 
eighteenth century. 

For many hundreds of years the magnet-stone was of 
little use to mankind save as a curiosity which possessed 
the power of attracting small pieces of iron and steel and 
other magnets like itself. Then some one, no one knows 
who, discovered that if a magnet-stone were hung by a 
thread in a suitable manner it would always tend to point 
North and South; and so the "Magnes-stone" became 
also called the "lodestone," or "leading-stone." 

These simple bits of lodestone suspended by a thread 
were the forerunners of the modern compass and were of 
great value to the ancient navigators, for they enabled 
them to steer ships in cloudy weather when the sun was 
obscured and on nights when the pole-star could not be 
seen. 

The first real *compasses* were called *gnomons*, and 
consisted of a steel needle which had been rubbed upon a 
lodestone until it acquired its magnetic properties. Then it 
was thrust through a reed or short piece of wood which 
supported it on the surface of a vessel of water. If the 
needle was left in this receptacle, naturally it would move 
against the side and not point a true position. Therefore 
it was given a circular movement in the water, and as soon 
as it came to rest, the point on the horizon which the north 
end designated was carefully noted and the ship’s course 
laid accordingly. 

The modern mariners’ compass is quite a different 
arrangement. It consists of three parts, the *bowl*, the *card*, 
and the *needle*. The bowl, which contains the card and 
needle, is usually a hemispherical brass receptacle, 
suspended in a pair of brass rings, called *gimbals*, in such a 
manner that the bowl will remain horizontal no matter 
how violently the ship may pitch and roll. The card, which 
is circular, is divided into 32 
equal parts called the *points 
of the compass*. The needles, 
of which there are generally 
from two to four, are fastened 
to the bottom of the card. 

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   :alt: Fig. 1.—The Card of a Mariner's Compass, Showing the "Points."
   
   Fig. 1.—The Card of a Mariner's Compass, Showing the "Points."

In the center of the card is 
a conical socket poised on an 
upright pin fixed in the 
bottom of the bowl, so that the 
card hanging on the pin turns 
freely around its center. On 
shipboard, the compass is so placed that a black mark, 
called the *lubber’s line*, is fixed in a position parallel to the 
keel. The point on the compass-card which is directly 
against this line indicates the direction of the ship’s head. 

Experiments with Magnetism
##########################

The phenomena of magnetism and its laws form a very 
important branch of the study of electricity, for they play 
an important part in the construction of almost all 
electrical apparatus. 

Dynamos, motors, telegraphs, telephones, wireless 
apparatus, voltmeters, ammeters, and so on through a practically 
endless list, depend upon magnetism for their operation. 

**Artificial Magnets** are those made from steel by the 
application of a lodestone or some other magnetizing force. 

The principal forms are the 
Bar and Horseshoe, so called 
from their shape. The 
process of making such a 
magnet is called *Magnetization*. 

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   :alt: Fig. 2.—A Bar Magnet
   
   Fig. 2.—A Bar Magnet

Small horseshoe and bar magnets can be purchased at 
toy-stores. They can be used to perform 
very interesting and instructive 
experiments. 

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   :alt: Fig. 3.—A Horseshoe Magnet
   
   Fig. 3.—A Horseshoe Magnet

Stroke a large darning-needle from end to 
end, always in the same direction, with one 
end of a bar magnet. Then dip the needle 
in some iron filings and it will be found that 
the filings will cling to the needle. The 
needle has become a magnet. 

Dip the bar magnet in some iron filings 
and it will be noticed that the filings cling 
to the magnet in irregular tufts near the ends, 
with few if any near the middle. 

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   :alt: Fig. 4.—A Magnetized Needle and a Bar Magnet which have been dipped in Iron Filings.
   
   Fig. 4.—A Magnetized Needle and a Bar Magnet which have been dipped in Iron Filings.

This experiment shows that the attractive power of a 
magnet exists in *two opposite* places. These are called the 
poles. 

There exists between magnets and bits of iron and steel 
a peculiar unseen force which can exert itself across 
space. 

The power with which a magnet attracts or repels 
another magnet or attracts bits of iron and steel is called 

**Magnetic Force.** The force exerted by a magnet upon a 
bit of iron is not the same at all distances. The force is 
stronger when the 
magnet is near the 
iron and weaker 
when it is farther 
away. 

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   :alt: Fig. 5.—The Lifting Power of a Bar Magnet. *It must be brought closer to the nails than the tacks because they are heavier*. 
   
   Fig. 5.—The Lifting Power of a Bar Magnet. *It must be brought closer to the nails than the tacks because they are heavier*. 

Place some small 
carpet-tacks on a 
piece of paper and hold a magnet above
them. Gradually 
lower the magnet until the tacks jump up to meet it. 

Then try some nails in place of the tacks. The nails are 
heavier than the tacks, and it will require a greater force 
to lift them. The magnet will have to be brought much 
closer to the nails than to the tacks before they are lifted, 
showing that the force exerted by the magnet is strongest 
nearest to it. 

Magnetize a needle and lay it on a piece of cork floating 
in a glass vessel of water. It will then be seen that the 
needle always comes to rest lying nearly in a north and 
south line, with the same end always toward the north. 

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   :alt: Fig. 6.—A Simple Compass. 
   
   Fig. 6.—A Simple Compass.

The pole of the magnet which tends to turn towards the 
north is called the 
*north-seeking pole* 
and the opposite one 
is called the 
*south-seeking pole*. 

The name is 
usually abbreviated to 
simply the north and 
south poles. The north pole of a magnet is often indicated 
by a straight line or a letter N stamped into the metal. 

A magnetized needle floating on a cork in a basin of 
water is a simple form of 

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   :alt: Fig. 7.—Several Different Methods of Making a Simple Compass. 
   
   Fig. 7.—Several Different Methods of Making a Simple Compass.

**Compass.** Figure 7 shows several other different ways 
of making compasses. The first method is to suspend a 
magnetized needle from a fine silk fiber or thread. 

The second method illustrates a very sensitive compass 
made from paper. Two magnetized needles are stuck 
through the sides with their north poles both at the same 
end. The paper support is mounted upon a third needle 
stuck through a cork. 

A compass which more nearly approaches the familiar 
type known as a pocket compass may be made from a small 
piece of watch-spring or clock-spring. 

The center of the needle is annealed or softened by 
holding it in the flame of an alcohol lamp and then allowing it 
to cool. 

Lay the needle on a piece of soft metal such as copper 
or brass, and dent it in the center with a punch. 

Balance the needle on the end of a pin stuck through the 
bottom of a pill-box. 

**Magnetic Substances** are those which are attracted by a 
magnet. Experiment with a number of different materials, 
such as paper, wood, brass, iron, copper, zinc, rubber, steel, 
chalk, etc. It will be found that only iron and steel are 
capable of being attracted by your magnet. Ordinary 
magnets attract but very few substances. Iron, steel, 
cobalt, and nickel are about the only ones worthy of 
mention. 

**Attraction through Bodies.** A magnet will attract a nail 
or a tack through a piece of paper, just as if nothing 
intervened. 

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   :alt: Fig. 8.—The Attraction of an Iron Nail through Glass. 
   
   Fig. 8.—The Attraction of an Iron Nail through *Glass*.

It will also attract through glass, wood, brass, and all 
other substances. Through an iron plate, however, the 
attraction is reduced or entirely checked because the iron 
takes up the magnetic effect itself and prevents the force 
from passing through and reaching the nail. 

A number of carpet-tacks may 
be supported from a magnet in 
the form of a chain. Each 
individual tack in the series 
becomes a *temporary* magnet by 
*induction*. 

If the tack in contact with 
the magnet be taken in the 
hand and the magnet suddenly withdrawn, the tacks will 
at once lose their magnetism and fall apart. 

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   :alt: Fig. 9.—A Magnetic Chain. 
   
   Fig. 9.—A Magnetic Chain. 

It will furthermore be found that a certain magnet will 
support a certain number of tacks 
in the form of a chain, but that if a 
*second* magnet is placed beneath the 
chain, so that its south pole is under 
the north pole of the original 
magnet, the chain may be lengthened by 
the addition of several other tacks.

The reason for this is that the magnetism in the tacks 
is increased by induction. 

**Magnets will Attract or Repel** each other, depending upon 
which poles are nearest. 

Magnetize a sewing-needle and hang it from a thread. 
Bring the north pole of a bar magnet near the lower end of 
the needle. If the lower end of the needle happens to 
be a south pole it will be attracted by the north pole 
of the bar magnet. If, on the other hand, it is a north 
pole, it will be repelled and you cannot touch it with 
the north pole of the bar magnet unless you catch it and 
hold it. 

This fact gives rise to the general law of magnetism: 
*Like poles repel each other and unlike poles attract each other.*

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   :alt: Fig. 10.—An Experiment Illustrating that Like Poles Repel Each Other and Unlike Poles Attract. 
   
   Fig. 10.—An Experiment Illustrating that Like Poles Repel Each Other and Unlike Poles Attract. 

Another interesting way of illustrating this same law is 
by making a small boat from cigar-box wood and laying a 
bar magnet on it. Place the north pole of the bar magnet 
in the bow of the boat. 

Float the boat in a basin of water. Bring the south pole 
of a second magnet near the stern of the boat and it will 
sail away to the opposite side of the basin. Present the north 
pole of the magnet and it will sail back again. 

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   :alt: Fig. 11.—A Magnetic Boat. 
   
   Fig. 11.—A Magnetic Boat. 

If the south pole of the magnet is presented to the bow 
of the boat the little ship will follow 
the magnet all 
around the basin. 

The repulsion of 
similar poles may 
be also illustrated 
by a number of 
magnetized sewing-needles fixed in small corks so that they 
will float in a basin of water with their points down. 

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   :alt: Fig. 12.—Repulsion between Similar Poles, Shown by Floating Needles. 
   
   Fig. 12.—Repulsion between Similar Poles, Shown by Floating Needles. 

The needles will then arrange themselves in different 
symmetrical groups, according to their number. 

A bar magnet thrust among them will attract or repel 
them depending upon its polarity. 

The upper ends of the needles should all have the same 
polarity, that is, all be either north or south poles. 

Magnetism flows along certain lines called 

**Lines of Magnetic Force.** These lines always form closed 
paths or circuits. The region in the neighborhood of a 
magnet through which these lines are passing is called the
*field of force*, and the path through which they flow is called 
the 

**Magnetic Circuit.** The paths of the lines of force can be 
easily demonstrated by placing a piece of paper over a 
bar magnet and then sprinkling iron filings over the paper, 
which should be jarred slightly in order that the filings may 
be drawn into the magnetic paths. 

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   :alt: Fig. 13.—A Magnetic "Phantom," Showing the Field of Force about a Magnet. 
   
   Fig. 13.—A Magnetic "Phantom," Showing the Field of Force about a Magnet. 

The filings will arrange themselves in curved lines, 
diverging from one pole of the magnet and meeting again at 
the opposite pole. The lines of force are considered as 
extending outward from the north pole of the magnet, curving 
around through the air to the south pole and completing 
the circuit back through the magnet. 

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   :alt: Fig. 14.—Magnetic Phantom showing the Lines of Force about a Horseshoe Magnet. 
   
   Fig. 14.—Magnetic Phantom showing the Lines of Force about a Horseshoe Magnet.  

Figure 14 shows the lines of force about a horseshoe 
magnet. It will be noticed that the lines cross directly 
between the north and south poles. 

The difference between the magnetic fields produced by 
like and unlike poles is shown in Figure 15. 

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   :alt: Fig. 15.—Lines of Force between Like and Unlike Poles.  
   
   Fig. 15.—Lines of Force between Like and Unlike Poles. 

A study of this illustration will greatly assist the mind in 
conceiving how attraction and repulsion of magnetic poles 
take place. 

It will be noticed the lines of force between two north 
poles resist each other and meet abruptly at the center. 
The lines between a north and a south pole pass in regular 
curves. 

**The Earth is a Great Magnet.** The direction assumed by 
a compass needle is called the *magnetic meridian*. 

The action of the earth on a compass needle is exactly 
the same as that of a permanent magnet. The fact that a 
magnetized needle places itself in the magnetic meridian 
is because the earth is a great magnet with lines of force 
passing in a north and south direction. 

The compass needle does not generally point exactly 
toward the true North. This is because the magnetic pole 
of the earth toward which the needle points is not situated 
at the same place as the geographical pole. 

**Magnetic Dip.** If a sewing-needle is balanced so as to be 
perfectly horizontal when suspended from a silk thread 
and is then magnetized, it will be found that it has lost its 
balance and that the *north* end points slightly downward. 

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   :alt: Fig. 16.—A Simple Dipping Needle.  
   
   Fig. 16.—A Simple Dipping Needle. 

This is due to the fact that the earth is round and that 
the magnetic pole which is situated in the far North is 
therefore not on a horizontal line 
with the compass, but below such 
a line. 

A magnetic needle mounted so 
as to move freely in a vertical 
plane, and provided with a scale 
for measuring the inclination, is 
called a 

**Dipping Needle.** A dipping needle 
may be easily made by thrusting 
a knitting-needle through a cork 
before it has been magnetized. 

A second needle is thrust through at right angles to the 
first and the arrangement carefully balanced, so that it will 
remain horizontal when resting on the edge of two glasses. 

Then magnetize the first needle by stroking it with a bar 
magnet. When it is again rested on the glasses it will be 
found that the needle no longer balances, but dips downward. 

**Permanent Magnets** have a number of useful applications 
in the construction of scientific instruments, voltmeters, 
ammeters, telephone receivers, magnetos and a number of 
other devices. 

In order to secure a very powerful magnet for some 
purposes a number of steel bars are magnetized separately, and 
then riveted together. A magnet made in this way is called 
a compound magnet, and may have either a bar or a 
horse-shoe shape. 

Magnets are usually provided with a soft piece of iron 
called an armature or "keeper." The "keeper" is laid 
across the poles of the magnet when the latter is not in use 
and preserves its magnetism. 

A blow or a fall will disturb the magnetic arrangement of 
the molecules of a magnet and greatly weaken it. The most 
powerful magnet becomes absolutely demagnetized at a 
red heat, and remains so after cooling. 

Therefore if you wish to preserve the strength of a 
magnetic appliance or the efficiency of any electrical instrument 
provided with a magnet, do not allow it to receive rough 
usage. 

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   :alt: CHAPTER II STATIC ELECTRICITY
   
CHAPTER II STATIC ELECTRICITY
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If you take a glass rod and rub it with a piece of flannel 
or silk, it will be found to have acquired a property which 
it did not formerly possess: namely, the power of attracting 
to itself such light bodies as dust or bits of thread and 
paper. 

Hold such a rod over some small bits of paper and watch 
them jump up to meet it, just as if the glass rod were 
a magnet attracting small pieces of iron instead of 
paper. 

The agency at work to produce this mysterious power is 
called *electricity*, from the Greek word "Elektron," which 
means *amber*. Amber 
was the first substance 
found to possess this 
property. 

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   :alt: Fig. 17.—An Electrified Glass Rod will Attract Small Bits of Paper.
   
   Fig. 17.—An Electrified Glass Rod will Attract Small Bits of Paper.
   
The use of amber 
begins with the dawn 
of civilization. Amber 
beads have been found 
in the royal tombs at 
Mycenae and at 
various places throughout Sardinia, dating from at least two 
thousand years before our era. 

Amber was used by the ancient world as a jewel and for 
decoration. 

The ancient Syrian woman used distaffs made of amber 
for spinning. As the spindle whirled around it often rubbed 
against the spinner’s garments and thus became *electrified*, 
as amber always does when it is rubbed. Then on nearing 
the ground it drew to itself the dust or bits of chaff or leaves 
lying there, or sometimes perhaps attracted the fringe of 
the clothing. 

The spinner easily saw this, because the bits of chaff 
which were thus attracted would become entangled in her 
thread unless she were careful. The amber spindle was, 
therefore, called the "harpaga" or "clutcher," for it 
seemed to seize such light bodies as if it had invisible talons, 
which not only grasped but held on. 

This was probably the first intelligent observation of an 
electrical effect. 

In the eighteenth century, when Benjamin Franklin 
performed his famous kite experiment, electricity was 
believed to be a sort of fiery atmospheric discharge which 
could be captured in small quantities and stored in 
receptacles such as Leyden jars. 

Franklin was the first to prove that the lightning 
discharges taking place in the heavens are electrical. 

The story of his experiment is very interesting. 

He secured two light strips of cedar wood, placed 
cross-wise and covered with a silk handkerchief for a kite. To 
the top of the upright stick of the kite was fastened a sharp 
wire about a foot long. The twine was of the usual kind, 
but was provided with a short piece of silk ribbon and a 
key. The purpose of the ribbon was possible protection 
against the lightning running through his body, silk being 
a "non-conductor," as will be explained a little farther on. 
The key was secured to the junction of the silk ribbon 
and the twine, to serve as a convenient conductor from 
which to draw the sparks—if they came. He did not have 
to wait long for a thunderstorm, and as he saw it gathering 
he went out with his son, then a young man twenty-two 
years of age. The great clouds rolled up from the horizon, 
and the gusts of wind grew fitful and strong. The kite felt 
a swishing blast and began to rise steadily, swooping this 
way and that as the breeze caught it. The thunder 
muttered nearer and nearer and the rain began to patter on the 
grass as the kite flew higher. 

The rain soon began to fall heavily, compelling Franklin 
and his son to take refuge under a near-by shed. The heavy 
kite, wet with water, was sailing sluggishly when suddenly 
a huge low-lying black cloud traveling overhead shot forth 
a forked flame and the flash of thunder shook the very 
earth. The kite moved upward, soaring straight into 
the black mass, from which the flashes began to come 
rapidly. 

Franklin watched the silk ribbon and the key. There 
was not a sign. Had he failed? Suddenly the loose fibers 
of the twine erected themselves. The moment had come. 
Without a tremor he advanced his knuckle to the key. 
And between his knuckle and the key passed a spark! then 
another and another. They were the same kind of little 
sparks that he had made hundreds of times with a *glass 
tube.* 

And then as the storm abated and the clouds swept off 
towards the mountains and the kite flew lazily in the blue, 
the face of Franklin gleamed in the glad sunshine. The 
great discovery was complete, his name immortal. 

The cause of lightning is the accumulation of the electric 
charges in the clouds, the electricity residing on the surface 
of the particles of water in the cloud. These charges grow 
stronger as the particles of water join together and become 
larger. As the countless multitude of drops grows larger and 
larger the "potential" is increased, and the cloud soon 
becomes heavily charged. 

Through the effects of a phenomenon called *induction,* 
and which we have already stumbled against in the 
experiment with the tacks and the magnetic chain, the force 
exerted by the charge grows stronger because of a charge 
of the opposite kind on a neighboring cloud or some object 
on the earth beneath. These charges continually strive 
to burst across the intervening air. 

As soon as the charge grows strong enough a vivid flash 
of lightning, which may be from one to ten miles long, takes 
place. The heated air in the path of the lightning expands 
with great force; but immediately other air rushes in to 
fill the partial vacuum, thus producing the terrifying sounds 
called *thunder*. 

In the eighteenth century, electricity was believed to be 
a sort of fiery atmospheric discharge, as has been said. 
Later it was discovered that it seemed to flow like water 
through certain mediums, and so was thought to be a fluid. 
Modern scientists believe it to be simply a vibratory 
motion, either between adjacent particles or in the ether 
surrounding those particles. 

It was early discovered that electricity would travel 
through some mediums but not through others. These 
were termed respectively "conductors" and 
"non-conductors" or insulators. Metals such as silver, copper, gold, 
and other substances like charcoal, water, etc., are good 
conductors. Glass, silk, wool, oils, wax, etc., are 
non-conductors or insulators, while many other substances, 
like wood, marble, paper, cotton, etc., are partial 
conductors. 

There seems to be two kinds of electricity, one called 
"static" and the other "current" electricity. The former 
is usually produced by friction while the latter is generated 
by batteries or dynamos. 

A very simple and well-known method of generating 
static electricity is by shuffling or sliding the feet over 
the carpet. The body will then become *charged*, and if the 
knuckles are presented to some metallic object, such as a 
gas-jet or radiator, a stinging little spark will jump out to 
meet it. 

.. figure:: images/Image23.jpg
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   :alt: A Double Lightning Discharge from a Cloud to the Earth.
   
   *From the author's "Wireless Telegraphy and Telephony" by permission.*
   **A Double Lightning Discharge from a Cloud to the Earth.**
 
The electricity is produced by the friction of the feet 
sliding over the carpet and causes the body to become 
electrified. 

Warm a piece of writing-paper, then lay it on a wooden 
table and rub it briskly with the hand. It soon will become 
stuck to the table and will not slide along as it did at first. 
If one corner is raised slightly it will tend to jump right 
back. If the paper is lifted off the table it will tend to cling 
to the hands and the clothing. If held near the face it 
will produce a tickling sensation. All these things happen 
because the paper is electrified. It is drawn to the other 
objects because they are *neutral*, that is, do not possess an 
electrical charge. 

.. figure:: images/Image24.jpg
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   :alt: Fig. 19.—A Piece of Dry Writing-Paper may be Electrified by Rubbing. 
   
   Fig. 19.—A Piece of Dry Writing-Paper may be Electrified by Rubbing. 

All experiments with static electricity perform better in 
the winter time, when it is cool and clear, than in the 
summer. The reason is that the air in winter is drier than in 
summer. Summer air contains considerable moisture and 
water vapor. Water vapor is a *partial* conductor of 
electricity, and the surrounding air will therefore conduct the 
static electricity away from your apparatus almost as fast 
as it can be produced in the summer time. 

.. figure:: images/Image25.jpg
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   :alt: Fig. 20.—A Surprise for the Cat.
   
   Fig. 20.—A Surprise for the Cat.

Some day during the winter time, when it is cool and clear, 
and the cat is near a fire or a stove, stroke the cat rapidly 
with the hand. The fur will stand up towards the hand 
and a faint crackling noise will be heard. The crackling 
is caused by small sparks passing between the cat and the 
hand. If the experiment is performed in a dark room, the 
sparks may be plainly seen. If you present your knuckle 
to the cat's nose a spark will jump to your knuckle and 
somewhat surprise the cat. 

If the day is brisk and cool, so that everything outside is 
frozen and dry, try combing the hair with a rubber comb. 
Your hair will stand up all over your head instead of lying 
down flat, and the faint crackling noise, showing that 
sparking is taking place as the comb passes through the 
hair, will be plainly heard. The electricity is produced by 
the friction between the hair and the comb. 

Electricity may be produced by friction between a 
number of substances. A hard rubber rod, a glass rod, a rubber 
comb or a stick of sealing-wax may be very easily 
electrified by rubbing them briskly with a piece of dry, warm 
flannel. 

**Electroscopes** are devices for detecting the presence of 
static electricity. 

.. figure:: images/Image26.jpg
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   :alt: Fig. 21.—A Paper Electroscope.
   
   Fig. 21.—A Paper Electroscope.

A very simple form of electroscope may be made in much 
the same manner as the paper compass described in the 
last chapter. It may be cut out of writing-paper and 
mounted on a pin stuck through a cork. If an electrified 
rod is held near the electroscope it may be made to whirl 
around in the same manner as a compass needle when a bar 
magnet is brought to it. 

**The Pith-Ball Electroscope** is a very simple device, in 
which a ball of cork or elder pith is hung by a fine silk 
thread from an insulated support. A suitable electroscope 
may be made from a glass bottle having a piece of wire thrust 
into the cork to support the pith ball. When the 
electrified rod is presented to the pith ball, it will fly out towards 
the rod. 

.. figure:: images/Image27.jpg
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   :alt: Fig. 22.—A Pith-Ball Electroscope.
   
   Fig. 22.—A Pith-Ball Electroscope.

If the pith ball is permitted to touch the glass rod, the 
latter will transfer some of its electricity and charge the 
ball. Almost immediately the pith ball 
will fly away from 
the glass rod, and 
no matter how near 
the rod is brought, 
it will refuse to be 
touched again. 

This action is 
much the same as 
that of the magnetized needle suspended from a thread when the similar pole of the magnet 
is presented to it. 

When the rod is first presented to the pith ball, the latter 
is neutral and does not possess an electrical charge. When 
the rod has touched the ball, however, some of the 
electricity from the rod passes to the ball, and after this they will 
repel each other. 

The reason is that the rod and the ball are *similarly* 
charged and *similarly charged bodies will repel each other*. 

.. figure:: images/Image28.jpg
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   :alt: Fig. 23.—A Double Pith-Ball Electroscope. 
   
   Fig. 23.—A Double Pith-Ball Electroscope. 

If you are a good observer you might have noticed when 
experimenting with an electrified rod and the small bits of 
paper, that some of the little papers were first attracted and 
flew upwards to the 
rod, but having once 
touched it, were 
quickly repelled. 

The repulsion 
between two similarly 
electrified bodies 
may be shown by a 
double electroscope. 

A double electroscope is made by 
hanging two pith 
balls on two silk threads from the same support. 

Electrify a glass rod and touch it to the pith balls. They 
will immediately fly apart 
because they are electrified with the same kind 
of electricity. 

**The Gold-leaf Electroscope** is one of the most 
sensitive means which 
can be employed to detect small amounts of static electricity. 

.. figure:: images/Image29.jpg
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   :alt: Fig. 24.—A Gold-Leaf Electroscope. 
   
   Fig. 24.—A Gold-Leaf Electroscope. 

It is a very simple instrument and is easily made in a short time. A couple of 
narrow strips of the thinnest tissue paper, or, better still, 
two strips of gold leaf, are hung from a support in a 
wide-mouthed glass bottle which serves at once to insulate and 
protect the strips from draughts of air. 

The mouth of the jar is closed by a plug of paraffin wax, 
through the center of which passes a small glass tube. A 
stiff copper wire passes through the tube. The lower end of 
the wire is bent at right angles to furnish support for the 
strips of gold leaf. A round sheet metal disk about the size 
of a quarter is soldered to the upper end of the rod. 

If an electrified stick of sealing-wax or a glass rod is 
presented to the disk of the electroscope, the strips will repel 
each other very strongly. If the instrument is sensitive, 
the strips should begin to diverge some time before the rod 
reaches the disk. It is possible to make an electroscope so 
sensitive that chips formed by sharpening a pencil will 
cause the strips to diverge. 

**There are two kinds of static electricity.** Rub a glass 
rod with a piece of silk and then suspend it in a wire stirrup 
as shown in Figure 25. Excite a 
second rod also with a piece of 
silk and bring it near one end of 
the suspended one. The suspended 
rod is *repelled* and will swing away 
from the one held in the hand. 

.. figure:: images/Image30.jpg
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   :alt: Fig. 25.—Method of Suspending an Electrified Rod in a Wire Stirrup. 
   
   Fig. 25.—Method of Suspending an Electrified Rod in a Wire Stirrup. 

Now rub a stick of *sealing-wax* 
with a piece of *flannel* until the 
sealing-wax is electrified. Then 
bring the stick of sealing-wax near the end of the suspended 
rod. The rod will be *attracted* to the sealing-wax. 

If you experiment further you will find that two sticks 
of sealing-wax will repel each other. 

.. figure:: images/Image31.jpg
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   :alt: Fig. 26.—Similarly Electrified Bodies Repel Each Other. Dissimilarly Electrified Ones Attract Each Other. 
   
   Fig. 26.—Similarly Electrified Bodies Repel Each Other. Dissimilarly Electrified Ones Attract Each Other. 

This experiment indicates that there are two kinds of 
electrification: one developed by rubbing glass with silk 
and the other developed by rubbing sealing-wax with 
flannel. 

In the first instance, the glass rod is said to be *positively* 
electrified, and in the latter case the sealing-wax is 
*negatively* electrified. 

The same law that applies to magnetism also holds true 
in the case of static electricity, and similarly electrified 
bodies will repel each other and dissimilar ones attract. 

**The Electrophorus** is an instrument devised by Volta in 
1775 for the purpose of obtaining static electricity. 

.. figure:: images/Image32.jpg
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   :alt: Fig. 27.—The Electrophorous
   
   Fig. 27.—The Electrophorous

It is easily constructed and will furnish a source of 
electricity for quite a number of interesting experiments. An 
electrophorus consists of two parts, a round cake of 
resinous material cast in 
a metal dish or pan, 
and a round metal 
disk which is provided with an insulating handle. 

To make an electrophorus, first procure an old cake or 
pie tin, and fill it with bits of resin or sealing-wax. Place 
the pan in a warm spot upon the stove where the resin will 
melt, taking care not to overheat or it will spatter and 
possibly take fire. As the resin melts, add more until the pan 
is nearly full. When all is melted, remove from the fire and 
set it away where it may cool and harden in the pan 
without being disturbed. 

Cut a circular disk out of sheet tin, zinc, or copper, 
making the diameter about two inches less than that of the 
pie pan. Solder a small cylinder of tin or sheet brass to 
the center of the disk to aid in supporting the handle. The 
latter is a piece of glass tubing about three-quarters of an 
inch in diameter and four or five inches long, placed in the 
center of the cylinder and secured with molten sealing-wax. 

In order to use the electrophorus the resinous cake must 
first be beaten or briskly rubbed with a piece of warm 
woolen cloth or flannel. Then place the disk on the cake 
holding the insulating handle with the right hand. Touch 
the cover or the disk momentarily with the forefinger of 
the left hand. After the finger is removed, raise the disk 
from the cake by picking it up with the glass insulating 
handle. The disk will now be found heavily charged with 
positive electricity, and if the knuckles are presented to 
the edge, a spark will jump out to meet them. 

.. figure:: images/Image33.jpg
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   :alt: Fig. 28.—An Electric Frog-Pond.
   
   Fig. 28.—An Electric Frog-Pond.

The cover may then be replaced, touched, and once more 
removed. It will yield any number of sparks, the resinous 
cake only needing to be recharged by rubbing once in a long 
while. 

**An Electric Frog-Pond** may be experimented with by 
cutting out some small tissue-paper frogs. Moisten them 
a little and lay them on the cover of the electrophorus. 
Touch the electrophorus with the finger and then raise it 
with the insulating handle. If the "frogs" are not too wet 
they will jump from the cover upon the table as soon as the 
cover is raised. 

.. figure:: images/Image34.jpg
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   :alt: STATIC ELECTRIC MACHINES
   
CHAPTER III STATIC ELECTRIC MACHINES
````````````````````````````````````

A Cylinder Electric Machine 
###########################

The electrophorus described in the last chapter is capable 
of furnishing sufficient electricity for many interesting 
experiments, but for the purpose of procuring larger supplies 
of electricity, a static electric machine is necessary. 

An electric machine is composed of two parts, one for 
producing the electricity by the friction of two surfaces 
rubbing against each other, and the other an arrangement 
for collecting the electricity thus formed. 

The earliest form of electric machine consisted of a ball 
of sulphur fixed upon a spindle which could be rotated 
by means of a crank. When the dry hands were pressed 
against the sulphur by a person standing on a cake of 
resin, which insulated him, sparks could be drawn from 
his body. 

Later a leather cushion was substituted for the hands, and 
a glass cylinder for the ball of sulphur, so that the frictional 
electric machine now consists of a cylinder or a disk of glass 
mounted upon a horizontal axis capable of being turned by 
a handle. A leather cushion, stuffed with horsehair and 
covered with a powdered amalgam of zinc or tin, presses 
against one side of the cylinder. A "prime" conductor in 
the shape of an elongated cylinder presents a row of fine 
metal spikes, like the teeth of a rake, to the opposite side. 
A flap of silk attached to the leather cushion passes over 
the cylinder and covers the upper half. 

.. figure:: images/Image35.jpg
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   :alt: Fig. 29.—Front View of a Cylinder Electric Machine. 
   
   Fig. 29.—Front View of a Cylinder Electric Machine. 
   
When the handle of the machine is turned, the friction 
produced between the leather cushion and the glass 
generates a supply of positive electricity on the glass, which is 
collected, as the cylinder revolves, by 
the row of sharp 
points, and transferred to the prime 
conductor. 

The first thing required in the construction of an 
electric machine is a 
large glass bottle having a capacity of from two to four 
quarts. 

The insulating power of glass varies considerably. 
Common green glass (not white glass colored green by copper, 
but glass such as the telegraph insulators are made from) 
generally insulates the best. Some sorts of white glass, the 
Bohemian especially, are good insulators, but this quality 
will not usually be found in ordinary bottles. 

.. figure:: images/Image36.jpg
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   :alt: Fig. 30.—Method of Finding the Center of a Circle. 
   
   Fig. 30.—Method of Finding the Center of a Circle. 

Select a smooth bottle which has no lettering embossed 
upon it, and stand it upon a piece of white paper. Trace 
on the paper a line around the circumference of the bottle 
so that the circle thus formed is of the same size as the 
bottom of the bottle. Lay a carpenter’s square on the 
circle so that the point *C* just touches the circumference. 
Draw a line from *A* to *B* where the sides of the square cut 
the circumference. The point in the middle of this line is 
the center of the circle. 

Place the paper on the bottom of the bottle so that the 
circle coincides with the circumference, and mark the center 
of the bottle. 

The bottle must now be drilled. This is accomplished 
with a small three-cornered file, the end of which has been 
broken off so as to form a ragged cutting edge. The file is 
set in a brace and used like an ordinary drill. During the 
boring process the drill must be frequently lubricated with 
a mixture of gum camphor and turpentine. The drilling, 
which will require almost an hour before the glass is pierced, 
if the bottle is a thick one, should be performed slowly and 
carefully, so as to avoid all danger of cracking the glass. 
The hole, when finished, should be from one-quarter to 
three-eighths of an inch in diameter. 

After the hole has been bored, fit a wooden plug into the 
neck of the bottle and cement it there with a mixture 
composed of one-half a pound of resin, five ounces of beeswax, 
one-quarter of an ounce of plaster of Paris, and 
three-quarters of an ounce of red ocher, melted together over a 
moderately warm stove. Dip the plug in the molten cement 
and force it into the neck of the bottle. When the cement 
dries it will be impossible to remove it. 

The sizes of bottles vary, so that it is quite impossible to 
give dimensions which must be closely followed in 
constructing the machine. Those in the text are approximate. The 
drawings have been made to scale so as to show the 
proportions the parts bear to each other. 

A heavy wooden base will be required to mount the 
machine on. Two uprights are mounted on the base to 
support the axis of the bottle. Through one of these bore 
a hole of the same diameter as the wooden plug fitted in 
the neck of the bottle. The end of the wooden plug 
projecting through the upright is notched and fitted with a 
crank so that the bottle may be revolved. The handle of 
the crank is an ordinary spool having one flange cut off and 
mounted with a screw and a washer. 

.. figure:: images/Image37.jpg
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   :alt: Fig. 31.—The "Rubber."
   
   Fig. 31.—The "Rubber."

The machine is now ready for the "rubber" and "prime 
conductor." The rubber is a piece of wood one inch square 
and from six to eight inches long. A piece of undressed 
leather is tacked on as shown in the illustration and stuffed 
with horsehair. The wood is 
shellacked and covered with 
tin-foil previous to tacking on the 
leather. A strip of wood, two 
inches wide and one-half an inch 
thick, is fastened to the back 
of the rubber. The strip should 
be just long enough so that when 
the lower end rests on the base 
the rubber is level with the axis 
of the bottle. The lower end 
may be fastened to the base by 
means of a small brass hinge. 
Two rubber bands stretch from 
hooks between the rubber and the base so as to pull the 
former tightly against the bottle. The illustration shows 
a method of mounting the rubber on a foot-piece held to 
the base with a thumb-nut so that it may be slid back and 
forth and the pressure varied at will. 

The prime conductor is formed from a piece of 
curtain-pole two inches in diameter and eight inches long. The 
ends are rounded with a rasp and then smoothed with 
sandpaper. The whole surface is then shellacked and covered with 
a layer of tinfoil. The heads of a number of dressmaker’s 
pins are cut off, and the pins forced into the side of the prime 
conductor with a pair of pincers. They should form a row 
like the teeth of a rake about three-eighths of an inch apart. 
A hole is bored in the center of the under side of the prime 
conductor to receive a glass rod one-half inch in diameter. 
A second hole of the same size is bored in the base in such a 
position that when the glass rod is in place, the teeth on 
the prime conductor are on a level with the axis of the 
bottle, and their points about 3-32 of an inch away from 
the glass. The glass rod must be used in order to insulate 
the prime conductor and prevent the escape of the 
electricity. It is secured with some 
of the cement described on page 
33. A piece of water-gauge 
glass may be used in place of a 
glass rod. 

.. figure:: images/Image38.jpg
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   :alt: Fig. 32.—The Prime Conductor or Collector. 
   
   Fig. 32.—The Prime Conductor or Collector. 

A strip of oiled silk, or in its 
place a strip of silk which has 
been shellacked, eight or nine 
inches wide, and long enough 
to reach half-way around the bottle, is tacked to the 
rubber so that the silk covers the upper half of the cylinder 
and comes over to within one-quarter of an inch of the 
steel points. 

The machine is now complete, and when the handle is 
turned rapidly, you will be able to draw sparks from the 
prime conductor. The sparks will probably be very short, 
about one-half of an inch long. These can be increased, 
however, to three inches, if the glass is of the right quality, 
by treating the rubber with amalgam. 

The amalgam is formed by melting one ounce of tin and 
adding to it one ounce of zinc in small bits. As soon as 
the zinc has also melted add to the mixture two ounces of 
mercury which has been previously warmed. Be careful 
not to inhale any of the vapor during this operation. Pour 
the mixture into a vessel of cold water, which will reduce 
the metal to small grains. Pour off the water and grind 
the amalgam to a powder by pounding the grains with a 
hammer. 

The leather rubber should be *thinly* smeared with lard 
and the powdered amalgam rubbed on it. 

In order to obtain the greatest effect from an electric 
machine, it must be carefully freed from dust and particles 
of amalgam adhering to the glass, and the insulating column 
rubbed with a warm woolen cloth. The best results are 
obtained by placing the machine near a stove or radiator 
where it is warm. 

.. figure:: images/Image39.jpg
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   :alt: Fig. 33.—The Complete Cylinder Electric Machine. 
   
   Fig. 33.—The Complete Cylinder Electric Machine. 
   
A Wimshurst Machine
###################

The Wimshurst Machine consists of two varnished glass 
plates revolving in opposite directions. On the outside of 
each of these plates are cemented a number of tinfoil 
"sectors," arranged radially. Two conductors at right 
angles to each other extend obliquely across the plates, one 
at the back and the other at the front. These conductors 
each terminate in brushes of tinsel which electrically excite 
the "sectors" as the plates revolve. The electricity is 
collected by a set of "collectors" arranged in a somewhat 
similar manner to the collector on the cylinder electric 
machine. 

**The Glass Plates** are each eighteen inches in diameter. 
Purchase two panes of clear glass twenty inches square 
from a glass dealer. The white glass is far preferable to the 
green glass and will make the best electric machine. The 
plates should be of the thickness known as "single light" 
and should be perfectly free from wavy places, bubbles, or 
other imperfections. 

.. figure:: images/Image40.jpg
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   :alt: Fig. 34.—Paper Pattern for laying out the Plates. 
   
   Fig. 34.—Paper Pattern for laying out the Plates. 
   
The work is first laid out on a piece of stiff paper twenty 
inches square as a pattern. Describe a circle four inches in 
diameter. Using the same center, draw other circles, making 
them respectively eight, sixteen, and eighteen inches in 
diameter. Then mark sixteen radial lines, from the center, 
making them equal distances apart, as shown in Figure 34. 

.. figure:: images/Image41.jpg
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   :alt: Fig. 35.—Plate with Sectors in Position, and a Pattern for the Sectors. 
   
   Fig. 35.—Plate with Sectors in Position, and a Pattern for the Sectors. 
   
Lay one of the glass panes over the pattern and cut out a 
glass circle eighteen inches in diameter, or perhaps you may 
be able to have a glazier do the cutting for you and so save 
considerable trouble and possible breakage. Two such 
plates should be made. 

The Sectors are cut from heavy flat tinfoil according to 
the pattern shown in Figure 35. They should be made one 
inch and one-half wide at the wide end and three-quarters 
of an inch at the other end. They are each four inches 
long. Thirty-two such sectors are required. The easiest 
way to make them is to cut out a pattern from heavy 
cardboard to serve as a guide. 

Clean and dry both of the glass plates very carefully and 
then give them each two thin coats of white shellac. After 
they have been dried, lay one of the plates on the paper 
pattern so that the outside of the plate will coincide with 
the largest circle on the paper. 

Then place a weight in the center of the plate so that it 
will not move, and stick sixteen of the tinfoil sectors on the 
plate with thick shellac. The sectors are arranged 
symmetrically on the plate, using the eight-inch and sixteen-inch 
circles and the radial lines as guides. Both plates should 
be treated in this manner. Each sector should be 
carefully pressed down on the glass, so that it will stick 
smoothly without air-bubbles or creases. When all the 
sectors are in place the plates will appear like that shown 
in Figure 35. 

**The Bosses** will have to be turned out at a wood-working 
mill or at some place where they have a turning-lathe. The 
bosses are four inches in diameter at the large end and one 
inch and one-half at the other. A groove is turned near 
the small end of each to accommodate a round leather 
belt. 

A hole should be made in each boss about half-way 
through from the small end. These holes should be bushed 
with a piece of brass tubing having an inside diameter of 
one-half inch. The tubing should go into the hole very 
snugly and be a "driven fit." 

.. figure:: images/Image42.jpg
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   :alt: Fig. 36.—A Side View of one of the Bosses, showing the Brass Bushing used. 
   
   Fig. 36.—A Side View of one of the Bosses, showing the Brass Bushing used. 
   
The bosses should both be given a coat of shellac, and 
after this is dry, fastened to the glass plates on the same side 
to which the tinfoil sectors 
are attached. The best plan 
is to lay the disks on the 
paper pattern and adjust 
them until the outer edge 
coincides with the largest 
circle. 

Then apply some *bichromate glue* to the flat surface 
of one of the bosses and place the latter in the center of the 
plate in line with the smallest circle. 

Place a weight on the boss to hold it down firmly against 
the plate and leave it over night, or for ten or twelve hours, 
until thoroughly dry. 

The glue is prepared by placing some high-grade glue in 
a tin cup and covering it with cold water. Allow it to stand 
until the glue absorbs all the water it will and becomes 
soft. Then pour the water off and add enough *glacial acetic 
acid* to cover the glue. 

Heat the mixture until it is reduced to a liquid, stirring it 
until it is perfectly smooth. Add a teaspoonful of powdered 
bichromate of potash to the glue. 

The glue must now be kept in the dark, for sunlight will 
"set" the glue so that it becomes insoluble. 

The Frame of the machine is composed of two strips 
twenty-five inches long, three inches wide, and an inch and 
one-half in thickness, and two cross-pieces of the same 
thickness and width fifteen inches long. 

.. figure:: images/Image43.jpg
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   :alt: Fig. 37.—The Frame. 
   
   Fig. 37.—The Frame. 
   
Notches are cut at both sides of the base to admit the 
feet of the uprights. 

**The Uprights** are seventeen inches long, three inches wide, 
and one and one-half inches thick. 

.. figure:: images/Image44.jpg
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   :alt: Fig. 38.—The Upright.
   
   Fig. 38.—The Upright.
   
The notch at the foot is cut the same width as the 
thickness of the long members of the frame and is arranged so 
that when fitted in 
place, the foot of the 
upright will rest on 
the table in line with 
the bottom of the 
cross-pieces. 

**The Driving-Wheels** are turned out of wood on a lathe. 
They are seven inches in diameter and seven-eighths of an 
inch thick. A groove should be turned in the edge to carry 
a small round leather belt. The wheels are mounted on a 
wooden axle made from a round curtain-pole. They are 
glued to the axle and arranged so that the grooves will fall 
directly underneath the pulleys turned in the bosses. 

.. figure:: images/Image45.jpg
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   :alt: Fig. 39.—The Driving-Wheels and Axle. 
   
   Fig. 39.—The Driving-Wheels and Axle. 

The ends of the axle pass through the uprights, five inches 
above the bottom. 

The front end of the axle is fitted with a crank and a handle. 

.. figure:: images/Image46.jpg
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   :alt: Fig. 40—The Boss and Axle. For sake of clearness, the Plate is not shown.
   
   Fig. 40—The Boss and Axle. For sake of clearness, the Plate is not shown.

The plates are mounted on short iron axles passing through 
the top of the upright into the brass bushings. One end of 
each of the axles is 
filed flat where it 
passes through the 
wood upright so that 
it may be firmly held 
by a set-screw and 
prevented from revolving. 

Fasten a small fiber washer to the center of one glass disk 
so that it will separate the plates and prevent them from 
touching when revolving. 

The collectors, quadrant rods, etc., are mounted on glass 
rods one inch in diameter. The bottoms of the rods fit in 
holes (*H H*) bored in the cross-pieces of the base, Figure 37. 
The upper ends are each fitted with a brass ball two inches 
in diameter. The balls are mounted on the rods by 
soldering a piece of brass tubing to the ball and slipping it over 
the rod. The rods should 
be of the proper length to 
bring the center of the balls 
on a line with the center of 
the plates. 

.. figure:: images/Image47.jpg
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   :alt: Fig. 41—Showing how the Ball, Comb, etc., are mounted on the Glass Rod.
   
   Fig. 41—Showing how the Ball, Comb, etc., are mounted on the Glass Rod.

Make two forks as shown 
in Figure 42 out of brass 
rod, three-sixteenths of an 
inch in diameter and solder 
brass balls at the ends. 
The forks are eleven inches 
long. 

A number of small holes 
must be bored in the "prongs" and pins made by cutting 
ordinary dressmakers’ pins in half and soldering them in 
place. These pins, mounted on the forks, form the combs 
or collectors. 

Bore a horizontal hole through each of the brass rods on 
the top of the glass rods and pass the shanks of the forks 
through and solder them in place. 

One of the shanks may be provided with a discharge ball 
at the end as shown by *D B* in Figure 44. The other is 
provided with a hard rubber handle made from a piece of 
rod. Bore a three-eighths hole directly in the top of each 
brass ball to receive the quadrant rods forming the 
spark-gap. 

.. figure:: images/Image48.jpg
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   :alt: Fig. 42.—A Comb or Collector. 
   
   Fig. 42.—A Comb or Collector. 

The quadrant rods extend over the top of the plates and 
are three-eighths of an inch in diameter. They are loose 
in the tops of the balls so that they may be moved about or 
removed entirely. 

A small brass ball three-quarters of an inch in diameter 
should be soldered to the top of one of the quadrant rods 
and a similar ball two inches in diameter to the other. 

.. figure:: images/Image49.jpg
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   :alt: Fig. 43.—Showing how the Tinsel Brushes are arranged on the "Neutralizer" Rods.

   Fig. 43.—Showing how the Tinsel Brushes are arranged on the "Neutralizer" Rods.

Two large brass balls, two inches in diameter, are fitted 
over the ends of the axles, which project through the 
uprights. Bore a one-quarter-inch hole through each ball at 
right angles to the axle and slip a one-quarter-inch brass 
rod through and solder it fast. 

.. figure:: images/Image50.jpg
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   :alt: Fig. 44.—The Complete Wimshurst Electric Machine.

   Fig. 44.—The Complete Wimshurst Electric Machine. B B B B, *Brushes*. C C, *Combs*. D B, *Discharge Ball*. I I, *Glass Rods*. H, *Handle*. Q Q, *Quadrant Rods*. S S S S S, *Sectors*. S G, *Spark-Gap*. P P, *Driving-Wheels*. For the sake of clearness, several of the sectors are not shown. 

The ends of the rods should be tipped with a bunch of 
tinsel or fine copper wires and be curved so that the brushes 
so formed will just touch the sectors on the disks when the 
latter are revolved. 

These are the neutralizers and are arranged in the 
approximate positions shown in Figure 44. 

The driving-wheels are connected to the bosses by means 
of small round leather belts. The belt at the rear of the 
machine is crossed in order to make the plates revolve in 
opposite directions. 

If the machine has been properly built it is now ready for 
operation. It may be necessary to charge the machine the 
first time that it is used by touching several of the sectors 
with the charged cover of an electrophorus. Then if the 
handle is turned the accumulated electricity should 
discharge across the spark-gap at the top of the machine in 
the form of bright blue sparks. 

Experiments with an Electric Machine
#################################### 

Many interesting experiments can be performed with an 
electric machine. The number is almost unlimited. A few 
of the most instructive ones are described below. Others 
can be found in almost any text book on physics. 

**The Leyden jar** consists of a glass jar coated with tinfoil 
part way up on both the outside and inside. Through the 
wooden stopper passes a brass rod or a heavy copper wire 
which connects with the inner coating of tinfoil by means 
of a small brass chain. The upper and outside end of the 
rod usually terminates in a brass ball or knob. 

It is a very simple matter to make a good Leyden jar. 

.. figure:: images/Image51.jpg
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   :alt: Fig. 45.—The Leyden Jar.

   Fig. 45.—The Leyden Jar.
   
The jar must be thoroughly cleaned and dried before 
coating. The inside is then given a thorough brushing over with 
shellac or varnish. Before it is dry, carefully insert the 
tin-foil and press it smoothly against the glass. The outside 
of the jar is treated and coated in the same manner. The 
inside and outside of the bottom are also coated by cutting 
the tinfoil in circular pieces and shellacking them on. 

In order to charge the Leyden jar, grasp it in the hand 
near the bottom and hold the knob against the prime 
conductor while turning the handle of the machine. 

.. figure:: images/Image52.jpg
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   :alt: Fig. 46.—A Wooden Mortar for Igniting Gunpowder. 

   Fig. 46.—A Wooden Mortar for Igniting Gunpowder. 
   
**Igniting gunpowder.** Bore a hole one-half inch in 
diameter and one inch deep in a block of hardwood. Pass two 
small brass wires through holes in the sides, letting the ends 
of the wires be about one-eighth of an inch apart. Pour 
a little gunpowder in loosely over the wires. Tie a piece 
of thoroughly moistened cotton twine, three inches long, to 
one of the wires and 
attach it to the outside coating of a 
charged Leyden jar. 

Connect the knob of 
the jar to the other 
wire. The gunpowder will immediately explode. Keep the face and hands 
away from the gunpowder when performing this experiment. 

.. figure:: images/Image53.jpg
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   :alt: Fig. 47.—An Electric Umbrella. 

   Fig. 47.—An Electric Umbrella. 
   
**Electric Umbrella.** The repulsion of similarly electrified 
bodies which was illustrated 
by the action of the pith ball 
electroscope may be better 
illustrated by pasting some narrow 
streamers of tissue paper about 
one-eighth of an inch wide and 
four inches long to a small cork 
covered with tinfoil. The cork 
is mounted on the upper end of 
a stiff copper wire supported in 
a bottle. When the wire is 
connected to the prime conductor and the machine set in 
motion, the strips will spread out like an umbrella. 

**Lightning Board.** A pane of glass is thoroughly cleaned 
and then given a coat of shellac or varnish. Before the 
varnish is dry, press on a piece of tinfoil large enough to 
cover one side of the glass and rub it down smoothly. 

.. figure:: images/Image54.jpg
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   :alt: Fig. 48.—A Lightning Board. 

   Fig. 48.—A Lightning Board. 
   
After the shellac or varnish is dry, cut the tinfoil up into 
innumerable little squares with a sharp knife and ruler, 
leaving two solid strips of tinfoil at the ends of the glass 
pane. 

The pane is mounted by cementing it in a slot in the cork 
of a bottle. Connect one of the tinfoil strips to the prime 
conductor and the other to the earth or the body. When 
the machine is turned, innumerable little sparks will pass 
between the tinfoil squares and give an appearance very 
similar to that of lightning. 

.. figure:: images/Image55.jpg
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   :alt: Fig. 49.—An Electric Dance. 

   Fig. 49.—An Electric Dance. 
   
**The Electrical Dance.** A number of little balls of cork 
or pith are enclosed in a cylinder of glass about two and 
one-half or three inches high formed by cutting off the top of a 
lamp chimney. The top and bottom of the cylinder are 
closed by two circular pieces of sheet brass or copper. The 
top disk is connected to the prime conductor while the 
bottom one is connected to the rubber. When the machine 
is set in motion, the little 
balls will dance up and 
down. Bits of feather or 
paper cut to represent figures of men and women 
may be used as well as pith 
or cork balls. 

**The Electric Whirl.** The 
whirl consists of an S 
shaped piece of brass wire, 
pointed at both ends and supported on a needle by a 
little conical depression made in the center with a 
punch. 

.. figure:: images/Image56.jpg
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   :alt: Fig. 50.—An Electric Whirl.

   Fig. 50.—An Electric Whirl.
   
The needle is stuck in a cork in the top of a bottle 
and connected with 
the prime conductor of 
the electric machine. 
When the latter is set 
in motion, the whirl 
will commence to revolve at a high rate of 
speed. 

**Lichtenberg’s Figures** can be produced by charging a Leyden jar by connecting 
the knob or inside coating with the prime conductor and 
holding the outside coating in the hand. 

Then trace a small circle on the electrophorus bed with 
the knob. 

Charge a second Leyden jar by connecting the outside 
coating with the prime conductor. 

The inside coating should be connected to the rubber 
by means of a wire fastened 
to the knob. The same result 
may be obtained by connecting 
the outside coating with the 
prime conductor and touching 
the knob with the hand. 

Then trace a cross on the 
electrophorus bed with the 
knob, making the cross inside 
of the circle. 

.. figure:: images/Image57.jpg
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   :alt: Fig. 51.—Lichtenberg's Figures. 

   Fig. 51.—Lichtenberg's Figures. 
   
Shake a mixture of red lead 
and sulphur through a muslin bag from a height of several 
inches over the electrophorus. 

The red lead will accumulate around the cross and the 
sulphur around the circle. 

.. figure:: images/Image58.jpg
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   :alt: CELLS AND BATTERIES

CHAPTER IV CELLS AND BATTERIES
``````````````````````````````

In order that the young experimenter may obtain 
electricity for driving his various electrical devices it is 
necessary to resort to batteries, a small dynamo, or the 
house-lighting current. 

All houses are not supplied with electric current. 
Furthermore, many boys have no source of power from which to 
drive a small dynamo. Batteries must therefore be 
resorted to in the majority of cases. 

A number of different cells and batteries are described in 
this chapter. All of them are practical, but after buying 
zinc, chemicals, etc., for any length of time, figure out what 
your batteries *cost* you to make. The real value is not their 
cost in dollars and cents but in what you have *learned* in 
making them. If you have a continuous use for electrical 
current for running *small* electrical devices it is cheaper to 
buy dry cells, or what is better, a *storage battery*, and have 
it *recharged* when necessary. 

*Build your own batteries first*. Then after you have learned 
how they are made and something about their proper care 
buy them from some reliable electrical house. 

Batteries are always interesting to the average 
experimenter, and when properly made are one of the most useful 
pieces of apparatus around the home, laboratory, or shop 
that it is possible to construct. Many hundreds of 
thousands of experiments have been carried out by capable men 
in an effort to discover or devise a perfect battery, and the 
list of such cells is very great. 

Only the most common forms, which are simple and 
inexpensive to construct but will at the same time render fair 
service, have been chosen for description. 

Cells are usually considered *one* element or jar of a 
battery. A *cell* means only one, while a *battery* is a *group* of 
cells. It is not a proper use of the word to say "battery" 
when only *one* cell is implied. This 
is a very common error. 

**The Voltaic cell** is called after its 
inventor, Volta, a professor in the 
University of Pavia, and dates 
back to about the year 1786. 

.. figure:: images/Image59.jpg
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   :alt: Fig. 52.—The Voltaic Cell. 
   
   Fig. 52.—The Voltaic Cell. 

A simple voltaic cell is easily 
made by placing some water mixed 
with a little sulphuric acid in a glass 
tumbler and immersing therein two 
clean strips, one of zinc and the 
other of copper. The strips must be kept separate from 
each other. The sulphuric acid must be diluted by mixing 
it with about ten times its volume of water. In mixing acid 
with water always remember never to pour water into acid 
but to perform the operation the other way and pour the 
acid into the water. A copper wire is fastened with a screw 
or by soldering to the top of each of the strips, and care 
must be exercised to keep the wires apart. 

As has been said, the zinc and copper must never be 
allowed to touch each other in the solution, but must be kept 
at opposite sides of the jar. 

The sulphuric acid solution attacks the zinc, causing it 
slowly to waste away and disappear. This action is called 
*oxidation*, and in reality is a very slow process of 
burning. The consumption of the zinc furnishes the electric 
energy, which in the case of this cell will be found to be 
sufficient to ring a bell or buzzer, or run a very small toy 
motor. 

As soon as the plates are immersed in the acid solution, 
bubbles will begin to rise from the zinc. These bubbles 
contain a gas called hydrogen and they indicate that a 
chemical action is taking place. The zinc is being dissolved 
and the *hydrogen* gas is being set free from the acid. It will 
be noticed that no bubbles arise from the copper plate and 
that there is little if any chemical action there. In other 
words, it seems that the chemical action at one plate is 
stronger than that at the other. 

A cell might be likened to a furnace in which the zinc is 
the fuel which is burned to furnish the energy. We know 
that if the zinc is burned or oxidized in the open air it will 
give out energy in the form of *heat*. When it is burned or 
oxidized slowly in acid in the presence of another metal it 
gives out its energy in the form of *electricity*. The acid 
might be likened to the fire, and the copper to a hand which 
dips into the cell to pick up the current and takes no part 
chemically. 

If a wire is connected to each of the plates and the free 
ends of the wires touched to the tip of the tongue it will 
produce a peculiar salty taste in the mouth indicating the 
presence of a current of electricity. 

If the wires are connected to an electric bell, the bell will 
ring, or, instead, the current may be used to run a small 
motor. If the cell is made of two zinc plates or two copper 
plates, the bell will not ring, because no electricity will be 
produced. In order to produce a current, the electrodes 
must be made of two different materials upon which the 
acid acts differently. Current may be obtained from a cell 
made with a zinc and carbon plate or from one with zinc and 
iron. 

Therefore, in order to make a battery it is necessary to 
have a metal which may be consumed, a chemical to 
consume or oxidize it, and an inactive element which is merely 
present to collect the electricity. 

When the wires connected to the two plates are joined 
together, a current of electricity will flow from the copper 
plate through the wire to the zinc. The copper is known as 
the *positive* pole and the zinc as the *negative*. 

A simple voltaic cell may be easily made by cutting out a 
strip of zinc and a strip of copper, each 3 1/2 inches long, and 
one inch wide. A small hole bored through the upper end 
of the strips will permit them to be mounted on a wooden 
strip with a screw as shown in Figure 53. The connecting 
wires are placed under the heads of the screws. Care should 
be exercised to arrange the screws used for mounting the 
electrodes to the wooden strip so that they do not come 
exactly opposite, and there is no danger of the points 
touching and forming a short circuit. 

.. figure:: images/Image60.jpg
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   :alt: Fig. 53.—The Elements of Simple Voltaic Cell. 
   
   Fig. 53.—The Elements of Simple Voltaic Cell.

.. figure:: images/Image61.jpg
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   :alt: Fig. 54.—A Home-Made Voltaic Cell. 
   
   Fig. 54.—A Home-Made Voltaic Cell.

An ordinary tumbler or jelly glass will make a good 
battery jar. The exciting liquid should be composed of 

| One part of sulphuric acid 
| Ten parts of water 

One of the disadvantages of the voltaic cell is that it 
becomes *polarized*, that is, small bubbles of hydrogen which 
are liberated by the chemical action collect on the copper 
plate and cause the strength of the battery to fall off 
rapidly. 

There are a great number of *elements*, as the zinc and 
copper are called, and an even greater number of different 
solutions or *excitants* which can be employed in place of 
sulphuric acid to make a cell, forming an almost endless 
number of possible combinations. 

**Leclanche Cell.** One of the most common forms of cell 
employed for bell-ringing, telephones, etc., is called the 
Leclanche cell, after its inventor, and consists of two 
elements, one of zinc and the other of carbon, immersed in a 
solution of *sal ammoniac* or *ammonium chloride*. This cell 
has an E. M. F. of 1.4 volts, which is about half as much 
again as the voltaic cell. 

.. figure:: images/Image62.jpg
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   :alt: Fig. 55.—Carbon-Cylinder Cell, and Cylinder.
   
   Fig. 55.—Carbon-Cylinder Cell, and Cylinder.

The most common form of Leclanche cell is illustrated 
in Figure 55. This type is usually known as a "carbon 
cylinder" cell because the positive 
element is a hollow carbon 
cylinder. The zinc is in the 
form of a rod passing through 
a porcelain bushing set in the 
center of the carbon cylinder. 
A battery of such cells can 
only be used successfully for 
open circuit work. The 
"open circuit" is used for bells, burglar alarms, telephone 
circuits, etc., or wherever the circuit is such that it is 
"open" most of the time and current is only drawn 
occasionally and then only for short periods. 

If the current is drawn for any appreciable length of 
time hydrogen gas will collect on the carbon cylinder and 
the cell will become *polarized*. When polarized it will not 
deliver much current. 

Many methods have been devised for overcoming this 
difficulty, but even the best of them are only partially successful. 

The usual method is to employ a chemical *depolarizing* 
agent. Figure 56 shows a Leclanche cell provided with a 
*depolarizer*. 

The carbon is in the form of a plate placed in a *porous 
cup* made of earthenware and filled with *manganese dioxide*. 

Chemists class *manganese dioxide* as an *oxidizing* agent, 
which means that it will furnish 
oxygen with comparative ease. 
Oxygen and hydrogen have a 
strong *chemical affinity* or 
attraction for each other. 

.. figure:: images/Image63.jpg
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   :alt: Fig. 56.—A Leclanche Cell, showing the Porous Cup. 
   
   Fig. 56.—A Leclanche Cell, showing the Porous Cup.

If the carbon plate is packed 
in manganese dioxide any 
hydrogen which tends to collect on 
the carbon and polarize the cell 
is immediately *seized* by the 
oxygen of the manganese dioxide and united with it to form 
water. 

This form of Leclanche cell is called the disk type. It is 
capable of delivering a stronger current for a longer period 
of time than the carbon cylinder battery. The zinc is 
usually made in the form of a cylinder, and fits around the 
outside of the porous cup. 

**Dry Cells** are used extensively nowadays for all open circuit 
work on account of their convenience and high efficiency. 

The dry cell is not, as its name implies, "dry," but the 
exciting agent or electrolyte, instead of being a liquid, is a 
wet paste which cannot spill or run over. 
The top of the cell is poured full of molten 
pitch, thus effectively sealing it and 
making it possible to place the cell in any 
position. 

Dry cells can be purchased from almost 
any electrical house or garage for 
twenty-five cents each. It will therefore hardly 
pay the young experimenter to make his 
own *dry cells*. For the sake of those who 
may care to do so, however, directions for 
building a simple but efficient dry cell of 
the type used for door-bells and ignition 
work, will be found below. 

.. figure:: images/Image64.jpg
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   :alt: Fig. 57.—A Dry Cell. 
   
   Fig. 57.—A Dry Cell. 

The principle of a dry cell is the same 
as that of a Leclanche cell of the disk type. The exciting 
solution is *ammonium chloride*, the electrodes or elements 
are zinc and carbon, and the carbon is surrounded by 
manganese dioxide as a depolarizing agent. 

Obtain some sheet zinc from a plumbing shop or a 
hardware store and cut out as many rectangles, 8 x 6 inches, 
as it is desired to make cells. Also cut out an equal number 
of circles 2 3/8 inches in diameter. 

Roll the sheets up into cylinders 2 3/8 inches in diameter 
inside and 6 inches long. The edges are lapped and 
soldered. Fit one of the round circles in one end of each of the 
cylinders and solder them securely into place, taking care 
to close up all seams or joints which might permit the 
electrolyte to escape or evaporate. 

Secure some old carbon rods or plates by breaking open 
some old dry cells. The carbons will be in the form of a 
flat plate, a round rod, or a star-shaped corrugated rod, 
depending upon the manufacture of the cell. Any of these 
types of carbons will serve the purpose well, provided that 
they are fitted with a thumb-screw or a small bolt and nut 
at the top so as to make wire connections with the carbon. 

Make a wooden plunger of the same shape as the carbon 
which you may select, but make it slightly larger. Smooth 
it with sandpaper and give it a coat of shellac to prevent it 
from absorbing moisture. 

This wooden plunger is temporarily inserted in the center 
of one of the zinc cups and supported so that it will be about 
one-half inch above the bottom. 

The electrolyte is prepared by mixing together the 
following ingredients in the proportions shown: 

| Sal Ammoniac. 1 part
| Zinc Chloride. 1 part 
| Plaster of Paris. 3 parts 
| Flour. 3/4 part 
| Water. 2 parts 

.. figure:: images/Image65.jpg
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   :alt: Fig. 58.—The Different Operations involved in Making a Dry Cell. 
   
   Fig. 58.—The Different Operations involved in Making a Dry Cell. 

The above paste is then firmly packed into the zinc shell 
around the wooden plunger, leaving a space of about 3/4 of 
an inch at the top. The paste can be poured in very readily 
when first mixed but sets and hardens after standing a short 
while. 

After it has set, withdraw the wooden plunger, thus 
leaving a space inside of the dry cell a little larger than the 
carbon. The carbon is now inserted in this hole and the 
surrounding space is filled with a mixture composed of: 

| Sal Ammoniac. 1 part 
| Zinc Chloride. 1 part 
| Manganese Dioxide. 1 part 
| Granulated Carbon. 1 part 
| Flour. 1 part 
| Plaster of Paris. 3 parts 
| Water. 2 parts 

The granular carbon may be had by crushing up some old 
battery carbons. The parts given in both of the above 
formulas are proportioned so that they may be measured 
by bulk and not by weight. An old teaspoon or a small cup 
will make a good measure. 

Each one of the zinc shells should be filled in this 
manner. After they have all been filled, clean off the top edge 
of the zinc and pour the remaining space in the cell full of 
molten tar or pitch so as to seal it over. 

Solder a small binding-post to the top edge of the zinc to 
facilitate connection. Then wrap the cells in several 
thicknesses of heavy paper to prevent them from short 
circuiting, and they are ready for use. 

A small hole bored through the sealing material 
after it is dry will provide a vent for the escape of 
gases. 

**Recharging dry cells** is a subject that interests most 
experimenters. 

Dry cells very often become useless before the zinc shell 
is used up or the chemicals are exhausted, due to the fact 
that the water inside of the cell dries up and the resistance 
therefore becomes so great that it is practically impossible 
for the current to pass. 

The life of such cells may be partially renewed by drilling 
several holes in the cell and permitting it to soak in a strong 
solution of sal ammoniac until some of the liquid is absorbed. 
The holes should then be plugged up with some sealing wax 
in order to prevent evaporation. 

An old dry cell may be easily turned into a "wet" cell by 
drilling the zinc full of holes and then setting it in a jar 
containing a sal ammoniac solution. The battery should be 
allowed to remain in the solution. 

**Wet batteries** are very much easier to make than dry 
batteries and are capable of delivering more current. 

They have the disadvantage, however, of wasting away 
more rapidly, when not in service, than dry cells. 

The Leclanche cell is the type generally first attempted 
by most experimenters. 

**Carbon plates** for making such a battery are most easily 
and cheaply obtained from old dry cells. About the only 
way that a dry cell can be broken open is with a cold-chisel 
and a hammer. Care must be taken, however, in order not 
to break the carbon. 

Ordinary jelly-glasses make good jars for small cells. 
Fruit-jars may be used for larger batteries by cutting the 
tops off so that the opening is larger. The carbon plate 
contained in a dry cell is usually too long for a jar of this 
sort and must be 
broken off before it 
can be used. The 
lower end is the one 
which should be 
broken because the 
top carries a 
binding-post, with which 
connections can be 
made. A small hole 
is bored in the 
carbon rod at a distance 
from the bottom 
equal to the height 
of the jar which is 
to be used. 

.. figure:: images/Image66.jpg
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   :alt: Fig. 59.—A Zinc-Carbon Element, made from Heavy plates.
   
   Fig. 59.—A Zinc-Carbon Element, made from Heavy plates.

Considerable care 
must be used in boring carbon because 
it is very brittle and 
easily cracks. Only very light pressure should be used on 
the drill. The carbon is fastened to a strip of wood, about 
an inch and one-quarter wide, one-half an inch thick, and a 
little longer than the top of the glass jar is wide. 

.. figure:: images/Image67.jpg
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   :alt: Fig. 60.—A Method of making a Cell Element from Carbon Rods. 
   
   Fig. 60.—A Method of making a Cell Element from Carbon Rods. 
   
A piece of heavy sheet zinc is fastened on the other side 
opposite the carbon, with a screw. It is a good idea to paint 
the screws and the surrounding portions of both the zinc 
and the carbon with hot paraffin wax so that the solution 
will not "creep" and attack the screws. It is also a good 
plan first to soak the 
wooden strip in some 
hot paraffin until it is 
thoroughly impregnated. 

Ammonium chloride, 
or, as it is more 
commonly called, sal 
ammoniac, should be added 
to a jar of water until it 
will dissolve no more. 
The zinc and carbon 
elements may then be 
placed in the solution. 

One of the great disadvantages of the voltaic cell is that 
the zinc is attacked by the acid when the battery is not in 
use and cannot be allowed to remain in the solution without 
quickly wasting away. This is true in the case of the 
Leclanche cell only to a very small extent. The voltaic cell is 
more powerful than the Leclanche cell, but the elements 
must be carefully lifted out and rinsed with water every 
time that you are through using the cell. By using several 
carbon plates instead of one, the cell may be made more 
powerful. The illustrations show several ways of 
accomplishing this. The simplest method is to place 
a carbon plate on each 
side of the wooden strip 
and use a zinc in the 
form of a rod which 
passes through a hole 
between the two. Care 
must always be used to 
keep any screws which 
are used to hold the carbons or zincs in position 
in the cells from touching each other. 

.. figure:: images/Image68.jpg
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   :alt: Fig. 61. An Element made from two Carbon Plates and a Zinc Rod. 
   
   Fig. 61. An Element made from two Carbon Plates and a Zinc Rod. 
   
In Figure 62 an arrangement of using four 
carbons is shown. The 
drawing is self-explanatory. In any of the cells using more than one carbon 
element, the carbons should all be connected. 

In discussing the voltaic cell we mentioned the fact that 
it becomes polarized, and explained this phenomenon as 
being caused by hydrogen bubbles collecting on the copper 
or positive pole. The same thing happens in the case of 
carbon or any other material which is used as a positive. 

**Polarization** is the "bugbear" of batteries. It can be 
eliminated to a certain extent, however, by the use of a 
"depolarizer" *placed in the solution*. There are several such 
substances, the most common being *sodium bichromate* and 
*potassium bichromate*. These are used in battery 
preparations on the market called "Electric Sand," "Electropoian 
Fluid," etc. 

.. figure:: images/Image69.jpg
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   :alt: Fig. 62. A Method of Mounting four Carbon Plates.
   
   Fig. 62. A Method of Mounting four Carbon Plates.
   
When one of these is added to a sulphuric acid solution, 
using zinc and carbon as the battery elements, it forms 
a very powerful cell, having E. M. F. of two volts. 

A battery solution of this kind may be prepared by 
adding four ounces of bichromate of potash to a solution 
composed of four ounces of sulphuric acid mixed with sixteen 
ounces of water. The battery will give a more powerful 
current for a longer time when this solution is used 
instead of the plain sulphuric acid and water or sal 
ammoniac. 

.. figure:: images/Image70.jpg
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   :alt: Fig. 63.—A Battery Element arranged for three Cells. 
   
   Fig. 63.—A Battery Element arranged for three Cells. 
   
It might be well at this time to caution the experimenter 
against the careless handling of sulphuric acid. It is not 
dangerous if handled properly, but if spilled or spattered 
around carelessly it is capable of doing considerable damage 
to most things with which it comes in contact. Do not 
attempt to use it in any place but a shop or cellar. The 
smallest drop coming in contact with any organic matter such as 
woodwork, clothing, carpets, etc., will not only discolor 
any of the latter, but eat a hole in it. The best thing to use 
to counteract the effects of the acid which has been spilled 
or spattered is water in sufficient quantity to drench things 
and dilute the acid enough to render it harmless. A little 
strong ammonia will neutralize the acid and sometimes 
restore the color to clothing which has been burned by 
acid. 

.. figure:: images/Image71.jpg
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   :alt: Fig. 64.—A Plunge Battery, with Windlass. 
   
   Fig. 64.—A Plunge Battery, with Windlass. 
   
All acid batteries of this sort have the one objection that 
it is impossible to leave the elements in the solution without 
wasting the zinc. The usual way to arrange the battery 
cells so that the elements may be removed from the solution 
most easily is to fasten the elements to a chain or cord 
passing over a windlass fitted with a crank so that when 
the crank is turned the elements may be raised or lowered 
as desired. 

A "plunge battery" of this sort is illustrated in 
Figure 64. The construction is so plainly shown by the drawing 
that it is hardly necessary to enter into the details. The 
crank is arranged with a dowel-pin which passes through 
into a hole in the frame, so that when the elements are lifted 
out of the solution the pin may be inserted in the hole and 
the windlass prevented from unwinding. 

.. figure:: images/Image72.jpg
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   :alt: Fig. 65.—A Plunge Battery adapted to a Set of Elements, as shown in Figure 63. 
   
   Fig. 65.—A Plunge Battery adapted to a Set of Elements, as shown in Figure 63. They may be lifted out and placed on the "Arms" to drain. 
   
A somewhat easier method of accomplishing the same 
result is that shown by Figure 65. In this, the elements are 
simply raised up out of the jars and laid across the two 
"arms" to drain. 

**The Edison-Lalande** cell employs a block of pressed 
copper oxide as the positive element, while two zinc plates form 
the negative. The exciting liquid is a strong solution of 
caustic soda. 

.. figure:: images/Image73.jpg
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   :alt: Fig. 66.—An Edison-Lalande Cell. 
   
   Fig. 66.—An Edison-Lalande Cell. 
   
The copper oxide acts both as the positive element and 
as a depolarizer, for the oxygen of the oxide immediately 
combines with any hydrogen tending to form 
on the plate. 

This type of cell has some advantages but 
also many disadvantages, chief among which is 
the fact that the E. M. F. is very low. It is 
used principally for railway signal work, 
slot-machines, etc. 

**A Tomato-Can Battery** using caustic soda as 
the exciting liquid is a simple form of home-made battery 
whose only disadvantage is the low voltage that it delivers. 

.. figure:: images/Image74.jpg
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   :alt: Fig. 67.—A Tomato-Can Cell; Sectional View. 
   
   Fig. 67.—A Tomato-Can Cell; Sectional View. 

.. figure:: images/Image75.jpg
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   :alt: Fig. 68.—The Tomato-Can Cell Complete. 
   
   Fig. 68.—The Tomato-Can Cell Complete. 

The cell is liable to polarization, but the large surface of 
its positive elements protects it to some extent. 

The positive element and the outer vessel is a tomato can. 
Within it is a porous cup made out of blotting paper or 
unglazed earthenware such as a flower pot. 

The space between the can and the porous cup is filled 
with fine scrap-iron such as borings and turnings. A zinc 
plate is placed in the porous cup. 

The cell is filled with a ten-per-cent solution of caustic 
soda. 

The following table gives the names, elements, fluids, 
voltage, etc., of the most useful batteries, all of which may 
be easily constructed by the experimenter. 

.. figure:: images/Image76.jpg
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   :alt: Table of Useful Batteries

Secondary or Storage Batteries 
##############################

The storage battery is a very convenient means of taking 
energy at one time or place and using it at some other time 
or place. 

Small storage batteries are used in automobiles to supply 
current for the headlights and spark-coils. Many 
automobiles are now equipped with "electric starters," 
consisting of a dynamo-motor and a storage battery. Throwing a 
switch will cause the current from the storage battery to 
drive the motor and "crank" the engine. After the engine 
is started, the motor acts as a dynamo and generates a 
current for recharging the storage battery. 

Storage batteries are also used to drive electric vehicles 
and cars. 

Many central lighting and power stations employ storage 
batteries to supply the extra current demanded during 
rush hours. In the middle of the day, when the "load" is 
light, the surplus current of the dynamos is used to recharge 
the storage batteries. 

What is really effected in the storage battery is the 
electrical storage of *energy*, not the storage of electricity. 
Properly speaking, the energy is put into the form of 
chemical energy, and there is really *no more electricity in the cell*
when it is charged than after it is discharged. 

.. figure:: images/Image77.jpg
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   :alt: Fig. 69.—Two Methods of Connecting Cells so as to obtain Different Voltage and Amperage Values. 

   Fig. 69.—Two Methods of Connecting Cells so as to obtain Different Voltage and Amperage Values. 

Storage batteries are made up of plates of lead (the 
electrodes) or an alloy of lead cast into a "grid" or framework. 

The framework may be one of a large number of patterns, 
but usually consists of a set of bars crossing one another 
at right angles, leaving a space between. 

The spaces are filled with a paste of *lead oxide*. They are 
then "formed" by placing in a tank of acid solution and 
connected to a source of electric current. 

.. figure:: images/Image78.jpg
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   :alt: Fig. 70.—Small Storage Cells. 

   Fig. 70.—Small Storage Cells. 

The plate connected to the positive wire gradually turns 
dark-brown in color, due to the changes in the paste, which 
gradually turns into *lead peroxide*. The paste in the 
negative plate becomes 
gray in color and 
changes into a form 
of metallic lead 
called *spongy lead*. 

The positive and 
negative plates are 
placed in a bundle 
after the forming 
process has been 
completed. They are kept apart by strips of wood or 
rubber called separators. 

The negative plates of one cell are all connected in parallel 
at one end of the cell. The positive plates are connected at 
the other end. The liquid surrounding the plates is diluted 
sulphuric acid. 

When the battery has been exhausted, it is charged by 
connecting a dynamo with the terminals of the battery and 
sending a current through it. This current reverses the 
chemical action, which goes on during the discharge of the 
battery. 

**A Storage Battery** furnishes the most convenient source 
of current for performing all sorts of electrical experiments. 
It is capable of giving more current for a longer period than 
dry cells and is not expensive, for it merely requires 
recharging and does not have to be thrown away each time 
the current is used up. 

The storage cell described below is made in a very simple 
manner and will well repay any time or expense spent in 
its construction. 

.. figure:: images/Image79.jpg
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   :alt: Fig. 71.—How to make the Plates for a Storage Cell. 

   Fig. 71.—How to make the Plates for a Storage Cell. 

The plates are cut out of a large sheet of lead, one-quarter 
of an inch thick. They may be made any convenient size 
to fit the jars which the experimenter may have at hand. 
We will assume that they are to be made two and 
seven-eighths inches wide and three and one-half inches long. 
They will then fit the rectangular glass storage cell which 
is already on the market and can be procured from dealers 
in electrical supplies. 

A long terminal or lug is left projecting from the plate 
as shown in Figure 71. 

Any number of plates may be placed in a single cell, 
depending of course upon the size of the glass jar. We will 
suppose that three will just fit the jar nicely. An odd 
number of plates should always be used, so that a positive plate 
may come between two negatives. 

Each cell will give two volts regardless of the number of 
plates. Increasing the number of plates, however, will give 
the cell a greater amperage capacity and make the charge 
last longer. Three cells (six volts) will form a convenient 
set for running small fan-motors, miniature lights, etc. 

Cut out nine plates and pile them up in sets of three with 
a piece of thin wood (cigar-box wood) between each pair of 
plates. Clamp them together in a vise and bore full of 
one-quarter-inch holes. 

The plates are now ready for pasting. They are placed on 
a smooth slab of stone or glass and pasted with a stiff 
mixture of red lead and sulphuric acid (two parts water to one 
part acid). The paste must be pressed carefully into the 
recesses of the plates with a flat 
stick. They are then laid aside to 
dry and harden. 

.. figure:: images/Image80.jpg
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   :alt: Fig. Fig. 72.—The Wood Separator.

   Fig. 72.—The Wood Separator.

After they have thoroughly dried 
they should be assembled as in 
Figure 73 with one positive plate 
between two negative ones. The 
wooden "separators" are easily cut out of wood with a saw 
and penknife. The thin wood used in the construction of 
peach baskets is the best for the purpose. The separators 
should be made the same size as the lead battery plates. 

Each group of plates is then placed in a jar containing a 
mixture of sulphuric acid and water (4 parts water to one 
part acid). In mixing the 
acid be very careful to pour 
the acid into the water, 
stirring the mixture slowly at 
the same time, and not the 
water into the acid. 

.. figure:: images/Image81.jpg
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   :alt: Fig. 73.—The Complete Element for a Storage Cell.

   Fig. 73.—The Complete Element for a Storage Cell.

The plates are now ready 
for "forming." The binding-posts on the lugs of the 
plates may be secured from 
the carbons of some old dry 
cells. The simplest method 
of "forming" the plates is 
to use four gravity cells and "form" one storage cell at 
a time. 

.. figure:: images/Image82.jpg
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   :alt: Fig. 74.—A Battery of Home-Made Storage Cells. 

   Fig. 74.—A Battery of Home-Made Storage Cells. 

Connect the positive pole (copper) of the gravity battery 
to the positive pole (center-plate) of the storage cell and the 
negative (zinc) of the gravity battery to the negative 
(outside plates) of the storage cell. Allow the current to flow 
through the storage battery for several days or until the 
positive plate turns to a dark chocolate-brown color and the 
negatives to a gray-slate. 

.. figure:: images/Image83.jpg
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   :alt: Fig. 75.—Gravity Cells. 

   Fig. 75.—Gravity Cells. These consist of zinc and copper elements, immersed in a zinc-copper sulphate solution. They cannot be easily made, and are best purchased. The illustration also shows the star-shaped copper and "crowfoot" zinc element used in a gravity cell. 

After the cells have once been "formed" all that they 
require is occasional recharging from gravity cells or from a 
dynamo, by connecting the positive pole of the charging 
current to the positive plates of the storage cells and the 
negative pole to the negative plates. 

When the cells are fully charged, bubbles of gas will rise 
freely from the plates. If a dynamo is used it must be 
"*shunt*" wound and not a "*series*" machine. Recharging 
will only require about one-quarter of the time consumed 
in forming. 

It is a very good plan to connect twelve gravity cells in 
series and use them to recharge the storage battery. The 
gravity cells can always be kept connected to the storage 
cells when the latter are not in use and thus remain fully 
charged and ready to supply their maximum current. 

After the cells have been in use for some time, it is a good 
plan to lift out the plates and remove all sediment which 
has settled to the bottom of the jars. 

A set of three such storage cells will have an E. M. F. of 
over six volts. Any number may be connected up in series 
in order to obtain a higher voltage. 

Storage batteries are usually rated in "ampere hours." 
An ampere hour is the amount of current represented by 
one ampere flowing for one hour. A ten-ampere-hour 
storage battery will deliver: 

| One ampere for ten hours
| Two amperes for five hours 
| Five amperes for two hours 
| Ten amperes for one hour 

In other words, the result obtained by multiplying the 
number of amperes by the time in hours is the *ampere hour 
capacity*. 

A dynamo must have an E. M. F. of about ten volts in 
order to charge a three-cell storage battery. 

.. figure:: images/Image84.jpg
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   :alt: ELECTRO-MAGNETISM AND MAGNETIC INDUCTION 

CHAPTER V ELECTRO-MAGNETISM AND MAGNETIC INDUCTION 
``````````````````````````````````````````````````

Connect two copper wires to a voltaic cell and stretch 
a portion of the wire over a compass needle, holding it 
parallel to it and as near as possible without touching. Then 
bring the free ends of the wires together and observe that 
the needle is deflected and after a few movements back 
and forth comes to rest at an angle with the wire. 

.. figure:: images/Image85.jpg
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   :alt: Fig. 76.—A Current of Electricity flowing through a Wire will deflect a Compass Needle. 

   Fig. 76.—A Current of Electricity flowing through a Wire will deflect a Compass Needle. 
   
Next form a rectangular loop of wire and place the needle 
within it as in Figure 77. A greater deflection will now be 
obtained. If a loop of several turns is formed, the 
deflection will be still greater. 

These experiments were first performed by Oersted, in 
1819, and show that the region around a wire carrying a 
current of electricity has *magnetic* properties. 

.. figure:: images/Image86.jpg
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   :alt: Fig. 77.—If a Loop of Wire is formed about a Compass Needle, the Deflection will be greater. 

   Fig. 77.—If a Loop of Wire is formed about a Compass Needle, the Deflection will be greater. 
   
Another interesting experiment showing the magnetic 
effect of a current of electricity when passing through a 
wire may be performed by connecting a heavy copper 
wire to two or three 
bichromate-of-potash 
cells. Dip the wire 
into a pile of fine iron 
filings and a thick 
cluster of them will adhere to the wire as in Figure 78. 

As soon as the circuit is broken so that the current of 
electricity ceases flowing, the filings will fall off, showing 
that the magnetic effect ceases with the current. 

.. figure:: images/Image87.jpg
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   :alt: Fig. 78.—Iron Filings clustered on a Wire carrying a Current of Electricity. 

   Fig. 78.—Iron Filings clustered on a Wire carrying a Current of Electricity. 
   
These three simple experiments have shown that if a 
current of electricity is passed through a copper wire, the 
wire will deflect a compass needle, attract to itself iron 
filings, etc., as long as the current continues to flow. As 
soon as the current is shut off, the magnetic effect is 
*destroyed*. 

The region in the neighborhood of a wire carrying a 
current is a *field of force* through which lines of magnetism 
are flowing in exactly the same way that they do in the 
neighborhood of a bar or 
horseshoe magnet.

.. figure:: images/Image88.jpg
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   :alt: Fig. 79.—Magnetic Phantom formed about a Wire carrying a Current of Electricity.

   Fig. 79.—Magnetic Phantom formed about a Wire carrying a Current of Electricity.
   
This is readily shown by 
punching a small hole in a 
piece of cardboard, and passing a wire carrying a strong 
current of electricity through 
the hole. 

If a few iron filings are sifted 
on the cardboard and the latter jarred slightly with a pencil as they fall, 
they will arrange themselves in circles  
with the wire at the center, 
forming a magnetic phantom and showing the paths of the 
lines of magnetic force. 

.. figure:: images/Image89.jpg
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   :alt: Fig. 80.—Magnetic Phantom formed about several Turns of wire.

   Fig. 80.—Magnetic Phantom formed about several Turns of wire.
   
By forming the wire into a coil as in Figure 80 the magnetic 
field generated is 
much stronger and 
more plainly seen, 
for then the combined effect of the 
wires is secured. 

.. figure:: images/Image90.jpg
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   :alt: Fig. 81.—Paper Tube wrapped with Wire for Experimental Purposes. 

   Fig. 81.—Paper Tube wrapped with Wire for Experimental Purposes. 
   
Roll up a small 
paper tube about 
1/2 inch in diameter 
and four inches long. Wind neatly on the tube three 
layers of No. 18 insulated copper wire. Pass an electric 
current through it from two or three cells of a battery, and 
test its magnetic properties by bringing it near a compass 
needle. It will be found that the coil possesses very marked 
magnetic properties, and will readily cause the needle to 
swing about, even though it is held quite a distance 
away. 

If an iron bar is placed inside of the paper tube, the 
magnetic effect will be greatly increased. 

.. figure:: images/Image91.jpg
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   :alt: Fig. 82.—Showing how the Lines of Force "Leak" at the sides of the coil, from a Coil of Wire, and how they are concentrated by an Iron Core. 

   Fig. 82.—Showing how the Lines of Force "Leak" at the sides of the coil, from a Coil of Wire, and how they are concentrated by an Iron Core. 
   
The presence of the iron bar inside of the coil of wire 
greatly increases the 
number of lines of 
force running through 
the coil. 

.. figure:: images/Image92.jpg
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   :alt: Fig. 83.—The Principle of an Electro-Magnet.

   Fig. 83.—The Principle of an Electro-Magnet.

When a bar is not 
used, many of the 
lines of force leak out at the sides of the coil,
and but few extend
from end to end. The effect of the iron core is not only to 
diminish the leakage of the lines of force, but also to add 
many more to those previously existing. Hence the 
magnetic strength of a coil is greatly increased by the iron 
core. 

A coil of wire wrapped around an iron core forms an 
*electro-magnet*. 

.. figure:: images/Image93.jpg
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   :alt: Fig. 84.—if you wrap some insulated Wire around an Ordinary Nail and connect it to a Battery, it will become an Electro-Magnet. 

   Fig. 84.—if you wrap some insulated Wire around an Ordinary Nail and connect it to a Battery, it will become an Electro-Magnet. 

If you wrap some insulated wire around an ordinary 
nail and connect it to one or 
two cells of a battery it will 
become an electro-magnet and 
pick up bits of iron and steel. 

If you wind the wire around a 
small paper tube into which a 
nail will slide easily, the coil will 
draw the nail in when the current is turned On. A hollow coil 
of this sort is called a solenoid. 

Electro-magnets and solenoids play a part in the 
construction of almost all electrical machinery. They form 
the essential parts of dynamos, motors, telephone receivers, 
telegraph relays and sounders, and a host of other devices. 

The form usually given to an electro-magnet depends 
upon the use to which it is to be put. The horseshoe is 
the most common. This 
consists of two electro-magnets mounted on a yoke 
and connected so that the 
two free poles are North and South.

.. figure:: images/Image94.jpg
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   :alt: Fig. 85.—If you wind the Wire around a small Paper Tube into which a Nail will slide easily, the Coil will draw the Nail in when the Current is turned on.

   Fig. 85.—If you wind the Wire around a small Paper Tube into which a Nail will slide easily, the Coil will draw the Nail in when the Current is turned on.

Electro-magnets are made on a huge scale for 
lifting large castings and heavy pieces of iron. Such 
magnets are used in the great steel mills and in factories where 
nails, bolts, etc., are manufactured. 

Monster electro-magnets can be seen in wonderful 
perfection at the great steel mill at Gary, Indiana. 

Ships bring the ore down the lakes to Gary, where great 
steel jaws lift it out of the hold of the boat and carry it 
to the furnaces. 

After being melted, great machines pour it out. It is 
divided into huge ingots, and these, while hot, are carried 
to the first part of the rolling mill. 

The ingot is squeezed by a machine, made longer and 
narrower, then squeezed again and made still longer and 
narrower. 

It is started on its journey along the rollers of the mill, 
squeezed and pressed here and there, as it travels hundreds 
of yards—no hand ever touching it. It finally arrives, a 
red-hot steel rail, the right shape and the right length. 

During this time the steel has made a long journey and 
changed from a shapeless ingot to a finished rail, handled 
entirely by machinery guided and controlled by one or two 
operators, pressing levers and switches. 

When almost finished, the rail slides down an incline 
before a man who grasps the rail with huge pinchers, and 
standing at one end, runs his eye along it. As he looks 
along the rail he sees the defects, moves the left or the right 
hand, and another man at the levers of the straightening 
machine, straightens the rail as directed. 

And soon there are ten rails, perfectly straight, side by 
side, with more coming down the incline to meet the glance 
of the man’s eye. 

They are still too hot for any man’s touch and so a man 
sitting in a tower touches an electric switch, and along 
the overhead rails there comes gliding a monster electro-magnet. 

The magnet moves along, drops down upon the ten rails, 
lying side by side and weighing thousands of pounds. The 
man in the tower presses another switch, thus turning on 
the current, and electricity glues the rails to the magnet. 

.. figure:: images/Image95.jpg
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   :alt: Lifting-Magnets of the type known as Plate, Billet, and Ingot Magnets. 
   
   *By permission, from "Solenoids" by C. R. Underhill.* Lifting-Magnets of the type known as Plate, Billet, and Ingot Magnets. 

The ten rails are lifted at once, as easily as you would 
lift a needle with your horseshoe magnet; they are carried 
to a flat-car, and when lowered in position, the current 
is turned off, releasing the rails, and the magnet travels back 
for another load. 

Induction
#########

In 1831, Michael Faraday, a famous English chemist 
and physicist, discovered that if a magnet be suddenly 
plunged into a hollow coil of wire, a momentary current of 
electricity is generated in the coil. As long as the magnet 
remains motionless, it induces no current in the coil, but 
when it is moved back and forth, it sets up the currents. 
The source of electrical energy is the mechanical work done 
in moving the magnet. 

.. figure:: images/Image96.jpg
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   :alt: Fig. 86.—Showing how a Current of Electricity may be induced by a Bar Magnet and a Coil. 
   
   Fig. 86.—Showing how a Current of Electricity may be induced by a Bar Magnet and a Coil. 

The medium which changes the mechanical energy into 
electricity is the magnetic field which we have already seen 
exists in the neighborhood of a magnet. 

A current of electricity produced in a coil in such a 
manner is said to be an *induced* current and the phenomenon is 
that known as *magnetic induction*. 

Magnetic induction is met in the dynamo, induction coil, 
telephone, transformer, some forms of motors, and a 
number of other electrical devices. 

.. figure:: images/Image97.jpg
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   :alt: Fig. 87.—A Horseshoe Magnet and a Coil arranged to produce Electric Currents by Induction. 
   
   Fig. 87.—A Horseshoe Magnet and a Coil arranged to produce Electric Currents by *Induction*. 

A simple experiment in which electricity is produced by 
magnetic induction may be performed by winding a number 
of turns of fine insulated wire around the armature or 
keeper of a horseshoe magnet, leaving the ends of the iron 
free to come in contact with the poles of the permanent 
magnet. Connect the ends of the coil to a sensitive 
galvanometer, [#]_ the ends of the armature being in contact with the 
poles of the horseshoe magnet as shown in Figure 87. 

Keeping the magnet fixed, suddenly pull off the armature. 
The galvanometer will show a momentary current. 
Suddenly bring the armature up to the poles of the magnet; 
another momentary current in the reverse direction will 
flow through the circuit. 

The fact that it is a reverse current is shown by the 
actions of the galvanometer for it will be noticed that the 
needle swings in the opposite direction this time. 

It will also be noticed that no current is produced when 
the coil and magnet are stationary. Current is only 
generated when the coil and magnet are approaching one 
another or moving apart suddenly. 

This is because it is only then that the magnetic field is 
changing. The field is strongest nearest the magnet, and 
therefore if either the magnet or the coil of wire is moved, 
the strength of that part of the field which intersects the 
coil is changed. Induced currents can only be generated 
by a *changing* magnetic field. 

.. [#] See chapter on Measuring Instruments. 

.. figure:: images/Image98.jpg
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   :alt: ELECTRICAL UNITS

CHAPTER VI ELECTRICAL UNITS
```````````````````````````

The Ampere 
##########

There are certain terms used in the electrical field to 
distinguish various properties and qualities of the electrical 
current with which it is well for the young experimenter to 
acquaint himself. 

One of the first units usually required, in order to make 
intelligent comparisons, is a unit of measure. The *quart* is 
the unit of *measure* commonly applied to liquids and is based 
upon the amount of space occupied by a certain volume. 
The *pound* is a unit of weight which determines a certain 
amount of any substance by comparing the force which 
gravity exerts in pulling it to the earth with the same effect 
of gravity on another standard "weight." 

Electric current is invisible and weightless, and for these 
and other reasons cannot be measured by the quart or 
weighed by the pound. The only way that it can be 
measured is by means of some of the effects which it produces. 
Either the chemical, electro-magnetic, or the heating 
effects may be made the basis of a system of measurement. 

The first method used to measure electric current was the 
chemical one. 

If a current is passed through a solution of a chemical 
called copper *sulphate* (blue vitriol) by means of two copper 
plates, *copper* will be deposited on one plate and dissolved 
from the other. If the current is furnished by a battery the 
copper will be deposited on the plate connected with the 
zinc of the battery. If the current is allowed to flow for a 
short time and the two copper plates are then taken out and 
weighed it will be found that one plate is considerably 
heavier than the other. 

The copper has been taken from one plate and deposited 
on the other by the *electric currents*. The amount of 
electric current which will deposit 1.177 grammes of copper in 
an hour is called an *ampere*. The ampere is the unit of 
electrical current measurement, and implies quantity or 
amount. 

The chemical method of measuring current was at one 
time put to practical service in the distribution of electric 
current for lighting and power. Many years ago the house 
meters, used to measure the current, consisted of a jar 
containing two copper plates. The current used in the house 
would cause copper to deposit on one plate, and by weighing 
the plate the power company could determine the amount 
of current used, and thereby the amount of the bill. The 
meters nowadays make use of the magnetic effects of the 
current instead of the chemical, as described later on. 

The Volt
######## 

For purposes of explanation the electric current may be 
likened to a stream of water flowing through a pipe. 

If you hold your thumb over the end of a water-pipe 
through which water is flowing it will push your thumb 
away because of the *pressure* which the water exerts. 

Electric currents also exert a *pressure*, only it is not called 
pressure in electrical parlance, but, spoken of as 
*electromotive force* or *potential*. 

The pressure of the water enables it to pass through 
small openings and to overcome the resistance offered by 
the pipe. 

Wires and other electrical conductors do not offer a 
perfectly free path to an electric current, but also possess a 
resistance. It is the potential of the electro-motive force 
which overcomes the resistance and pushes the current 
through the wire. 

Advantage has been taken of the fact to fix a unit of 
electrical pressure called the *volt*. The pressure of the water 
in a water-pipe is measured in pounds, but the pressure of 
an electric current in a wire is measured by *volts*. The volt 
is the unit of electrical force which will cause a current of 
one ampere to flow through a resistance of one *ohm*. 

The Ohm
#######

The ohm is the unit of electrical resistance. The standard 
ohm is the resistance offered by a column of pure mercury 
having a section of one square millimeter and a length 
of 106.28 centimeters at a temperature of 0° centigrade. 

The pressure which will force sufficient current through 
such a column of mercury to deposit 1.177 grammes of 
copper in one hour is a volt, and in doing so has passed a 
current of one ampere through a resistance of one ohm. 

The units ohm, ampere, and volt, were named in honor 
of the three great electricians: Ohm, Ampère, and Volta. 

These three units bear a very close relation to each other 
which is explained by Ohm’s Law. 

Ohm’s Law is a simple statement of facts which it is well 
for the young electrician thoroughly to understand, for it 
might almost be said to be the basis of design of almost all 
electrical instruments. 

It is simply this: The strength of a current equals the 
voltage divided by the resistance. It may be expressed in 
symbols by: *C = E/R*. Where C is the current in amperes, 
E is the potential in volts, and R the resistance in ohms. 

By way of a simple example, we will suppose that a small 
telegraph sounder is connected to a battery and that the 
voltage of the battery is *ten volts*. We will further suppose 
that the resistance of the sounder connecting wires and the 
battery itself is *five ohms*. Knowing these two facts, it is 
very easy to find out how many amperes are flowing 
through the sounder by substituting these values in the 
equation as follows: 

| C = E/R
| E = 10 volts and R = 5 ohms 
| therefore C = 10/5 or 2 amperes 

In order to indicate fractions or very large values of the 
ampere, volt, and ohm, it is customary to use the following terms: 

| Milli-volt = 1/1000 of a volt
| Mill-ampere = 1/1000 of an ampere 
| Kilo-volt = 1000 volts 
| Meg-ohm = 1,000,000 ohms 

The Watt 
########

It is no doubt perfectly plain that the water in a certain 
size of pipe at a pressure of 100 lbs. is more powerful than a 
stream of water in the same size of pipe at 25 lbs. pressure. 

Likewise a current of electricity represents more power 
at 100 volts potential than the same current would at 25 
volts. The unit of electrical power is called the *watt*. A 
watt is represented by a current of one ampere flowing 
through a wire at a potential of one volt. 

The number of watts is found by multiplying the voltage 
by the amperage. In the case of the sounder and battery 
used as an example to explain Ohm’s Law, and where the 
voltage was 10 and the amperage found to be 2, the number 
of watts is 10 x 2, or 20 watts. 

Seven hundred and forty-six watts represent one 
electrical horse-power. One thousand watts are called a 
*kilo-watt*. 

The Coulomb
###########

So far, none of the units have taken into consideration the 
element of time. 

If water should be permitted to run out of a pipe into a 
tank until ten gallons had passed it would not be possible 
to tell at what rate the water was flowing by knowing that 
ten gallons had passed unless it were also known how long 
the water had been flowing. Ten gallons per minute or ten 
gallons per hour would indicate the rate of flow. 

One ampere flowing for one second is the electrical unit 
of flow. This unit is called the *coulomb*. 

One ampere flowing for one hour is called an *ampere hour*. 
The number of ampere hours is found by multiplying the 
current in amperes by the time in hours. 

A battery may be said to have a capacity of 10 ampere 
hours. This means that it will deliver one ampere for 10
hours (1 ampere x 10 hours = 10 ampere hours) or 2 
amperes for 5 hours (2 amperes x 5 hours = 10 ampere hours). 

The same element of time enters into consideration in 
connection with the watt. One watt flowing for one hour is 
a *watt hour* and one kilowatt flowing for one hour is a 
*kilo-watt hour*. 

The Difference between Alternating and Direct Currents
######################################################

There are two distinct kinds of electric current supplied 
for lighting and power, one known as *direct* current and the 
other as *alternating*. 

A *direct current* is one which passes in one direction only. 
It may be represented by a straight line, as *A* in Figure 88. 

An alternating current is one which reverses its direction 
and passes first one way and then the other. It may be 
represented by a curved line, shown in Figure 88. It starts 
at *zero*, and gradually grows stronger and stronger. Then it 
commences to die away until no current is flowing. At this 
point it reverses and commences to flow in the opposite 
direction, rising gradually and then dying away again. 

This is repeated a definite number of times per second; 
when the current rises from zero, reverses and returns to 
zero, it is said to pass through a *cycle*. 

.. figure:: images/Image99.jpg
   :align: center
   :scale: 85 %
   :alt: Fig. 88.—Graphic Representation of a Direct and an Alternating Current.
   
   Fig. 88.—Graphic Representation of a Direct and an Alternating Current.

The part of the curved line from *a* to *b* in Figure 88 
represents the first part of the current, when it is rising. From 
*b* to *c* represents its fall. The point at which the curved line 
crosses the straight line is zero. At *c* the current crosses the 
line and commences to flow in the opposite direction until 
it reaches *d*, at which point it dies away and again crosses 
the line to flow in its original direction and *repeat the cycle*. 

In electrical parlance, that part of the current from *a* to *c* 
or from *c* to *e* is known as an *alternation*. From *a* to *e* is 
called a cycle. 

The reason why alternating current is often used in place 
of direct current is that it can be sent over the wires for 
long distances more economically than direct current. This 
is more fully explained farther on in the chapter dealing 
with a step-down transformer. 

The number of *cycles* taking place in one second is known 
as the *frequency* of the current. The usual *frequency* of 
commercial alternating currents is 60 cycles per second or 
7200 alternations per minute. 

.. figure:: images/Image100.jpg
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   :scale: 85 %
   :alt: ELECTRICAL APPURTENANCES

CHAPTER VII ELECTRICAL APPURTENANCES
````````````````````````````````````

Wires 
#####

Electric currents are usually led from place to place, 
at will, by means of conductors called *wires*. There are a 
great many kinds of wires, each adapted to some special 
purpose. 

Wires are usually covered with a material called an 
*insulator*, in order to prevent the loss of electric current 
due to the wires coming into contact with other bodies or 
circuits. Insulators are substances which do not conduct 
electricity. 

Wires which are *insulated* by heavy braids of cotton 
fiber and then impregnated with some compound, such as 
creosote, are called *weather-proof* wires, and are best adapted 
to outside service, where they must be exposed to the action 
of the elements. 

The wires used for interior wiring in buildings, etc., are 
usually insulated with rubber, over which is placed a 
cotton braid to protect the rubber. 

Rubber cannot well be used as an insulator for all wires, 
although its insulating value is very great, owing to the 
fact that it deteriorates under many conditions. 

Rubber-covered and weather-proof wires are made in a 
variety of insulations. Some may have only one insulating 
layer, while others have a great many. Different 
substances are used as insulators to adapt the wire to some 
special purpose. Copper is usually the only metal used 
to form the wire or conductor itself. The reason for this 
is that copper is a better conductor than any other metal 
except those known as precious metals, such as gold and 
silver, the cost of which prohibits their use for such 
purposes. The wire may be solid, or made up of a number of 
small conductors so that it is flexible. 

The various combinations of insulating layers, together 
with either a solid or a stranded conductor, have made 
possible a variety of current-carriers, known as: 

| Theater or Stage Cable 
| Elevator Cable 
| Fixture Wire 
| Telephone Wire 
| Mining Cable 
| Feeder Cable 
| Brewery Cord 
| Heater Cord, etc. 

depending upon the special use for which they were 
designed. 

.. figure:: images/Image101.jpg
   :align: center
   :scale: 85 %
   :alt: NUMBER, DIAMETER, WEIGHT, LENGTH, AND RESISTANCE OF COPPER WIRE

The wires which the young experimenter is likely to use 
in his work the most are known as *magnet wires*, and are 
used for making electro-magnets, coils, and various 
windings. Magnet wires may be insulated with either silk, 
cotton, or enamel. 

.. figure:: images/Image102.jpg
   :align: center
   :scale: 85 %
   :alt: NUMBER, DIAMETER, WEIGHT, LENGTH, AND RESISTANCE OF COPPER WIRE

Silk-covered and cotton-covered wires may be obtained 
with either a single or double covering. 

Wires with a single covering of silk or enamel are used 
when it is desirable to save space, for the covering of these 
two classes of magnet wires is thinner than either the 
cotton or double-silk-covered wire, and consequently they 
require less room for winding. 

The size of the wire is indicated by its diameter, and in 
the United States is measured by the Brown and Sharpe 
gauge, often indicated by the term, "B. & S." 

The preceding table shows the various sizes of wire of the 
Brown and Sharpe gauge, and also several of their 
characteristics, such as weight, resistance, etc. 

Insulators 
##########

The covering placed over wires is not the only 
precaution taken to insulate them, but in the case of permanent 
wiring they are usually mounted on glass or porcelain 
supports. 

.. figure:: images/Image103.jpg
   :align: center
   :scale: 50 %
   :alt: Fig. 89.—Staples and Wooden Cleat used for running Low Voltage Wires. 
   
   Fig. 89.—Staples and Wooden Cleat used for running Low Voltage Wires. 

Wires used for batteries, bells, telephones, etc., 
operated by batteries and where the voltage is not over 20 
volts, may be run under *insulated* staples or wooden 
cleats inside of a building. If outside and exposed to the 
weather, they should be mounted on suitable glass or 
porcelain knobs. 

.. figure:: images/Image104.jpg
   :align: center
   :scale: 50 %
   :alt: Fig. 90.—Porcelain Insulators to support Electric Light Wires. 
   
   Fig. 90.—Porcelain Insulators to support Electric Light Wires. 

Electric-light wires for inside use are commonly 
supported by insulators made of porcelain 
and known as cleats, knobs, and tubes 
according to the shape. 

Telegraph, telephone, and power 
lines are usually supported by glass 
knobs or large porcelain insulators 
which screw on to wooden pins. 

.. figure:: images/Image105.jpg
   :align: center
   :scale: 50 %
   :alt: Fig. 91.—Glass Insulator Binding-Posts and Pin used to support Telegraph and Telephone wires.
   
   Fig. 91.—Glass Insulator Binding-Posts and Pin used to support Telegraph and Telephone wires.

Binding-Posts
#############

Binding-posts are the most convenient device to make quick 
connections between wires and other parts of electrical 
apparatus. 

Binding-posts may be either made or purchased. Those 
which are purchased are of course the best, and will add 
greatly to the appearance of any instrument upon which 
they are mounted. 

Several of the best-known types of manufactured posts 
are shown in Figure 92. 

.. figure:: images/Image106.jpg
   :align: center
   :scale: 85 %
   :alt: Fig. 92.—Types of Binding-Posts. 
   
   Fig. 92.—Types of Binding-Posts. 

Figure 93 shows different ways of making simple 
binding-posts and connectors from screws, washers, screw-eyes, 
and strips of metal. The drawings are self-explanatory and 
should need no comment. 

.. figure:: images/Image107.jpg
   :align: center
   :scale: 85 %
   :alt: Fig. 93.—Home-made Binding-Posts. 
   
   Fig. 93.—Home-made Binding-Posts. 

The screws and nuts obtainable from old dry cells are 
very convenient to use for binding-posts and other similar 
purposes. 

Switches and Cut-Outs
#####################

Switches and cut-outs are used in electrical 
work for turning the current on and off. 

If the experimenter chooses to make them 
himself, care should be taken, to construct them in a 
strong and durable fashion, for they usually are 
subjected to considerable use, with consequent 
wear and tear. 

.. figure:: images/Image108.jpg
   :align: center
   :scale: 85 %
   :alt: Fig. 94.—Binding-Post removed from the Carbon of a Dry Cell.
   
   Fig. 94.—Binding-Post removed from the Carbon of a Dry Cell.

Several very simple home-made switches are  
illustrated in Figure 95.

.. figure:: images/Image109.jpg
   :align: center
   :scale: 85 %
   :alt: Fig. 95.—Simple Switches. 
   
   Fig. 95.—Simple Switches. *A*, Single-Point Switch. *B*, Two-Point Switch. *C*, Three-Point Switch. *D*, Five-Point Switch. *E*, Lever with End Rolled up to form Handle. *F*, Lever with Handle made from part of a Spool.

The first one shown (*A*) has one contact, formed by 
driving a brass-headed tack through a small strip of copper 
or brass. 

The movable arm is a strip of copper or brass, rolled up 
to form a handle at one end. The other end is pivoted 
with a brass screw. The brass screw passes through a small 
strip of copper or brass having a binding-post mounted 
on the end. A small copper washer should be placed 
between the movable arm and the copper strip to make the 
switch work more easily. 

A somewhat similar switch is shown by *B* in the same 
illustration, only in this case a handle made from half 
of a spool is used, instead of rolling up the end of the 
arm. 

The other illustrations show how the same method of 
construction may be applied to make switches having more 
than one "point" or contact. 

No dimensions have been given for constructing these 
switches, because it is doubtless easier for the young 
experimenter to use materials which he may have at hand, 
and construct a switch of his own proportions. Only one 
suggestion is necessary, and that is to bevel the under edges 
of the arm with a file, so that it will slip over the head of 
the brass tack more easily. 

The switches shown in Figure 96 are capable of 
carrying heavier currents than those just described, and more 
nearly approach the type used on lighting and power 
switchboards. 

The base may be made of wood, but preferably should 
be made of some insulating substance such as fiber or 
slate. 

.. figure:: images/Image110.jpg
   :align: center
   :scale: 85 %
   :alt: Fig. 96.—Knife Switches. 
   
   Fig. 96.—Knife Switches. 

The patterns for the metal parts are shown in 
Figure 97. These are cut from heavy sheet-brass or 
sheet-copper, and then bent into shape with a pair of flat-nosed 
pliers. 

The handle of the single-pole switch is driven on over 
the metal tongue. 

The double-pole switch is almost a duplicate of the 
single-pole type, but has two sets of levers and contacts, actuated 
by the handle, in place of one. 

.. figure:: images/Image111.jpg
   :align: center
   :scale: 50 %
   :alt: Fig. 97.—Metal Parts for the Knife Switches.
   
   Fig. 97.—Metal Parts for the Knife Switches.

The ends of the blades to which the handle is connected 
are turned over at right angles and a hard-wood cross-bar 
fastened between 
the ends. The 
handle is fastened 
to the center of the 
cross-bar. 

After the switch 
is assembled, bend 
the various parts until they "line up" that is, are in 
proper position in respect to each other, so that the blades 
will drop into the contacts without bringing pressure to 
bear on either one side or the other of the handle in order 
to force the blades into line. 

Fuses
#####

Fuses are used to prevent electrical instruments and wires 
from damage due to too much current flowing through. 
When an electric current passes through a resistance it 
produces *heat*. 

A fuse is usually a short piece of lead or some alloy which 
melts at a low temperature, and which is inserted in the 
circuit so that the current must flow through it. If too 
much current flows through the fuse it will become hot 
and melt, because of its low melting-point, thus 
interrupting the circuit and shutting the current off until the cause 
which occasioned the surplus current to flow can be 
ascertained. 

Fuses are rated according to the amount of current 
which is required to "blow" them out, and therefore are 
called 1, 3, 5, or 10 ampere fuses, as the case may be. 

.. figure:: images/Image112.jpg
   :align: center
   :scale: 85 %
   :alt: Fig. 98.—Simple Fuses. *A*, Fuse-Block with plain Wire Fuse. *D*, Fuse-Block with Mica Fuse in position. 
   
   Fig. 98.—Simple Fuses. *A*, Fuse-Block with plain Wire Fuse. *D*, Fuse-Block with Mica Fuse in position. 

When a fuse burns out in a trolley car or in a light or 
power circuit, it is because a greater amount of current is 
trying to pass than the circuit can safely carry. If a fuse 
were not placed in such a circuit so as to shut the current 
off before the danger point is reached, any electrical device 
might become "burned out," or in extreme cases, the wires 
become so hot as to cause a serious fire. 

Figure 98 shows several simple forms of fuses which the 
experimenter may easily make to protect the batteries, 
etc., from short circuits. 

The simplest possible fuse consists merely of a small piece 
of lead wire or a strip of thick tinfoil held between two 
binding-posts mounted upon a wooden block. 

The same form of fuse may be made from a strip of mica 
about two and one-half inches long and one-half an inch wide. 

A strip of thin sheet-copper is bent around the ends of 
the mica strip. 

A piece of fuse wire is stretched between the two copper 
contacts and fastened to each with a drop of solder. Fuse 
wire of any desired ampere-carrying capacity can be 
obtained from most electrical supply houses. 

Such a fuse is held in a mounting as shown by *D*. The 
contacts are made from sheet-copper or brass. They should 
spring together very tightly, so as to make perfect contact 
with the copper ends on the mica strip. 

Lightning-Arresters 
###################

Lightning-arresters are used to protect all wires which 
run into a building from outdoors, especially telegraph 
or telephone wires, so that static 
electricity due to lightning will not 
damage the instruments. 

Lightning-arresters may be 
constructed in many ways and of 
different materials, but there are only 
two types for which the average 
experimenter will have any use. 

.. figure:: images/Image113.jpg
   :align: center
   :scale: 50 %
   :alt: Fig. 99.—Lightning-Arrester and Ground-Wire Switch. 
   
   Fig. 99.—Lightning-Arrester and Ground-Wire Switch. 

The arrester shown in Figure 99 
is the type known as "lightning-arrester and ground-wire 
switch." It is used principally on telegraph lines. 

It consists of three pieces of sheet-brass about 
one-sixteenth of an inch thick, and shaped as shown by *A*, *B*, and 
*C* in Figure 100. 

The metal pieces are mounted on a wooden block with 
a narrow space of 
about one-thirty-second of an inch 
separating them. 

.. figure:: images/Image114.jpg
   :align: center
   :scale: 50 %
   :alt: Fig. 100.—Home-made Lightning-Arrester. 
   
   Fig. 100.—Home-made Lightning-Arrester. 

The two outside 
pieces are each 
fitted with two 
binding-posts, and 
the center 
triangular-shaped piece is 
fitted with one post. 

A hole about one-eighth of an inch in diameter is bored 
between each of the metal pieces. 

Make a tapered metal pin which can be placed tightly 
in the holes, and will make contact between the metal 
pieces.

The two outside line wires of the telegraph circuit are 
connected to the outside metal pieces *C* and *B*. *A* is 
connected to the earth or ground. 

In case of a lightning storm, if the wires become charged, 
the small space between the metal plates will permit the 
charge to jump across and pass harmlessly into the ground. 

If complete protection is desired, it is merely necessary to 
insert the plug in one of the holes, and thus "ground" 
either wire or short-circuit both of them. 

.. figure:: images/Image115.jpg
   :align: center
   :scale: 85 %
   :alt: Fig. 101.—Lightning-Arrester for Telephone Wires. 
   
   Fig. 101.—Lightning-Arrester for Telephone Wires. 

The lightning-arrester shown in Figure 101 is designed 
for service on telephone wires. It is an ordinary fuse 
provided with an arrester in the shape of two carbon blocks 
about one inch square. The blocks rest on a copper 
strip, and are held in place by a spring-strip connected 
to *B*. 

The carbon blocks are separated by a piece of thin 
sheet-mica, of the same size as the blocks. 

The post, *B*, is connected to one of the telephone-line 
wires near the point where it enters the building from 
outdoors. The post, *A*, is connected to the instrument; *C* is 
connected to the ground. 

An arrester of this kind should be connected to each one 
of the telephone wires. 

If the line wires should happen to come into contact 
with a power wire, there is danger of damage to the 
instruments, but if an arrester is connected in the circuit such an 
occurrence would be prevented by the blowing out of the 
fuse. If the lines become charged by lightning, the charge 
can easily pass over the edge of the mica between the two 
blocks and into the ground. 

.. figure:: images/Image116.jpg
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   :alt: ELECTRICAL MEASURING INSTRUMENTS

CHAPTER VIII ELECTRICAL MEASURING INSTRUMENTS
`````````````````````````````````````````````

An instrument designed to measure electromotive force 
(electrical pressure) is called a *voltmeter*. An 
instrument designed to measure volume of current is called an 
*ammeter*. 

There are many forms of reliable meters for measuring 
current and voltage, but all are more or less expensive and 
out of the reach of an ordinary boy. 

Some meters are more carefully made than a watch, and 
are provided with fine hair-springs and jeweled bearings, 
but all depend upon the same principle for their action, 
namely, the mutual effects produced between a magnetic 
needle and a coil of insulated wire carrying a current of 
electricity. 

The little meters described in this chapter are simple 
and inexpensive but quite sensitive. Unlike a meter 
making use of a hair-spring, they will stand considerable rough 
handling, but of course should not be subjected to such 
treatment unnecessarily. 

Two types of meters are described. Both operate on 
exactly the same principle, but one is more elaborate than 
the other. 

A Simple Voltmeter and Ammeter 
##############################

A base-board five inches long, two and one-half inches 
wide and one-half inch thick is cut out of hard wood. In its 
center, cut a slot three-eighths of an inch wide and one and 
one-half inches long, with the slot running lengthwise the 
board. Along each side of the slot glue two small wooden 
blocks one and one-half inches long, one-quarter of an inch 
thick, and one-half of an inch high. 

.. figure:: images/Image117.jpg
   :align: center
   :scale: 85 %
   :alt: Fig. 102.—*A*, Base, showing Slot. *B* and *C*, Sides and Top of the Bobbin. *D*, Base and Bobbin in Position. 
   
   Fig. 102.—*A*, Base, showing Slot. *B* and *C*, Sides and Top of the Bobbin. *D*, Base and Bobbin in Position. 

When they are firmly in position, glue a strip of wood, two 
and one-half inches long, three-quarters of an inch wide and 
one-eighth inch thick to the top as shown by D in Figure 102. 

Using these as a support, wind a horizontal coil composed 
of 200 feet of No. 36 B. & S. gauge silk-covered wire. 

A needle is next made from a piece of watch-spring. It 
should be about one and one-quarter inches long, and 
one-eighth of an inch wide. 

Straighten it out by bending, and then heat the center 
in a small alcohol flame until the center is red-hot, taking 
care to keep the ends as cool as possible. 

The spring is mounted on a small steel shaft made by 
breaking up an ordinary sewing-needle. Make the piece 
one-half of an inch long. It must have very sharp points 
at both ends. The ends may 
be pointed by grinding. 

.. figure:: images/Image118.jpg
   :align: center
   :scale: 50 %
   :alt: Fig. 103.—Arrangement of the Needle and Pointer. 
   
   Fig. 103.—Arrangement of the Needle and Pointer. 

Bore a small hole just large 
enough to receive the needle 
through the center of the 
spring. Insert the needle in 
the hole and fasten it in the 
center by two small circular 
pieces of wood which fit tightly 
on the needle. A little glue or 
sealing-wax will serve to help make everything firm. 

The pointer is a piece of broom-straw, about three inches 
long. Bore a small hole in the top of one of the wooden 
clamps and insert the pointer in the hole, fastening it with 
a little glue. The pointer should be perfectly straight, and 
in a position at right angles to the spring. 

Bore a small hole in the bottom of one of the wooden 
clamps and glue a small wire nail in the hole. The purpose 
of the nail is to serve as a counterweight and keep the 
pointer in a vertical position. 

The spring should be magnetized by winding ten or 
twelve turns of magnet wire around one end and 
connecting it with a battery for a moment. 

.. figure:: images/Image119.jpg
   :align: center
   :scale: 50 %
   :alt: Fig. 104.—A, Bearings. B, How the Needle is mounted. 
   
   Fig. 104.—*A*, Bearings. *B*, How the Needle is mounted. 

The needle is mounted in two small pieces of thin 
sheet-brass, one inch long and one-half inch wide. Bend each 
strip at right angles in the middle, and at one-quarter of 
an inch from one 
end make a small 
dent by means of 
a pointed nail and 
a hammer. 

The strips are 
now slipped down 
in the center of the 
slot in the coil with 
the dents inside of the coil and exactly opposite one another. 
After the exact position is found, they may be fastened 
into position by two very small screws. 

The sharp-pointed sewing-needle, together with the 
magnetized spring, pointer, and counterweight, should slip 
down into the dents made in the strips and swing freely 
there. It may require a little filing and bending, but the 
work should be done patiently, because the proper working 
of the meter will depend upon having the needle swing 
freely and easily in its place. 

Fasten an upright board, four inches wide and 
one-quarter of an inch thick, to the base-board, back of the 
bobbin. 

Attach a piece of thick cardboard to the upright by 
means of small blocks, in such a position that the pointer 
swings very close to it but does not touch it. 

The meter is now complete, except for marking or 
calibrating the scale. The method of accomplishing this will 
be described farther on. 

.. figure:: images/Image120.jpg
   :align: center
   :scale: 85 %
   :alt: Fig. 105.—The Completed Meter.
   
   Fig. 105.—The Completed Meter.

If the meter is wound with No. 36 B. & S. gauge wire it 
is a voltmeter for measuring voltage. If it is wound with 
No. 16 B. & S. gauge wire it will constitute an ammeter for 
measuring amperes. 

A Portable Voltmeter and Ammeter
################################ 

The bobbin upon which the wire is wound is illustrated in 
Figure 106. The wood is the Spanish cedar, of which cigar 
boxes are made. It should be one-eighth of an inch thick, 
and can be easily worked with a pocket-knife. In laying 
out the work, scratch the lines on the wood with the point 
of a darning-needle. Pencil lines are too thick to permit 
of accuracy in small work. The bobbin when finished must 
be perfectly true and square. 

The dimensions are best understood from the illustrations. 
In putting the bobbin together, do not use any nails. Use 
strong glue only. 

Two bobbins are required, one for the ammeter and one 
for the voltmeter. After completing the bobbins, 
sandpaper them and coat them with 
shellac. 

.. figure:: images/Image121.jpg
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   :alt: Fig. 106.—Details of the Bobbin.
   
   Fig. 106.—Details of the Bobbin.

The bobbin for the ammeter 
is wound with No. 14 B. & S. 
double-cotton-covered magnet 
wire. The voltmeter requires 
No. 40 B. & S. silk-covered 
wire. In both cases the wire should be wound carefully in 
smooth, even layers. A small hole is bored in the flange 
through which to pass the end of the wire when starting 
the first layer. After finishing the winding, about six 
inches of wire should be left at both ends to make 
connection with the terminals. The whole winding is then given 
a coat of shellac. A strip of passe-partout tape, one-half 
of an inch wide wound over the wire around the bobbin 
will not only protect the wire from injury, but also give 
the bobbin a very neat appearance. 

The armature is a piece of soft steel one inch long, 
one-eighth of an inch thick and three-eighths wide. A 
one-eighth-inch hole is bored one-sixteenth of an inch above the 
center for the reception of the shaft. The center of gravity 
is thus thrown below the center of the mass of the 
armature, and the pointer will always return to zero if the 
instrument is level. 

The shaft is a piece of one-eighth-inch Bessemer steel 
rod, seven-sixteenths of an inch long. The ends are filed to 
a sharp knife-edge on the under side, as indicated in the 
figure. 

.. figure:: images/Image122.jpg
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   :alt: Fig. 107.—The Bobbin partly cut away so as to show the Bearing. Details of the Armature and Shaft. 
   
   Fig. 107.—The Bobbin partly cut away so as to show the Bearing. Details of the Armature and Shaft. 

A one-sixteenth-inch hole is bored in the top of the 
armature to receive the lower end of the pointer, which is a 
piece of No. 16 aluminum wire, four and one-half inches 
long. 

After the holes have been bored, the armature is 
tempered so that it will retain its magnetism. It is heated to a 
bright red heat and dropped into a basin of strong salt 
water. The armature is then magnetized by rubbing one 
end against the pole of a strong magnet. 

The bearings are formed by two strips of thin 
sheet-brass, three-sixteenths of an inch wide, and one and 
one-quarter inches long, bent and glued to the sides of the bobbin. 

In the illustration, part of the bobbin is represented as cut 
away. The center of the bearing is bent out so that the end 
of the shaft will not come in contact with the sides of the 
bobbin. The top of the center is notched with a file to form 
a socket for the knife-edges of the shaft. 

.. figure:: images/Image123.jpg
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   :alt: Fig. 108.—Completed Voltmeter. 
   
   Fig. 108.—Completed Voltmeter. 

The bobbin is glued to the center of a wooden base, seven 
inches long, four inches wide and three-quarters of an inch 
thick. The terminals of the coil lead down through two 
small holes in the 
base and thence to 
two large 
binding-posts. The wires are 
inlaid on the under 
side of the base, i.e., 
they pass from the 
holes to the binding-posts through two 
grooves. This precaution avoids the 
possibility of their 
becoming short-circuited or broken. 

The case is formed of two sides, a back and top of 
one-half-inch wood. It is six inches high, four inches wide, 
and two inches deep. A glass front slides in two shallow 
grooves cut in the wooden sides, one-eighth of an inch from 
the front. 

The case is held down to the base by four round-headed 
brass screws, which pass through the base into the sides. 
It is then easily removable in case it ever becomes 
necessary to repair or adjust the instrument. 

The meter and case, as illustrated in Figure 108, are 
intended for portable use and are so constructed that they 
will stand up. A small brass screw, long enough to pass all 
the way through the base, serves to level the instrument. 
If a little brass strip is placed in the slot in the screw-head 
and soldered so as to form what is known as a "winged 
screw," the adjustment may be made with the fingers and 
without the aid of a screw-driver. 

Where the instrument is intended for mounting upon a 
switch-board, it can be given a much better appearance by 
fitting with a smaller base, similar in size and shape to the 
top. The binding-posts are then mounted in the center of 
the sides. 

To calibrate the meters properly, they are compared 
with some standard. The scale is formed by a piece of 
white cardboard glued by two small blocks on the inside 
of the case. The various values are marked with a pen and 
ink. The glass front, therefore, cannot be put in place until 
they are located. 

The zero value on the meters will normally be in the 
center of the scale. When a current is passed through the 
bobbin, the armature tends to swing around at right 
angles to the turns of wire. But since the armature is 
pivoted above the center of the mass, when it swings, the 
center of gravity is displaced and exerts a pull in opposition 
to that of the bobbin, and the amount of swing indicated 
by the pointer will be greater as the current is stronger. 
The pointer will swing either to the right or the left, 
depending upon the direction in which the current passes 
through the bobbin. The pointer of the instrument 
illustrated in Figure 108 is at zero when at the extreme left of 
the scale. The pointer is bent to the left, so that the current 
will be registered when passing through the meter only in 
one direction, but the scale will have a greater range of 
values. It will also be necessary to cut a small groove in 
the base of the instrument in this case so that the 
armature will have plenty of room in which to swing. 

.. figure:: images/Image124.jpg
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   :alt: Fig. 109.—Circuits for Calibrating the Ammeter and Voltmeter. 
   
   Fig. 109.—Circuits for Calibrating the Ammeter and Voltmeter. 

When calibrating the ammeter, it is placed in series with 
the standard meter, a set of strong batteries, and a 
rheostat. The rheostat is adjusted so that various current 
readings are obtained. The corresponding positions of the 
pointer on the meter being calibrated are then located for 
each value. 

The voltmeters must be placed in parallel, or shunt with 
each other, and in series with several battery cells. A 
switch is arranged so that the voltage of a varying number 
of cells may be passed through the meters. To secure 
fractional values of a volt, the rheostat is placed in shunt 
with the first cell of the battery. Then, by adjusting both 
the switch and the rheostat, any voltage within the 
maximum range of the battery may be secured. 

This means of regulating voltage is a common one, and 
of much use in wireless telegraph circuits, as will be 
explained later. 

When using the meters, it is always necessary that the 
ammeter shall be in series and the voltmeter in parallel or 
in shunt with the circuit. 

Galvanoscopes and Galvanometers
############################### 

In the first part of Chapter V it was explained that several 
turns of wire surrounding a compass-needle would cause 
the needle to move and show a deflection if a current of 
electricity were sent through the coil. 

Such an instrument is called a *galvanoscope* and may be 
used for detecting very feeble currents. A galvanoscope 
becomes a *galvanometer* by providing it with a scale so that 
the deflection may be measured. 

A galvanometer is really, in principle, an ammeter the 
scale of which has not been calibrated to read in amperes. 

.. figure:: images/Image125.jpg
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   :scale: 50 %
   :alt: Fig. 110.—Simple Compass Galvanoscope.
   
   Fig. 110.—Simple Compass Galvanoscope.

A very simple galvanoscope may be made by winding 
fifty turns of No. 36 B. & S. gauge single-silk-covered wire 
around an ordinary 
pocket compass. The 
compass may be set in 
a block of wood, and 
the wood provided 
with binding-posts so 
that connections are 
easily made. 

Another variety of 
the same instrument is 
shown in Figure 111. 

.. figure:: images/Image126.jpg
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   :alt: Fig. 111.—Galvanoscope.
   
   Fig. 111.—Galvanoscope.

Wind about twenty-five turns of 
No. 30  B. & S. gauge cotton-covered wire around the lower end of a 
glass tumbler. Leave about six inches of each end free for 
terminals, and then, 
after slipping the 
coil from the glass, 
tie the wire with 
thread in several 
places so that it 
will not unwind. 
Press two sides of 
the coil together 
so as to flatten it, and then attach it to a block of wood 
with some hot sealing-wax. 

Make a little wooden bridge as shown in Figure 111, and 
mount a compass-needle on it in the center. The 
compass-needle may be made out of a piece of spring-steel in the 
manner already described in Chapter I. 

Mount two binding-posts to the corners of the block, and 
connect the ends of the wire coil to them. Turn the block 
so that the needle points North and South and parallel to 
the coil of wire. 

If a battery is connected to the binding-posts, the needle 
will fly around to a position at right angles to that which 
it first occupied. 

An astatic galvanoscope is one having two needles with 
their poles in opposite directions. The word "astatic" 
means having no directive magnetic tendency. If the 
needles of an astatic pair are separated and pivoted 
separately, they will each point to North and South in the 
ordinary manner. But when connected together with the poles 
arranged in opposite directions they neutralize each other. 

An astatic needle requires but very little current in order 
to turn it either one way or the other, and for this reason 
an astatic galvanoscope is usually very sensitive. 

A simple instrument of this sort may be made by winding 
about fifty turns of No. 30-36 B. & S. gauge single-silk or 
cotton-insulated wire into a coil around a glass tumbler. 
After removing the coil from the glass, shape it into the 
form of an ellipse and fasten it to a small base-board. 

Separate the strands of wire at the top of the coil so that 
they are divided into two groups. 

.. figure:: images/Image127.jpg
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   :alt: Fig. 112.—Astatic Galvanoscope. 
   
   Fig. 112.—Astatic Galvanoscope. 

Make a bridge or 
standard in the 
shape of an inverted 
U out of thin wooden 
strips and fasten it 
to the block. 

The needles are 
ordinary sewing-needles which have 
been magnetized 
and shoved through 
a small carrier-bar, 
made from a strip 
of cardboard, with 
their poles opposite 
one another, as 
shown in the illustration. 

.. figure:: images/Image128.jpg
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   :alt: Fig. 113.—Astatic Needles. 
   
   Fig. 113.—Astatic Needles. 

They may be held in 
place in the cardboard strip by a 
small drop of sealing-wax. 

A small hole is punched in 
the top of the carrier, through 
which to pass the end of a 
thread. The upper end of the 
thread passes through a hole 
in the bridge and is tied to a 
small screw-eye in the center of 
the upper side of the bridge. 

The carrier-bar is passed through the space where the 
coil is split at the top. The lower needle should hang in 
the center of the coil. The upper needle should be above 
and outside the coil. 

The terminals of the coil are connected to two 
binding-posts mounted on the base-block. 

Owing to the fact that this galvanoscope is fitted with an 
astatic needle, the instrument does not have to be turned 
so that the coil may face North and South. The slightest 
current of electricity passing into the coil will instantly 
affect the needles. 

An astatic galvanometer for the detection of exceedingly 
weak currents and for use in connection with a 
"Wheatstone bridge" for measuring resistance, as described farther 
on, will form a valuable addition to the laboratory of the 
boy electrician. 

Make two small bobbins similar to those already 
described in connection with the volt and ammeter, but twice 
as long, as shown in Figure 114. 

Wind each of the bobbins in the same direction with 
No. 36 silk-covered or cotton-covered wire, leaving about six 
inches free at the ends for connection to the binding-posts. 

Fasten each of the bobbins to the base-board with glue. 
Do not nail or screw them in position, because the presence 
of nails or screws may impair the sensitiveness of the 
instrument. In mounting the bobbins, leave about 
one-sixteenth of an inch of space between the inside flanges, 
through which the needle may pass. 

Connect the coils wound on the bobbins so that the end 
of the outside layer of the first coil is connected to the inside 
layer of the other coil. This arrangement is so that the 
current will travel through the windings in the same 
continuous direction, exactly the same as though the bobbin 
were one continuous spool. 

.. figure:: images/Image129.jpg
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   :scale: 85 %
   :alt: Fig. 114.—Bobbin for Astatic Galvanometer. 
   
   Fig. 114.—Bobbin for Astatic Galvanometer. 

Magnetize two small sewing-needles and mount them in 
a paper stirrup made from good, strong paper, as shown in 
Figure 114. Take care that the poles are reversed so that 
the north pole of one magnet will be on the same side of 
the stirrup as the south pole of the other. They may be 
fastened securely by a drop of shellac or melted sealing-wax. 

Cut out a cardboard disk and divide it into degrees as in 
Figure 115. Glue the disk to the top of the bobbins. A 
small slot should be cut in the disk so that it will pass the 
lower needle. 

A wooden post should be glued to the back of the base. 
To the top of this post is fastened an arm from which are 
suspended the magnetic needles.

A fine fiber for suspending the needle may be secured by 
unraveling a piece of embroidery silk. 

.. figure:: images/Image130.jpg
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   :scale: 85 %
   :alt: Fig. 115.—Completed Astatic Galvanometer. 
   
   Fig. 115.—Completed Astatic Galvanometer. 
   
The upper end of the fiber is tied to a small hook in the 
end of the arm. The wire hook may be twisted so that 
the needles may be brought to zero on the scale. Zero 
should lie on a line parallel to the two coils. 

The fiber used for suspending the needles should be as 
fine as possible. The finer the fiber is, the more sensitive 
will the instrument be. 

The lower needle should swing inside of the two coils, 
and the upper needle above the disk. 

How to Make a Wheatstone Bridge
############################### 

The amateur experimenter will find many occasions 
when it is desirable to know the resistance of some of his 
electrical apparatus. Telephone receivers, telegraph 
relays, etc., are all graded according to their resistance in 
ohms. The measurement of resistance in any electrical 
instrument or circuit is usually accomplished by comparing 
its resistance with that of some known circuit, such as a 
coil of wire which has been previously tested. 

The simplest method of measuring resistance is by means 
of a device known as the Wheatstone bridge. This 
instrument is very simple but at the same time is remarkably 
sensitive if properly made. A Wheatstone bridge is shown 
in Figure 116. 

The base is a piece of well-seasoned hard wood, thirty 
inches long, six inches wide, and three-quarters of an inch 
thick. 

Secure a long strip of No. 18 B. & S. gauge sheet-copper, 
one inch wide, and cut it into three pieces, making two of 
the pieces three inches long, and the other piece 
twenty-three and one-half inches long. 

Mount the copper strips on the base, as shown, being 
very careful to make the distance between the inside edges 
of the end-pieces just twenty-five inches. The strips should 
be fastened to the base with small round-headed brass 
screws. Mount two binding-posts on each of the short 
strips in the positions shown in the illustration, and three 
on the long strip. These binding-posts should pass through 
the base and make firm contact with the strips. 

.. figure:: images/Image131.jpg
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   :alt: Fig. 116.—Wheatstone Bridge.
   
   Fig. 116.—Wheatstone Bridge. 
   
Then make a paper scale twenty-five inches long, and 
divide it into one hundred equal divisions one-quarter of 
an inch long. Mark every fifth division with a slightly 
longer line, and every tenth division with a double-length 
line. 

Start at one end and number every ten divisions, then 
start at the other end and number them back, so that the 
scale reads 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, from 
right to left at the top and 0, 10, 20, 30, 40, 50, 60, 70, 80, 
90, 100, from left to right at the bottom. 

Solder a piece of No. 30 B. & S. gauge German-silver wire 
to one of the short copper strips opposite the end of the 
scale, and then stretch it tightly across the scale and solder 
it to the strip at the other end. 

Make a knife-contact by flattening a piece of heavy 
copper wire as shown in Figure 117. Solder a piece of flexible 
wire, such as "lamp cord," at the other end. It is well to 
fit the contact with a small wooden handle, made by boring 
out a piece of dowel. 

The instrument is now practically complete. 

.. figure:: images/Image132.jpg
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   :alt: Fig. 117.—Knife-Contact.
   
   Fig. 117.—Knife-Contact.
   
In order to use the Wheatstone bridge, it is necessary to 
have a set of resistances of known value. The resistance of 
any unknown circuit or piece of apparatus is found by 
comparing it with one of the known coils. It is just like 
going to a store and buying a pound of sugar. The grocer 
weighs out the sugar by balancing it on the scales with an 
iron weight of known value, and taking it for granted that 
the weight is correct, we would say that we have one, five, 
or ten pounds of sugar, as the case may be. 

The Wheatstone bridge might be called a pair of 
"electrical scales" for weighing resistance by comparing an 
unknown coil with one which we know has a certain value. 

The next step is to make up some standard resistance 
coils. Secure some No. 32 B. & S. gauge 
single-cotton-covered wire from an electrical dealer and cut into the 
following lengths, laying it straight on the floor but using 
care not to pull or stretch it. 

| 1/2 ohm coil—3 feet 1/2 inch 
| 1 ohm coil—6 feet 1 1/4 inches 
| 2 ohm coil—12 feet 2 1/2 inches 
| 5 ohm coil—30 feet 6 1/4 inches 
| 10 ohm coil—61 feet
| 20 ohm coil—122 feet 
| 30 ohm coil—183 feet 
| 50 ohm coil—305 feet 

These lengths of wire are then wrapped on the spools in 
the following manner. 

.. figure:: images/Image133.jpg
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   :alt: Fig. 118.—Resistance-Coil.
   
   Fig. 118.—Resistance-Coil. *A* shows how the Wire is doubled and wound on the Spool. *B* is the completed Coil. 

This method of winding is known as the non-inductive 
method, because the windings do not generate a magnetic 
field, which might affect the galvanometer needle used in 
connection with the Wheatstone bridge as described later 
on. 

Each length of wire should be doubled exactly in the 
middle, then wrapped on the spools like a single wire, the 
two ends being left free for soldering to the terminals as 
shown in Figure 118, B. 

The spools may be the ordinary reels upon which cotton 
and sewing-silk are wrapped. 

The terminals of the spools are pieces of stout copper 
wire, No. 12 or No. 14 B. & S. gauge. Two pieces of wire 
about three inches long are driven into holes bored in the 
ends of each spool. A small drop of solder is used 
permanently to secure the ends of the coil to each of the heavy 
wire terminals. 

The spools are then dipped into a pan of molten paraffin 
and boiled until the air bubbles cease to rise. 

The spools should be marked 1, 2, 10, 20, 30, and 50, 
according to the amount of wire each one contains as 
indicated in the table above. 

How to Use a Wheatstone Bridge for Measuring Resistance
####################################################### 

The instrument is connected as in Figure 116. 

The unknown resistance or device to be measured is 
connected across the gap at *B*. One of the standard known 
coils is connected across the gap at *A*. A sensitive 
galvanometer or a telephone receiver and two cells of battery 
are also connected as shown. 

If a telephone receiver is used, place it to the ear. If a 
galvanometer is used instead, watch the needle carefully. 
Then move the sharp edge of the knife-contact over the 
scale along the German-silver "slide wire" until a point is 
reached when there is no deflection of the needle or no sound 
in the telephone receiver. 

If this point lies very far on one side or the other of the 
center division on the scale, substitute the next higher or 
lower known resistance spool until the point falls as near 
as possible to the center of the scale. 

When this point is found, note the reading on the scale 
carefully. Now comes the hardest part. Almost all my 
readers have no doubt progressed far enough in arithmetic 
to be able to carry on the following simple calculation in 
proportion which must be made in order to find out the 
resistance of the unknown coil. 

The unknown resistance, connected to *B*, bears the same 
ratio to the known coil, at *A*, that the number of divisions 
between the knife-contact and the right-hand end of the 
scale (lower row of figures) bears to the number of divisions 
between the knife-edge and the left-hand end of the scale 
(upper row of figures). 

We will suppose that a 5-ohm coil was used at *A* in a test, 
and the needle of the galvanometer stopped swinging when 
the knife-contact rested on the 60th division from the 
left-hand end, or on the 40th from the right. Then, in order to 
find the value of the unknown resistance at *B*, it is simply 
necessary to multiply the standard resistance at *A* by the 
number of left-hand divisions and divide the product by 
the number of right-hand divisions. The answer will be 
the resistance of *B* in ohms. 

The calculation in this case would be as follows: 

| 5 X 40 = 200 

| 200/60 = 3.33 ohms 
    
| 3.33 ohms is the resistance of *B*. 

This explanation may seem very long and complex, but 
if you will study it carefully you will find it to be very 
simple. When once you master it, you will be enabled to 
make many measurements of resistance which will add 
greatly to the interest and value of your experiments. 

.. figure:: images/Image134.jpg
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   :alt: BELLS, ALARMS, AND ANNUNCIATORS
   
CHAPTER IX BELLS, ALARMS, AND ANNUNCIATORS
``````````````````````````````````````````

An electric bell may be bought almost anywhere for 
twenty-five cents, and from the standpoint of economy it 
does not pay to build one. 

A bell is not a hard thing to construct, and the time and 
money spent will be amply repaid by the more intimate 
knowledge of this useful piece of apparatus which will be 
gained by constructing it. 

The base is four inches wide and five and one-half inches 
long. 

The magnets consist of two machine bolts, wound with 
No. 22 cotton-covered magnet wire. Fiber ends are fitted 
on the bolts to hold the wire in place. 

The wire is wound on each of the magnets separately. 
Cover the cores with two or three layers of paper before 
winding on the wire. The ends of the wire are led through 
holes in the core ends. The ends of the bolts are passed 
through the yoke, and the nuts applied to hold them in 
place. 

The magnets are clamped down to the bell-base by means 
of a hard-wood strip having a screw passed through it 
between the magnets into the base. 

.. figure:: images/Image135.jpg
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   :alt: Fig. 119.—Details of the Magnet Spools, and Yoke for an Electric Bell. 
   
   Fig. 119.—Details of the Magnet Spools, and Yoke for an Electric Bell. 
   
The armature of the bell is shown in Figure 120. It is 
made of a piece of iron having a steel spring riveted to 
it, as illustrated. The armature is fastened to a small block 
mounted on the lower left-hand corner of the base. 

.. figure:: images/Image136.jpg
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   :alt: Fig. 120.—Details of the Armature, and Contact Screw. 
   
   Fig. 120.—Details of the Armature, and Contact Screw. 
   
A second block with a contact-point made from an 
ordinary brass screw by filing the end into the shape shown 
in the illustration, is mounted on the base so that it is 
opposite the end of the contact-spring fastened to the armature. 
The gong may be secured from an old bell or alarm 
clock. It is mounted on the upper part of the base in such 
a position that the hammer will strike it on its lower edge. 

The instrument is connected as shown in Figure 121. 
One terminal of the magnets is connected to the 
contact-screw. The other end is connected to the binding-post. A 
second binding-post is connected to the armature. 

.. figure:: images/Image137.jpg
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   :alt: Fig. 121.—The Completed Bell.
   
   Fig. 121.—The Completed Bell.
   
The armature spring should be bent so that the armature 
is pushed over against the contact. 

If a battery is connected to the bell, the electromagnets 
will pull the armature and cause the hammer to strike the 
gong. As soon as the armature has moved a short distance, 
the spring will move away from the contact and break the 
circuit. The magnets cease pulling as soon as the current is 
cut off and the armature spring then causes the armature 
to move back and touch the contact. As soon as the 
contact is made, the armature is drawn in again and the 
process is repeated. 

.. figure:: images/Image138.jpg
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   :alt: Fig. 122.—Diagram showing how to connect a Bell, Battery, and Push-Button. 
   
   Fig. 122.—Diagram showing how to connect a Bell, Battery, and Push-Button. 
   
A little experimenting with the bell will soon enable one 
to find its best adjustment. Figure 122 shows how to 
connect a bell to a battery and a push-button. A push-button 
is simply a small switch which closes the circuit when 
pressed. Do not make the armature spring too weak, or 
the hammer will move very slowly and with very little 
life. Each time that the armature moves toward the 
magnets, it should barely touch the iron cores before the ball 
strikes the bell. 

After you get the bell in good working order, it is well to 
make a small box to serve as a cover for the working parts 
of the instrument, leaving only the gong exposed. 

.. figure:: images/Image139.jpg
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   :alt: Fig. 123.—Two Simple Push-Buttons. 
   
   Fig. 123.—Two Simple Push-Buttons. 

.. figure:: images/Image140.jpg
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   :alt: Fig. 124.—Diagram showing how to arrange a Bell System of Return Signals. 
   
   Fig. 124.—Diagram showing how to arrange a Bell System of Return Signals. 

It is sometimes desirable to arrange two bells and two 
push-buttons, so that a return signal can be sent. In that 
case the circuit shown in Figure 124 may be employed. It 
is then possible for the person answering the bell to indicate 
that he has heard the call by pushing the second button. 
For instance, one push-button and bell might be located 
on the top floor of a house and the other bell and button 
in the basement. A person in the basement wishing to call 
another on the top floor would push the button. The 
person answering could return the signal by pushing the 
button on the top floor and cause the bell in the basement 
to ring. 

A Burglar Alarm
############### 

A simple method of making an efficient burglar alarm 
is shown in Figure 125. The base is a piece of wood about 
five by six inches, and half an inch thick. A small brass 
strip, *A*, is fastened to the base by means of two 
round-headed wood screws and the ends turned up at right angles. 
The lever, *B*, is also a strip of brass. One end is bent out, 
so as to clear the strip and the screws that are under it. 
The lever is pivoted in the middle with a screw and a 
washer. A small hole, *D*, is bored in the lower end through 
which a spring and a string are passed. The other end of 
the spring is fastened under a screw and washer, *C*. 

.. figure:: images/Image141.jpg
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   :scale: 85 %
   :alt: Fig. 125.—Burglar-Alarm Trap.
   
   Fig. 125.—Burglar-Alarm Trap.

In order to set the alarm, first fasten the base in any 
convenient place. Carry the string across the room and fasten 
it. Adjust the string so that the lever is half-way between 
the two ends of the strip, *A*. 

If the string is disturbed, it will pull the lever over against 
the strip, *A*. If the string is cut, the spring will pull the 
lever over to the opposite side. In either case, if the alarm 
is properly connected to a bell and battery, the circuit 
will be closed if the string is disturbed, and the bell will 
ring. 

One wire leading from the bell and the battery should 
be connected to *A*, and the other to the screw and 
washer, *C*. 

The alarm may be arranged across a window or doorway 
and a black thread substituted for the string. Any one 
entering in the dark and unaware of the existence of the 
alarm is liable to break the thread and ring the bell. 

An Electric Alarm
#################

It is often desirable to arrange an electrical alarm clock 
so that a bell will ring continuously until shut off. 

.. figure:: images/Image142.jpg
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   :alt: Fig. 126.—An Early-Riser’s Electric Alarm Attachment for a Clock. 
   
   Fig. 126.—An Early-Riser’s Electric Alarm Attachment for a Clock. 

Figure 126 shows an electrical alarm attachment. It 
consists of a wooden box, large enough to receive an ordinary
dry cell. A bell is fastened 
on the outside of the box. 
Connect one terminal of the 
battery to one terminal of 
the bell. Connect the other 
bell and battery terminals, 
each to a short piece of brass 
chain, about four inches 
long. The ends of the 
chain are then fastened to 
a small piece of sheet fiber 
or hard rubber, so that 
they are insulated from 
each other. The opposite 
end of the fiber is fastened to a piece of wire spring 
having a garter or suspender clip soldered to the end. 

.. figure:: images/Image143.jpg
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   :alt: Fig. 127.—Details of the Chain Electrodes, etc. 
   
   Fig. 127.—Details of the Chain Electrodes, etc. 

The operation of this electrical attachment is very simple. 
Wind up the alarm key of an ordinary alarm clock and 
place the clip on the key. Place the clock in such a 
position that the two chains do not touch each other. Set the 
clock. When the mechanical alarm goes off, the key will 
revolve and twist the two chains, thus closing the electric 
circuit and causing the bell to ring. The bell will ring until 
the clamp is removed. The outfit can be attached to any 
ordinary alarm clock. 

An Annunciator 
##############

Annunciators are often placed in bell and burglar 
alarm-circuits to indicate where the button ringing the bell was 
pushed, in case there 
are several. 

The separate indicators used on an 
annunciator are 
called *drops*. 

.. figure:: images/Image144.jpg
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   :alt: Fig. 128.—An Annunciator Drop. 
   
   Fig. 128.—An Annunciator Drop. 

A drop may be 
made from an electromagnet and 
some brass strips, 
etc. 

The frame is cut 
from heavy sheet-brass and shaped as shown in Figures 128 and 129. 

The drop bar is a strip of metal which is pivoted on the 
frame at its lower end and has the upper end turned up to 
receive a numeral or letter. 

The armature is made from a strip of sheet-iron. It is 
pivoted on the frame at its upper end. The strip is bent 
at right angles so as to fall in front of the magnet. The 
lower part of the armature is bent into a hook. The hook 
fits into a slot cut in the drop bar. A fine wire spring is 
placed between the frame and the upper end of the 
armature so as to pull 
the armature away 
from the core when 
the current is not 
passing through the 
magnet. 

The electromagnet should be wound 
with No. 25 B. & S. 
cotton-covered magnet wire. 

When a current is 
sent through the magnet, it will draw the armature in. 
This action releases the hook from the edge of the slot in 
the drop bar and permits the bar to drop and bring the 
number or letter down into view. 

.. figure:: images/Image145.jpg
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   :alt: Fig 129.—Details of the Drop-Frame and Armature. 
   
   Fig 129.—Details of the Drop-Frame and Armature.

A number of "drops" may be arranged on a board and 
placed in different circuits so as to indicate which circuit is 
closed at any time. It is a good plan to arrange a bar to 
act as a stop, so that the numeral will not drop down too 
far. Each time that any one of the drops falls, it must be 
reset by pushing the bar back into position. 

.. figure:: images/Image146.jpg
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   :alt: ELECTRIC TELEGRAPHS
   
CHAPTER X ELECTRIC TELEGRAPHS
`````````````````````````````

Experiments in telegraphy were carried out as far back 
as the year 1753, when it was proposed to transmit 
messages by representing the letters of the alphabet by 
combinations of sparks produced by a static machine; but 
these were of little practical value and nothing of any 
importance was accomplished until after the discovery of 
galvanic current. 

Many of these old experiments were very crude and 
appear somewhat ridiculous when compared with the 
methods of nowadays. The earliest proposal for an electric 
telegraph appeared in the *Scots’ Magazine* for February, 
1753, and shows several kinds of proposed telegraphs acting 
by the attractive power of electricity, conveyed by a series 
of parallel wires, one wire corresponding to each letter of 
the alphabet and supported by glass rods at every twenty 
yards. Words were to be spelled by the action of the 
electricity in attracting paper letters, or by striking bells 
corresponding to letters. 

The modern telegraph consists essentially of four things, 
namely: 

A battery which produces an electric current. 

A wire which conducts the electric current from one 
point to another. 

A transmitter for shutting the current off and on. 

An electro-magnetic receiving apparatus, which gives 
out in sounds, the signals made by the pulsations of the 
current from a distant point. 

The battery may be almost any form of battery. Gravity 
cells are preferred, however, for telegraph work. 

Heavy galvanized iron wire is usually employed as the 
"line." It is necessary to use non-conductors wherever 
the wire is fastened. Glass insulators placed on a wooden 
pin or bracket, which is fastened to the pole or building 
on which the wire is to be supported, are used for outside 
work. Inside of buildings, rubber tubes are used where 
the wires pass through walls, etc. 

The operation of a telegraph is not, as many people 
suppose, a complicated or difficult matter to understand, but 
is quite simple. 

The key is a contrivance for controlling the passage of 
the electric current in much the same manner as an ordinary 
switch. It consists of a steel lever, swung on 
trunnion-screws mounted in a frame, and provided with a rubber 
knob which the operator grasps lightly with the thumb and 
forefinger. On pressing the lever downward, a platinum 
point fastened on the under side of the lever is brought into 
contact with another point set into a rubber bushing in the 
base of the key, so that there is no electrical connection 
between the two points unless the key is pressed down or 
"closed," as it is often termed. The key is usually fastened 
to the operating bench by two rods called "legs." The 
lever is provided with screws which permit the stroke of 
the key to be very closely adjusted. 

.. figure:: images/Image147.jpg
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   :alt: Fig. 130.—A Typical Telegraph Key, showing the Various Parts.
   
   Fig. 130.—A Typical Telegraph Key, showing the Various Parts.  

The line wire and battery are connected to the key, so 
that no current can flow until the key is pressed and the 
contacts brought together. 

A "sounder" consists of two electromagnets mounted 
on a base under a movable flat piece of iron which is 
attracted by the magnetism of the electromagnets when a 
current flows through them and is withdrawn by a spring 
when no magnetism excites the windings. 

This piece of iron, which is called the armature, is 
mounted upon a strip of brass or aluminum called the lever. 
The lever strikes against a brass "anvil" and produces the 
"clicks," which form the dots and dashes of the telegraph 
alphabet. 

.. figure:: images/Image148.jpg
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   :alt: Fig. 131.—A Typical Telegraph Sounder, showing the Various Parts. 
   
   Fig. 131.—A Typical Telegraph Sounder, showing the Various Parts. 

Every time that the key is pressed, an electric current is 
sent out into the line. The current flows through the 
magnets of the sounder and causes the armature to be drawn 
downward. The lever strikes the anvil and produces a 
"click." When the key lever is released, the current is 
shut up and the lever flies up and clicks against the top of 
the anvil. 

The period of time between the first click and the second 
click may be varied at will according to the length of time 
that the key is held down. A short period is called a *dot* 
and a long period a *dash*. Combinations of dots, dashes, 
and spaces arranged according to the Morse Alphabet, 
make intelligible signals. 

How To Make a Simple Key and Sounder
####################################

The little telegraph instruments shown in Figures 132 
and 133 are not practical for long lines but may be used 
for ticking messages from one room to another, and can be 
made the source of much instruction and pleasure. 

.. figure:: images/Image149.jpg
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   :alt: Fig. 132.—A Simple Home-made Telegraph Key. 
   
   Fig. 132.—A Simple Home-made Telegraph Key. 

The key is a strip of brass fastened to a wooden base in 
the manner shown in Figure 132. It is fitted with a knob 
of some sort on the front end, so that it is conveniently 
gripped with the fingers. 

The little bridge is made from heavy sheet-brass and 
prevents the lever from moving too far away from the 
contact on the upward stroke. 

Connections are made to the key lever at the back end 
and the contact in front by the binding-posts, *A* and *B*. 
The post, *C*, connects with the bridge. 

The sounder consists of two small electromagnets 
mounted in a vertical position on a wooden base. The 
magnets are connected at the bottom by a strip of heavy 
sheet-iron which acts as a yoke. 

.. figure:: images/Image150.jpg
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   :alt: Fig. 133.—A Simple Home-made Telegraph Sounder. 
   
   Fig. 133.—A Simple Home-made Telegraph Sounder. 

The armature is made out of sheet-iron, rolled up in the 
manner shown in the illustration. One end of the armature 
is fastened to a wooden block in such a position that the 
armature comes directly over the magnets and about 
one-eighth of an inch above them. The opposite end of the 
armature moves up and down for about an eighth of an 
inch between two screws, each fastened in a wooden block 
mounted on an upright board in the back of the magnets. 
The purpose of the screws is to make the "click" of the 
sounder louder and clearer than it would be if the armature 
only struck the wood. 

A rubber band or a small wire spring passing over a screw 
and connected at the other end to the armature will draw 
the latter away from the magnets when the current is not 
passing. 

The terminals of the magnets are connected to 
binding-posts mounted on the base. 

.. figure:: images/Image151.jpg
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   :alt: Fig. 134.—A Diagram showing how to connect two Simple Telegraph Stations. 
   
   Fig. 134.—A Diagram showing how to connect two Simple Telegraph Stations. 

The key and sounder should be placed in series with one 
or two cells of a battery. Pressing the key will then cause 
the armature of the sounder to be drawn down and make 
a click. When the key is released, the armature will be 
drawn up by the spring or rubber band and make a second 
click. 

Hardly a boy interested in mechanics and electricity has 
not at some time or other wished for a telegraph instrument 
with which to put up a "line" with his chum. 

A practical working set of such instruments can be very 
easily constructed, and with little expense, by following 
the sketches and instructions given here. 

The magnets for the sounder may either be constructed 
by the intending telegraph operator or secured from some 
old electrical instrument such as a magneto-bell. In the 
latter case, the hardest part of the work will be avoided. 

If they are to be home-made, the following suggestions 
may prove of value in carrying out their construction. 

.. figure:: images/Image152.jpg
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   :alt: Fig. 135.—A Complete Telegraph Set, consisting of a Keyboard and a Sounder. 
   
   Fig. 135.—A Complete Telegraph Set, consisting of a Keyboard and a Sounder. 

The cores are made from one-quarter-inch stove-bolts 
with the heads cut off. The magnet heads are cut out of 
hard-wood fiber, one-eighth of an inch thick and one inch 
in diameter. They should fit tightly and be held in place 
with glue. They are separated so as to form a winding 
space between of seven-eighths of an inch. The 
magnets should be wound full of No. 25 B. & S. gauge 
cotton-covered wire. 

.. figure:: images/Image153.jpg
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   :alt: Fig. 136.—Details of the Telegraph Set shown in Figure 135. 
   
   Fig. 136.—Details of the Telegraph Set shown in Figure 135. 

The yoke is made of enough strips of sheet-iron, one-half 
inch wide and two inches long, to form a pile one-quarter 
of an inch thick. Two one-quarter-inch holes are bored in 
the opposite ends of the yoke, one and one-half inches apart. 
The lower ends of the magnet cores are passed through 
these holes. The ends should project one-half of an inch 
beyond the yoke. 

They are passed through two holes in a base-board 
three-quarters of an inch thick. The holes are countersunk 
from the lower side, so that a nut can be screwed on the 
lower end of each and the magnets held tightly in an 
upright position. The remaining parts of the instrument are 
very easily made, and are so clearly shown by the drawing 
that it is hardly necessary to say more than a few words in 
explanation. 

The lever or tongue, the anvil, the standard, and the 
lever of the key are all cut out of hard-wood according to 
the pattern shown in the illustration. 

The armature is a piece of soft iron fastened to the lever 
with a small brass screw. 

Tacks are placed under the heads of the adjusting screws 
on the sounder so that it will click more loudly. 

The rubber band acts as a spring to counteract the weight 
of the armature and lever and draw it up as soon as the 
current is cut off. The movement of the lever should be 
so adjusted that it is only sufficient to make an audible 
click. 

Use care to avoid friction between the lever and the 
standard, so that the former will move with perfect 
freedom. 

All the screws used in the work should be round-headed 
brass wood screws with the points filed flat. Bore a small 
hole before screwing them into place so as to avoid splitting 
the wood. 

The construction of the key is even more simple than that 
of the sounder. It should move up and down without any 
side motion. 

The circuit-closer should be kept closed when the 
instruments are not in use, and when you are receiving a message. 
As soon as you are through receiving and wish to transmit, 
you should open your circuit-closer and your friend close 
his. 

The tension of the spring under the lever of the key must 
be adjusted to suit the needs of each individual operator. 

.. figure:: images/Image154.jpg
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   :alt: Fig. 137.—A Diagram showing how to connect two Complete Telegraph Sets, using one Line Wire and a Ground.
   
   Fig. 137.—A Diagram showing how to connect two Complete Telegraph Sets, using one Line Wire and a Ground. The Two-Point Switches throw the Batteries out of Circuit when the Line is not in use. 

The diagram for connecting the instruments is 
self-explanatory. In cities or towns where a "ground" is 
available by connecting to the gas or water pipes, one line wire 
may be easily dispensed with. Or, if desirable, a ground 
may be formed by burying a large plate of zinc (three or 
four feet square) in a moist spot and leading the wire to it. 

How To Build a Telegraph Relay
############################## 

In working a telegraph over a long line or where there 
are a large number of instruments on one circuit, the 
currents are often not strong enough to work the sounder 
directly. In such a case a *relay* is used. The relay is built 
on the same principle as a sounder, but the parts are made 
much lighter, so that the instrument is more sensitive. 
The armature of a relay is so small and its movement so 
little that its clicking is scarcely audible. It is therefore 
fitted with a second set of contacts and connected to a 
battery and a sounder, which is to set in operation every time 
the contacts close. The principle of a relay is that a weak 
current of insufficient strength to do the work itself may 
set a strong local current to do its work for it. 

There are many forms of relays, and while that which is 
described below is not of the type commonly used on 
telegraph lines, it has the advantage of being far more sensitive 
than any instrument of the regular line relay type that the 
average experimenter could build. 

.. figure:: images/Image155.jpg
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   :alt: Fig. 138.—Details of the Relay Parts. 
   
   Fig. 138.—Details of the Relay Parts. 

Make the magnets from one-quarter-inch stove-bolts, 
and cut them off so that they will form a core about two and 
one-quarter inches long. Fit each of the cores with two 
fiber heads to hold the wire in place. Insulate the legs with 
paper and wind each with about fifty layers of 
No. 30 B. & S. gauge single-cotton-covered magnet wire. The winding 
space between the magnet’s heads should be one and 
one-eighth inches. 

The upper ends of the magnet cores should be allowed to 
project about one-quarter of an inch beyond the fiber head. 
The end of the core is filed flat, as shown in the illustration. 

The magnets are mounted upon an iron yoke, 
three-sixteenths of an inch thick. The holes in the yoke should 
be spaced so that there is a distance of one and one-half 
inches between the centers of the magnet cores. 

The armature of the relay is mounted on a small steel 
shaft with sharp points at each end. The exact shape of 
the armature may be best understood from the illustrations. 

The lower end of the shaft rests in a small cone-shaped 
depression made by driving a center punch into the yoke 
half-way between the two magnets. 

The top bearing is a strip of brass projecting from a 
wooden support. The end of the shaft rests in a depression 
similar to that in the yoke. 

The contact lever is made of brass and forced on the 
shaft below the armature. It swings between a small brass 
clip fastened to one side of the support and a little screw 
held in a similar clip on the opposite side. 

The contact clip is made of spring brass about No. 22 
gauge in thickness. It may be adjusted by a screw passing 
through the support. 

The armature may be controlled in its movement so 
that the latter will be very slight by adjusting the screws. 

There should not be any friction in the bearings and the 
armature should move with perfect freedom. The 
armature should approach the ends of the magnet cores until a 
space about the thickness of heavy paper separates them 
and should not touch them. 

.. figure:: images/Image156.jpg
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   :alt: Fig. 139.—The Completed Relay.
   
   Fig. 139.—The Completed Relay.

The spring is made of fine brass wire. It is fastened to 
the armature shaft, and the screw mounted on the wooden 
support with a piece of silk thread. The thread is passed 
around the shaft once or twice so that the tension of the 
spring will cause the armature to move away from the pole 
pieces just as soon as the current flowing through the 
magnets ceases. 

.. figure:: images/Image157.jpg
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   :alt: Fig. 140.—A Diagram showing how to connect a Relay, Sounder, and Key.
   
   Fig. 140.—A Diagram showing how to connect a Relay, Sounder, and Key. Closing the Key will operate the Relay. The Relay will then operate the Sounder in turn. 

The tension of the spring may be adjusted by turning 
the screw with a screw-driver. If the armature tends to 
stick to the magnet poles fasten a small piece of paper to 
the poles with some shellac. 

The terminals of the magnets are connected to two 
binding-posts marked *A* and *B*. The binding-posts marked *C* 
and *D* are connected respectively to the contact clip and the 
brass bearing on the top of the wooden support. 

The diagram in Figure 140 shows how the relay is 
connected to a telegraph line. 

How To Learn To Telegraph
######################### 

The instruments so far described have been practical 
working telegraph instruments, but they lack the fine 
points of commercial apparatus and it is not possible to 
become as efficient an operator with their aid as with a real 
key and sounder. 

If the young experimenter desires to become a proficient 
telegraph operator, the first thing to do is to purchase a 
Learner’s telegraph key and sounder. 

Connect a dry cell to the binding-posts on the back of 
the instrument. Screw the set down on a table about 
eighteen inches from the front edge, so that there is plenty 
of room for the arm to rest. See that none of the various 
adjustment screws about the instrument are loose and 
that the armature of the sounder moves freely up and 
down through a distance of about one-sixteenth of an 
inch. 

The spring which draws the lever upwards away from the 
magnets should be set only at sufficient tension to raise the 
lever when no current is passing. If too tight, the spring 
will not allow the armature to respond to the current 
flowing through the magnets. 

The key is provided with several adjustment-screws to 
regulate the tension and the play of the lever to suit the 
hand of the operator. A little practice will enable the 
student to judge best for himself just how the key should 
be set. 

The next step is to memorize the alphabet, so that each 
character can instantly be called to mind at will. The 
punctuation marks are not used very frequently, and the 
period is the only one which the student need learn at first. 

The Morse alphabet consists of dots, dashes, and spaces. 
Combinations of these signals spell letters and words. 

Many of the characters are the reverse of others. For 
example, *A* is the reverse of *N*. *B* and *F*, *D* and *U*, *C* and 
*R*, *Q* and *X*, *Z* and *&*, are the other reverse letters, so if the 
formation of one of each of these letters is memorized the 
reverse is easily mastered. 

It is important that the beginner should learn how 
properly to grasp the key, for habits are easily formed and 
a poor position will limit the sending speed of the operator. 

Place the first or index finger on the top of the key-handle, 
with the thumb under the edge; and the second finger on 
the opposite side. The fingers should be curved so as to 
form a quarter-section of a circle. Bring the third and 
fourth fingers down so that they are almost closed on the 
palm of the hand. Rest the arm on the table in front of 
the key and allow the wrist to be perfectly limber. 

.. figure:: images/Image158.jpg
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   :alt: Fig. 141.—How to hold a Telegraph Key.
   
   Fig. 141.—How to hold a Telegraph Key. 

The grasp on the key should be firm but not rigid. Avoid 
using too much strength or a light hesitating touch. 
Endeavor to acquire a positive, firm up and down motion of 
the key. Avoid all side pressure, and do not allow the 
fingers to leave the key when making the signals. The 
movement is made principally with the wrist, with the 
fingers and hand perfectly elastic. 

A dot is made by a single instantaneous, downward 
stroke of the key. A dash is made by holding the key down 
for the same period of time that it takes to make three 
dots. A long dash is made by holding the key down for 
the same time that it takes to make five dots. 

A space in the letters, such as, for instance, the space 
between the first and last two dots in the letter *R* should 
occupy the time of one dot. The space between each letter 
should occupy the time required for two dots, and the space 
between words should occupy the time required for three 
dots. 

Commence the use of the key by making dots in 
succession, first at the rate of two every second, and increasing 
the speed until ten can be made. Practice should be 
continued until three 
hundred and sixty 
dots a minute can 
be made with perfect regularity. 

Then begin 
making dashes at 
the rate of two 
every three 
seconds, and continue until one 
hundred and 
twenty a minute 
can be made with 
perfect regularity. 

Practise the 
long dashes at the 
rate of one a second, and increase until ninety can be made 
in a minute. 

.. figure:: images/Image159.jpg
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   :alt: Fig. 142.—The Morse Telegraphic Code. 
   
   Fig. 142.—The Morse Telegraphic Code. 

When this has been accomplished, practise the following 
letters until they can be perfectly made. Each row of letters 
is an exercise which should be practised separately until 
mastered. 

| Dot Letters 

   E I S H P 6 

| Dot and Space Letters 

   O C R Y Z & 

| Dash Letters 

   T L M 5 O 

| Dots and Dashes 

   A U V 4 

| Dashes and Dots 

   N D B 8 

| Mixed Dots and Dashes 

   F G J K Q W X 1 2 3 7 9 Period 

After you can write these different letters, practise 
making words. Select a list of commonly used words. 
When words seem easy to write, practise sending pages 
from a book. 

Systematic and continual practice will enable the student 
to make surprising progress in mastering the art of sending. 

Reading and receiving messages must be practised with 
a companion student. Place two instruments in separate 
rooms or in separate houses so that the operators will be 
entirely dependent upon the instruments for their 
communication with each other. Start by transmitting and receiving 
simple messages. Then use pages from a book, and 
increase the speed until it is possible to send and receive at 
least 15 words a minute without watching the sounder but 
merely depending upon the clicks to determine the duration 
of the dots and dashes. 

Figure 140 shows how to arrange a regular telegraph line 
for two stations. Gravity batteries should be used for 
regular telegraph work. It is necessary that the key should 
be kept closed by having its circuit-closer shut when 
messages are not being sent. If one of the keys is left open the 
circuit is broken, and it is not possible for a person at the 
other end of the line to send a message. 

Every telegraph office has a name or call usually 
consisting of two letters; thus for New York the call might 
be N. Y. and for Chicago, C. H. 

If New York should desire to call Chicago, he would 
repeat the call letters, C H., until answered. Chicago 
would answer by sending I, several times and signing, 
C H. When so answered, New York would proceed with 
the message. 

.. figure:: images/Image160.jpg
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   :alt: MICROPHONES AND TELEPHONES

CHAPTER XI MICROPHONES AND TELEPHONES
`````````````````````````````````````

In 1878, David Edward Hughes discovered that the 
imperfect contact formed between two pieces of some such 
substance as carbon or charcoal is very sensitive to the 
slightest changes in pressure, and when included in an 
electric circuit with a battery and a telephone receiver, 
will transmit sounds. Such an instrument is called a 
*microphone*. It has various forms but in most of them one 
piece of carbon or charcoal is held loosely between two 
other pieces in such a manner as to be easily affected by 
the slightest vibrations conveyed to it through the air or 
any other medium. 

.. figure:: images/Image161.jpg
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   :alt: Fig. 143.—A Microphone connected to a Telephone Receiver, and a Battery. 
   
   Fig. 143.—A Microphone connected to a Telephone Receiver, and a Battery. 

Figure 143 illustrates a simple form of instrument 
embodying this principle. A small pencil of carbon is 
supported loosely between two blocks of the same substance 
glued to a thin wooden sounding-board of pine. The 
sounding-board is mounted in an upright position on a wooden 
base. The carbon pencil rests loosely in two small 
indentations in the carbon blocks. The blocks are connected, by 
means of a very fine wire or a strip of tinfoil, with one or 
two cells of battery and a telephone receiver. Any 
vibration or sounds in range of the microphone will cause the 
sounding-board to vibrate. This will affect the pressure 
of the contact between the carbon pencil and the two blocks. 
When the pressure between the two is increased the 
resistance in the path of the electric current is decreased and 
more current immediately flows through the circuit. On 
the other hand, when the pressure is decreased, the 
resistance is increased and less current flows through the 
telephone receiver. The amount of current flowing in the 
circuit thus keeps step with the changes in the resistance, and 
accordingly produces sounds in the telephone receiver. 
The vibrations emitted from the receiver are usually much 
greater than those of the original sounds, and so the 
microphone may be used to magnify weak sounds such as the 
ticking of clock-wheels or the footfalls of insects. If a 
watch is laid on the base of the microphone, the ticking of 
the escapement wheel can be heard with startling loudness. 
The sounds caused by a fly walking on a microphone may 
be made to sound as loud as the tramp of a horse. 

.. figure:: images/Image162.jpg
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   :alt: Fig. 144.—A Very Sensitive Form of Microphone, with which the Footsteps of a Fly can be heard. 
   
   Fig. 144.—A Very Sensitive Form of Microphone, with which the Footsteps of a Fly can be heard. 

The electrical *stethoscopes* used by physicians to listen 
to the action of the heart are in principle only a 
microphone and telephone receiver connected to a battery. 

The drawing in Figure 144 illustrates a very sensitive 
microphone that is quite easy to make. With this 
instrument it is possible to hear the tramping of a fly’s feet or 
the noise of its wings. 

The base upon which the apparatus is mounted serves as 
the sounding-board and is made in the form of a hollow 
wooden box. It can be made from an ordinary cigar-box 
by removing the paper and taking the box apart. The 
piece forming the top of the box must be planed down until 
it is only three thirty-seconds of an inch thick. The box 
should measure about five inches square and three-quarters 
of an inch thick when finished. Do not use any nails or 
small brads whatsoever in its construction, but fasten it 
together with glue. If you use any nails you will decrease 
the sensitiveness of the instrument quite appreciably. 
The bottom of the box should be left open. The result 
is a sounding-board of the same principles as that of the 
banjo head. Small feet, one-quarter of an inch square, are 
glued to the four under corners so as to raise the bottom 
clear of the table, or whatever the microphone may be 
placed upon. The bottom of each one of the small feet is 
cushioned with a layer of felt so that no jars will be 
transmitted to the instrument by any object upon which it is 
resting. 

The carbon pencil used on this type of instrument is 
pivoted in the center and rests at one end upon a carbon 
block. 

The carbon block is made about one inch long, 
one-quarter of an inch thick, and one-half of an inch wide. A 
small hole is drilled near each end to receive a screw which 
fastens the block to the sounding-board. A fine wire is 
led from one of these screws to a binding-post mounted at 
the side of the box. Another wire leads from a second 
binding-post to a standard which is also fastened to the 
sounding-board with a small screw. 

The standard is made from a sheet of thin brass and is 
bent into the shape shown in the illustration. 

The pencil is a piece of one-quarter-inch carbon rod, 
two and three-quarter inches long. A small hole is drilled 
one and five-eighths of an inch from one end with a 
sewing-needle, and a piece of fine brass wire, pointed at both ends, 
pushed in. The wire should be a tight fit in the hole. It 
should be about one-half of an inch long, and may be made 
from an ordinary pin. 

The slide-bar is used to regulate the pressure of the 
pencil upon the carbon block and is simply a piece of soft 
copper wire about one-eighth of an inch in diameter. It is 
bent into the shape shown in the illustration so that it 
will slide over the carbon pencil. The sides of the standard 
should press just tightly enough against the ends of the 
pivot which passes through the carbon pencil to hold it in 
position without slipping, and at the same time allow it to 
swing freely up and down. 

The two binding-posts should be connected in series with 
two dry cells and a pair of good telephone receivers. Place 
the receivers against the ears. Move the slide-bar gently 
back and forth until the voice of any one talking in another 
part of the room can be heard distinctly in the telephone 
receivers. In order to hear faint whispers, move the 
slide-bar away from the carbon block. 

In order to hear a fly walk it is necessary to have the 
carbons very dry and clean. The instrument must be very 
carefully adjusted. Cover the microphone with a large 
glass globe and place a fly inside of the globe. Whenever 
the fly walks on any part of the microphone you will be 
able to hear each footstep in the telephone receivers. When 
he flies about inside of the globe, his wings will cause a 
loud roaring and buzzing noise to be heard in the receivers. 

Telephones
########## 

Not many years ago, when the telephone made its first 
appearance, it was the wonder of the times just as wireless 
telegraphy is to-day. Starting as an exceedingly simple 
and inexpensive apparatus, it has gradually developed 
into a wonderful and complex system, so that at the 
present time, instead of experiencing difficulty in telephoning 
over distances of fifty or one hundred miles, as at first, it 
is possible to carry on a conversation over a line two 
thousand miles long as easily as it is face to face. 

Like the telegraph, the principle of the telephone is that 
of a current of electricity flowing over a line wire into a 
pair of electro-magnets, but with many important 
differences. 

When compared with telegraph apparatus, the telephone 
is found to be exceedingly sensitive. A telegraph relay 
requires perhaps about one-hundredth of an ampere to 
work it properly. A telegraph sounder will require about 
one-tenth of an ampere, but a telephone receiver will 
render speech audible with less than a millionth of an 
ampere, and therefore may almost be said to be a hundred 
thousand times more sensitive than a sounder. 

Another difference between the telephone and the 
telegraph lies in the fact that the currents flowing over a 
telegraph line do not usually vary at a rate greater than 
twenty or thirty times a second, whereas telephone 
currents change their intensity hundreds of times a second. 

The telephone is an instrument for the transmission of 
speech to a distance by means of electricity, wherein the 
speaker talks to an elastic plate of thin sheet-iron which 
vibrates and sends out a pulsating current of electricity. 

The transmission of the vibrations depends upon 
well-known principles of electricity, and does not consist of the 
actual transmission of sounds, but of electrical impulses 
which keep perfect accord or step with the sound waves 
produced by the voice in the transmitter. These electrical 
currents pass through a pair of small electro-magnets acting 
upon a plate or diaphragm, which in turn agitates the air 
in a manner similar to the original voice speaking into the 
transmitter and thus emits sounds. 

That part of the apparatus which takes up the sounds 
and changes them into electric currents composes the 
*transmitter*. When words are spoken into the mouthpiece 
they strike a diaphragm, on the back of which is fastened 
a small cup-shaped piece of carbon. A second cup is 
mounted in a rigid position directly back of the first. The 
space between them is filled with small polished granules 
of carbon. When these granules are in a perfectly loose 
state and are undisturbed, their resistance to an electric 
current is very great and they allow almost none to flow. [#]_ 
When slightly compressed their resistance is greatly 
lowered and they permit the current to pass. The 
vibrations of the diaphragm cause the carbon cup mounted on 
its back to move and exert a varying pressure upon the 
granules with a corresponding variation in their resistance 
and the amount of current which will pass through. 

.. figure:: images/Image163.jpg
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   :alt: Fig. 145.—A Telephone System, consisting of a Receiver, Transmitter, and a Battery connected in Series.
   
   Fig. 145.—A Telephone System, consisting of a Receiver, Transmitter, and a Battery connected in Series. Words spoken into the Transmitter are reproduced by the Receiver. 

The *receiver*, or that part of the apparatus which 
transforms the pulsating current back into sound waves consists 
of a thin iron disk, placed very near but not quite touching 
the end of a small steel bar, permanently magnetized, and 
about which is wound a coil of fine insulated wire. 

The transmitter and the receiver are connected together 
in series with a battery as in Figure 145. When words are 
spoken into the transmitter the little carbon granules are 
immediately thrown into motion, and being alternately 
compressed and released cause corresponding changes in 
the current flowing through the receiver from the battery. 
The magnetism of the receiver changes with each change 
in the electric current, and thus by alternately attracting 
and repelling the diaphragm causes it to vibrate and emit 
sounds. Such is the *principle* of the telephone. The 
telephones in actual service to-day are complicated with bells, 
magnetos, induction coils, condensers, relays, and various 
other apparatus, which fact renders them more efficient. 

The bells and magnetos are for the purpose of calling 
the central operator or the person at the other end of the 
line and drawing attention to the fact that some one wishes 
to get into communication with him. The older styles of 
telephones used what is known as a polarized bell and a 
hand magneto for this purpose. A polarized bell is a very 
sensitive piece of apparatus which will operate with very 
little current. A magneto is a small hand dynamo which 
when turned with a crank will generate a current causing 
the bell at the other end of the line to ring. When the 
telephone receiver is raised off its hook in order to place it 
to the ear the bell and magneto are automatically 
disconnected from the line and the receiver and the 
transmitter are connected in their place. The current necessary 
to supply the telephone and receiver is supplied by two or 
three dry cells placed inside of each telephone. 

The latest types of instruments employ what is known as 
the central energy system, wherein the current is supplied 
by a large storage battery located at the central office and 
serving as a current supply to all the telephones connected 
to that system. 

It would be impossible to enter into the history of the 
telephone far enough to explain the details of some of the 
various systems in every-day use in such a book as this 
because of the immense amount of material it would be 
necessary to present. Such a work would occupy a volume 
of its own. Additional information may be readily found 
in any reference library. However, the "boy electrician" 
who wishes to make a telephone for communicating 
between the house and barn, or with his chum down the 
street, will find the necessary information in the following 
pages. If this work is carried out carefully and a 
home-made telephone system built and installed it will not only 
prove a very interesting undertaking but will also serve to 
dispel all mystery which may surround this device in the 
mind of the young experimenter. 

How to Build a Telephone 
########################

Telephone receivers are useful for many purposes in 
electrical work other than to receive speech. They are 
used in connection with wireless instruments, in place of 
a galvanometer in measuring electrical circuits, and for 
testing in various ways. 

Telephone receivers are of two types. One of them is 
long and cumbersome, and is very similar to the original 
Bell telephone receiver. The other is small and flat, and 
is called a "watch-case" receiver. A watch-case receiver 
is shown in Figure 146. It consists of a U-shaped 
permanent magnet so mounted as to exert a polarizing influence 
upon a pair of little electro-magnets, before the poles of 
which is placed an iron diaphragm. For convenience, these 
parts are assembled in a small cylindrical casing, usually 
of hard rubber. The permanent magnet exerts a continual 
pull upon the diaphragm, tending to draw it in. When the 
telephone currents pass through the little magnets, they 
will either strengthen the permanent magnet and assist 
it in attracting the diaphragm, or detract from its strength 
and allow the diaphragm to recede, depending upon which 
direction the current flows. 

.. figure:: images/Image164.jpg
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   :alt: Fig. 146.—A Watch-Case Telephone Receiver. 
   
   Fig. 146.—A Watch-Case Telephone Receiver. 

Watch-case receivers are usually employed for wireless 
telegraph work because they are very light in weight and 
can easily be attached to a head-band in order to hold them 
to the ears and leave the hands free. Watch-case receivers 
can be purchased for forty-five to seventy-five cents at 
almost any electrical supply house. They are very useful 
to the amateur experimenter in many ways. 

A telephone receiver capable of giving fair results on a 
short telephone line can be very easily made, but of course 
will not prove as efficient as one which is purchased 
ready-made from a reliable electrical manufacturer. 

The first practical telephone receiver was invented by 
Alexander Graham Bell and was made somewhat along the 
same lines as that shown in Figure 147. 

Such a receiver may be made from a piece of 
curtain-pole, three and three-quarter inches long and about one 
and one-eighth inches in diameter. A hole, three-eighths 
of an inch in diameter, is bored along the axis throughout 
its entire length, to receive the permanent magnet. 

The shell of the receiver is a cup-shaped piece of hard 
wood, two and one-half inches in diameter and one inch 
deep. It will have to be turned on a lathe. Its exact shape 
and dimensions are best understood from the dimensions 
shown in the cross section in Figure 147. The shell is firmly 
attached to one end of the piece of curtain-pole by gluing. 

The permanent magnet is a piece of hard steel, 
three-eighths of an inch in diameter and four and five-eighths of 
an inch in length. The steel will have to be tempered or 
hardened before it will make a suitable magnet, and the 
best way to accomplish this is to have a blacksmith do it 
for you by heating the rod and then plunging it into water 
when just at the right temperature. 

.. figure:: images/Image165.jpg
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   :scale: 85 %
   :alt: Fig. 147.—A Simple Form of Telephone Receiver. 
   
   Fig. 147.—A Simple Form of Telephone Receiver. 

One end of the bar is fitted with two thick fiber washers 
about seven-eighths of an inch in diameter and spaced 
one-quarter of an inch apart. The bobbin so formed is 
wound full of No. 36 B. & S. gauge single-silk-covered 
magnet wire. The ends of the wire are passed through two 
small holes in the fiber washers and then connected to a 
pair of heavier wires. The wires are run through two holes 
in the curtain-pole, passing lengthwise from end to end, 
parallel to the hole bored to receive the bar magnet. 

This bar magnet is then pushed through the hole until 
the end of the rod on which the spool is fixed is just below 
the level of the edges of the shell. 

The two wires are connected to binding-posts, *A* and *B*, 
mounted on the end of the receiver. A hook is also 
provided so that the receiver may be hung up. 

The diaphragm is a circular piece of thin sheet-iron, two 
and one-half inches in diameter. It is placed over the 
shell, and the bar magnet adjusted until the end almost 
touches the diaphragm. The magnet should fit into the 
hole very tightly, so that it will have to be driven in order 
to be moved back and forth. 

The diaphragm is held in place by a hard-wood cap, two 
and three-quarter inches in diameter and having a hole 
three-quarters of an inch in diameter in the center. The 
cap is held to the shell by means of four small brass screws. 

The receiver is now completed and should give a loud 
click each time that a battery is connected or disconnected 
from the two posts, *A* and *B*. 

The original Bell telephone apparatus was made up 
simply of two receivers without any battery or transmitter. 
In such a case the current is generated by "induction." 
The receiver is used to speak through as well as to hear 
through. This method of telephoning is unsatisfactory 
over any appreciable distances. The time utilized in making 
a transmitter will be well spent. 

A simple form of transmitter is shown in Figure 148. 
The wooden back, *B*, is three and one-half inches square 
and three-quarters of an inch thick. The front face of the 
block is hollowed out in the center as shown in the 
cross-section view. 

The face-plate, *A*, is two and one-half inches square and 
one-half an inch thick. A hole, seven-eighths of an inch 
in diameter, is bored through the center. One side is then 
hollowed out to a diameter of one and three-quarter inches, 
so as to give space for the diaphragm to vibrate as shown 
in the cross-sectional drawing. 

The carbon buttons are one inch in diameter and 
three-sixteenths of an inch thick. A small hole is bored in the 
center of each to receive a brass machine screw. The 
hole is countersunk, so as to bring the head of the screw 
down as close to the surface of the carbon as is possible. 
Then, using a sharp knife or a three-cornered file, score 
the surface of the carbon until it is covered with criss-cross 
lines. 

The diaphragm is a piece of thin sheet-iron cut in the 
form of a circle two and one-half inches in diameter. A 
small hole is bored through the center of this. One of the 
carbon buttons is fastened to the center of the diaphragm 
with a small screw and a nut. 

Cut out a strip of flannel or thin felt, nine-sixteenths of 
an inch wide and three and one-half inches long. Around 
the edge of the carbon button mounted on the diaphragm, 
bind this strip with silk thread in such a manner that the 
strip forms a cylinder closed at one end with the button. 

Fill the cylinder with polished carbon telephone 
transmitter granules to a depth of about one-eighth of an inch. 
These granules will have to be purchased from an electrical 
supply house. They are finely polished small carbon balls, 
much like birdshot in appearance. 

Slip a long machine-screw through the hole in the second 
carbon button and clamp it in place with a nut. Then place 
the carbon button in the cylinder so that it closes up the 
end. The space between the two buttons should be about 
three-sixteenths of an inch. Bind the flannel or felt around 
the button with a piece of silk thread so that it cannot 
slip out of place. The arrangement of the parts should now 
be the same as that shown by the cross-sectional drawing 
in the upper right-hand corner of Figure 148. 

The complete transmitter is assembled as shown in the 
lower part of Figure 148. 

A small tin funnel is fitted into the hole in the 
face-plate, *A*, to act as a mouthpiece. 

A screw passes through the back, *B*, and connects to the 
diaphragm. The screw is marked "*E*" in the illustration. 
A binding-post is threaded on the screw so that a wire may 
be easily connected. The screw passing through the back 
carbon button also passes through a hole in the wooden 
back, and is clamped firmly in position with a brass nut 
so that the button is held very rigidly and cannot move. 
The front button, being attached to the diaphragm, is free 
to move back and forth with each vibration of the latter. 

.. figure:: images/Image166.jpg
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   :alt: Fig. 148.—A Home-made Telephone Transmitter. 
   
   Fig. 148.—A Home-made Telephone Transmitter. 

The carbon granules should fill the space between the 
buttons three-quarters full. They should lie loosely 
together, and not be packed in. 

When connected to a battery and a telephone receiver 
the current passes from the post, *D*, to the back button, 
through the mass of carbon granules into the front button 
and out at the post, *E*. When the voice is directed into the 
mouthpiece, the sound waves strike the diaphragm and 
cause it to vibrate. The front button attached to it then 
also vibrates and constantly changes the pressure on the 
carbon granules. Each change in pressure is accompanied 
by an immediate change in resistance and consequently 
the amount of current flowing. 

Figure 149 shows a complete telephone ready for 
mounting on the wall. It consists of a receiver, telephone 
transmitter, bell, hook, and push-button. The bell is mounted 
on a flat base-board. The transmitter is similar to that just 
described, but is built into the front of a box-like cabinet. 
The box is fitted with a push-button at the lower 
right-hand corner. A simple method of making a suitable 
push-button is shown in the upper left-hand part of the 
illustration. It consists of two small brass strips arranged so that 
pushing a small wooden plug projecting through the side 
of the cabinet will bring the two strips together and make 
an electrical connection. 

The "hook" consists of a strip of brass, pivoted at one 
end with a round-headed brass wood screw and provided 
with a small spring, so that when the receiver is taken off 
of the hook it will fly up and make contact with a screw, 
marked *C* in the illustration. When the receiver is on the 
hook, its weight will draw the latter down against the 
screw, *D*. The hook is mounted on the base-board of the 
telephone, and projects through a slot cut in the side of 
the cabinet. 

Four binding-posts are mounted on the lower part of the 
base-board. The two marked *B* and *B* are for the battery. 

.. figure:: images/Image167.jpg
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   :scale: 85 %
   :alt: Fig. 149.—A Complete Telephone Instrument. 
   
   Fig. 149.—A Complete Telephone Instrument. Two Instruments such as this are necessary to form a simple Telephone System. 

That marked *L* is for the "line," and *G* is for the ground 
connection or the return wire. 

.. figure:: images/Image168.jpg
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   :scale: 85 %
   :alt: Fig. 150.—Diagram of Connection for the Telephone Instrument shown in Fig. 149. 
   
   Fig. 150.—Diagram of Connection for the Telephone Instrument shown in Fig. 149. 
   
The diagram of the connections is shown in Figure 150. 
The line-wire coming from the telephone at the other 
station enters through the binding-post marked *L*, and 
then connects to the hook. The lower contact on the hook 
is connected to one terminal of the bell. The other 
terminal of the bell leads to the binding-post marked *G*, which 
is connected to the ground, or to the second line-wire, 
where two are used. 

The post, *G*, and one post, *B*, are connected together. 
The other post marked *B* connects to one terminal of the 
transmitter. The other terminal of the transmitter is 
connected to the telephone receiver. The other post of 
the telephone receiver leads to the upper contact on the 
hook marked *C*. The push-button is connected directly 
across the terminals of the transmitter and the receiver so 
that when the button is pushed it short-circuits the 
transmitter and the receiver. When the receiver is on the hook 
and the latter is down so that it makes contact with *D* any 
current coming over the line-wire will pass through the 
bell and down through the ground or the return-wire to the 
other station, thus completing the circuit. If the current 
is strong enough it will ring the bed. When the receiver is 
lifted off the hook, the spring will cause the hook to rise 
and make contact with the screw marked *C*. This will 
connect the receiver, transmitter, and the battery to the 
line so that it is possible to talk. If, however, it is desired 
to ring the bell on the instrument at the other end of the 
line, all that it is necessary to do is to press the push-button. 
This will short-circuit the receiver and the transmitter 
and ring the bell. The battery current is flowing over the 
line all the time when the receiver is up, but the 
transmitter and the receiver offer so much resistance to its flow 
that not enough current can pass to ring the bell until the 
resistance is cut out by short-circuiting them with the 
push-button. 

The instruments at both ends of the line should be 
similar. In connecting them together care should be taken to 
see that the batteries at each end of the line are arranged 
so that they are in series and do not oppose each other. 
One side of the line may be a wire, but the return may be 
the ground, as already explained in the chapter on 
telegraph apparatus. 

A transmitter of the "desk-stand" type may be made 
according to the scheme shown in Figure 151. It consists 
simply of a transmitter mounted upon an upright, and 
provided with a base so that it may stand on a desk or a table. 

.. figure:: images/Image169.jpg
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   :alt: Fig. 151.—A Desk-Stand Type of Telephone.
   
   Fig. 151.—A Desk-Stand Type of Telephone.
   
It is also fitted with a hook and a push-button, so that it 
is a complete telephone instrument with the exception of 
the bell and the battery. The battery and the bell may be 
located in another place and connected to the desk-stand 
by means of a flexible wire or "electrical cord." 

Figure 152 shows what is known as a telephone induction 
coil. Induction coils are used in telephone systems 
whenever it is necessary to work over a long distance. Such a 
system is more complicated, and requires considerable 
care in making the connections, but is far superior to the 
system just described. 

.. figure:: images/Image170.jpg
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   :alt: Fig. 152.—A Telephone Induction Coil.
   
   Fig. 152.—A Telephone Induction Coil.
   
An induction coil consists of two fiber or hard-wood 
heads, about one inch square and one-quarter of an inch 
thick, mounted on the ends of an iron core composed of a 
bundle of small iron wires about two and one-half inches 
long. The core should be about five-sixteenths of an inch 
in diameter. 

The core is covered with a layer of paper and then wound 
with three layers of No. 22 B. & S. single-cotton-covered 
wire. These three layers of wire form the *primary*. The 
primary is covered with a layer of paper and then the 
secondary is wound on. The secondary consists of twelve 
layers of No. 36 B. & S. single-silk-covered magnet wire. 
It is advisable to place a layer of paper between layers of 
the secondary winding, and to give each one a coating of 
shellac. The two secondary terminals of the coil are led 
out through holes in the fiber head and kept separate from 
the primary terminals. 

.. figure:: images/Image171.jpg
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   :alt: Fig. 153.—Diagram of Connection for a Telephone System employing an Induction Coil at each Station. 
   
   Fig. 153.—Diagram of Connection for a Telephone System employing an Induction Coil at each Station. 
   
The wiring diagram of a telephone system using an 
induction coil at each station is shown in Figure 153. The 
speech sent over a line using an induction coil system is 
much clearer and more easily understood than that on a 
line not using such a device. 

In building telephone instruments or connecting them up, 
care and accuracy will go a long way towards success. 
Telephony involves some very delicate and sensitive 
vibratory mechanical and electrical actions, and such 
instruments must be very carefully made. 

.. [#] A transmitter is really a microphone built especially to receive the sounds of the human voice, and operates on the same principle. 

.. figure:: images/Image172.jpg
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   :alt: INDUCTION COILS

CHAPTER XII INDUCTION COILS
```````````````````````````

A Medical Coil or shocking coil, as it is properly termed, 
is nothing more or less than a small induction coil, and 
consists of a core, a primary winding, a secondary 
winding, and an interrupter. The principle of an induction coil 
and that of magnetic induction have already been explained 
in Chapter V. It might be well for the readers to turn back 
to pages 89-91 and reread them. 

The human body possesses considerable resistance, and 
the voltage of one or two ordinary cells of battery is not 
sufficient to overcome that resistance and pass enough 
current through the body to be felt, unless under 
exceptional conditions. 

The simplest means employable for raising the voltage 
of a battery high enough to produce a shock is the medical 
coil. 

The first step in making such a coil is to roll up a paper 
tube, five-sixteenths of an inch in diameter inside, and 
two and one-half inches long. The outer end of the paper 
is carefully glued, so that it will not unroll. The tube is 
filled with pieces of iron wire two and one-half inches long 
which have been straightened by rolling between two 
boards. The size of the iron wire may vary from No. 20 
to No. 24 B. & S. gauge. Enough should be slipped into the 
tube to pack it tightly and admit no more. 

A square block, 1 x 1 x 5-16 inches, is cut out of fiber 
or a close-grained hard wood and a hole three-eighths of an 
inch in diameter bored through the center. One end of the 
tube containing the core is smeared with glue and slipped 
into the block. The end of the tube is allowed to project 
through about one-sixteenth of an inch. A second block, 
in the form of a circle three-quarters of an inch in diameter, 
one-quarter of an inch thick, and having a three-eighths of 
an inch hole through the center, is glued on the opposite 
end. 

.. figure:: images/Image173.jpg
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   :alt: Fig. 154.—Details of Various Parts of a Medical Coil. 
   
   Fig. 154.—Details of Various Parts of a Medical Coil. 

After the glue has dried, four small holes are drilled in 
the square head in the approximate positions shown by 
Figure 154. Four layers of No. 22 B. & S. gauge magnet 
wire (it may be either silk or cotton, double or single 
covered) is wound smoothly and carefully over the core. The 
terminals are led out of the holes *a* and *b*. The primary 
is covered with two or three layers of paper, and then 
enough secondary wound on to bring the total diameter of 
the coil to about eleven-sixteenths of an inch. The 
secondary wire must be much finer than the primary. It is 
possible to use any size from No. 32 to No. 36 B. & S. gauge 
and obtain good results. The insulation may be either 
single silk or single cotton. 

.. figure:: images/Image174.jpg
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   :scale: 85 %
   :alt: Fig. 155.—Details of Interrupter for Medical Coil. 
   
   Fig. 155.—Details of Interrupter for Medical Coil. 

The secondary terminals are led out through the holes *c* 
and *d*. It is perhaps a wise plan to re-enforce these leads 
with a heavier piece of wire, because otherwise they are 
easily broken. 

The interrupter is a simple arrangement capable of being 
made in several different ways. The drawing shows an 
arrangement which can be improved upon by any 
experimenters who are familiar with a medical coil. I have shown 
the simplest arrangement, so that all my readers will be 
able to build it, and those who want to improve it can do so. 

If a small piece of silver is soldered to the spring and to 
the contact-point it will give better results. The silver 
is easily secured by cutting up a ten-cent piece. One 
terminal of the primary is connected to the interrupter spring 
and the other to a binding-post. The contact-post is also 
connected to a binding-post. If a battery is connected to 
the two binding-posts, the current will flow from one post 
through the coil to the interrupter spring, through the 
spring to the contact post, and thence back to the battery, 
making a complete circuit. As soon as the current flows, 
however, it produces magnetism which draws the spring 
away from the contact and breaks the circuit, cutting off 
the magnetic pull. The spring flies back to the contact but 
is drawn forward again immediately and repeats the 
operation continuously at a high rate of speed. 

.. figure:: images/Image175.jpg
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   :alt: Fig. 156.—Completed Medical Coil.
   
   Fig. 156.—Completed Medical Coil.

The secondary terminals are led out to two binding-posts 
to which are connected two electrodes or handles by means 
of flexible wires. The electrode may be made of two 
ordinary flat strips of sheet-metal or a piece of tubing. In the 
latter case, the wires may be connected by wedging them 
in with a cork. If the handles are grasped while the battery 
is connected to the primary posts and the interrupter is 
in operation a powerful shock will be felt. The shock may 
be regulated from a weak current that can hardly be felt 
to a very powerful one by providing the coil with a piece 
of iron tubing of about seven-eighths of an inch inner 
diameter and two inches long which will slip on and oh the 
coil. When the tube is all the way on, the shock is very 
mild, and when all the way off, the shock is very strong. 
Of course any intermediate strength may be secured at 
stages between the two extremes. 

The current from medical coils is often prescribed by 
physicians for rheumatism and nervous disorders, but must 
be properly applied. The coil just described is harmless. 
It will give a strong shock, but the only result is to make the 
person receiving it drop the handles and not be anxious to 
try it again. 

Spark-Coils 
###########

A "spark-coil" is one of the most interesting pieces of 
apparatus an experimenter can possess. The experiments 
that may be performed with its aid are varied and many. 

The purpose of a "spark-coil" is to generate enormously 
high voltages which are able to send sparks across an air 
space that ordinary currents of low voltage could not 
possibly pierce. The spark-coil is the same in principle as the 
small induction coils used as medical or shocking coils, but 
is made on a larger scale and is provided with a condenser 
connected across the terminals of the interrupter. 

.. figure:: images/Image176.jpg
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   :alt: Fig. 157.—Diagram showing Essential Parts of Induction Coil. 
   
   Fig. 157.—Diagram showing Essential Parts of Induction Coil. 

It consists of a central iron core surrounded by a coil of 
heavy wire called the "primary," and by a second outside 
winding of wire known as the "secondary." The primary 
is connected to a few cells 
of battery in series with an 
interrupter. The interrupter makes and breaks 
the circuit, i. e., shuts the 
current on and off repeatedly. 

Every time that the current is "made" or broken, 
a high voltage is induced 
in the secondary. By means 
of the condenser connected across the interrupter 
terminals, the current at "make" is caused to take a 
considerable fraction of time to grow, while at "break," the 
cessation is instantaneous. The currents induced in the 
secondary at break are so powerful that they leap across 
the space in a brilliant torrent of sparks. 

Building a Spark-Coil 
#####################

Perhaps more attempts are made by experimenters to 
construct a spark-coil than any other piece of apparatus, 
and the results are usually poor. A spark-coil is not hard 
to construct, but it requires careful work and patience. 
It is not a job to be finished in a day, but time must be 
liberally expended in its construction. Satisfactory results 
are easily obtained by any one of ordinary mechanical 
ability if patience and care are used. 

Parts for spark-coils are for sale by many electrical 
houses, and it is possible to purchase a set of such 
machine-made parts for less than the separate materials usually 
cost. 

For the benefit of those who might wish to build a larger 
coil than the one described in the following text, a table 
showing the dimensions of two other sizes will be found. 

.. figure:: images/Image177.jpg
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   :alt: Fig. 158.—Empty Paper Tube, and Tube filled with Core Wire preparatory to winding on the Primary. 
   
   Fig. 158.—Empty Paper Tube, and Tube filled with Core Wire preparatory to winding on the Primary. 

**The core** is made of very soft iron wire about No. 20 
or 22 B. & S. gauge, cut to exact length. Each piece should 
be six inches long. Iron wire may be purchased from 
electrical supply houses already cut to various lengths for 
twenty cents a pound. In view of the amount of labor 
required carefully to cut each piece to length and then 
straighten it out so that it will form a neat bundle, it is 
cheaper to purchase the wire already cut. Such wire has 
been annealed, i.e., softened by bringing to a red heat and 
then cooling slowly. In case the wire is purchased at a 
plumbing shop or a hardware store it must be annealed 
before it can be used. This is accomplished by tying the 
wire in a compact bundle and placing it in a wood fire where 
it will grow red-hot. When this stage is reached, cover 
the wire with ashes and allow the fire to die away. 

.. figure:: images/Image178.jpg
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   :alt: Fig. 159.—Illustrating the Various Steps in winding on the Primary and fastening the Ends of the Wire. 
   
   Fig. 159.—Illustrating the Various Steps in winding on the Primary and fastening the Ends of the Wire. 

Cut a piece of tough wrapping paper into strips six inches 
long and about five inches wide. Wrap it around a stick or 
metal rod one-half of an inch in diameter, so as to form a 
tube six inches long and having a diameter of one-half of 
an inch. Glue the inside and outside edges of the paper 
so that the tube cannot unroll and then slip it off the stick. 

.. figure:: images/Image179.jpg
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   :alt: Fig. 160.—Complete Primary Winding and Core.
   
   Fig. 160.—Complete Primary Winding and Core.

Fill the tube with the six-inch wires until it is packed 
tightly and no more can be slipped in. 

**The primary** consists of two layers of No. 18 B. & S. 
gauge cotton-covered wire wound over the core for a 
distance of five inches. One-half pound of wire is more than 
enough for one primary. The wire must be wound on very 
smoothly and carefully. In order to fasten the inside end 
so that it will not become loose, place a short piece of tape 
lengthwise of the core and wind on two or three turns over 
it. Then double the end back and complete the winding. 
After the first layer is finished, give it a coat of shellac and 
wind on the second layer. The end of the wire is wound 
with a piece of tape and fastened by slipping through a 
loop of tape embedded under the last few turns. The 
illustrations will explain more clearly just how this is 
accomplished. The second layer is then given a coat of 
shellac and allowed to dry. After it is dry, wrap about 
fifteen layers of paper which have been soaked in paraffin 
around the primary. This operation should be performed 
in a warm place, over a fire or lighted lamp where the 
paraffin may be kept soft, so that the paper will go on 
tightly. 

.. figure:: images/Image180.jpg
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   :alt: Fig. 161.—The Primary covered with Insulating Layer of Paper ready for the Secondary. 
   
   Fig. 161.—The Primary covered with Insulating Layer of Paper ready for the Secondary. 

The coil is now ready to receive the secondary winding. 
The core and primary which have been described are 
suit-able for a secondary giving sparks from one-half to 
three-fourths of an inch long. 

**The secondary** winding consists of several thousand 
turns of very fine wire wound on in smooth even layers 
with paper between each two layers. 

The following table shows the size and amount of wire 
required. In addition, about two pounds of paraffin and 
a pad of linen paper or typewriter paper will be required. 
The wire may be either enamel, cotton, or silk insulated. 
Single silk-covered wire is preferred. 

+------------+-------------+-----------+
|SIZE OF COIL| SIZE OF WIRE|  AMOUNT   |
+------------+-------------+-----------+
|1/2 inch    | 36 B. & S.  |  10 ounces|
+------------+-------------+-----------+
|1 inch      |  34 B. & S. |   1 lb.   |
+------------+-------------+-----------+
| 1/2 inch   | 34 B. & S.  |  2 lbs.   |
+------------+-------------+-----------+

The means for supporting and turning the coil in order 
to wind on the secondary may be left somewhat to the 
ingenuity of the young experimenter. The following 
suggestion, however, is one which experience has proved to 
be well worth following out, and may be applied to other 
things than the construction of an induction coil. It seems 
to be the nature of most boys, for some reason or other, to 
be unwilling to spend time and labor on anything which 
will aid them in their work. They are always in such a 
hurry and so anxious to see something completed that they 
direct all their energy to that end rather than spend part 
of their time in constructing some little device which 
would really lighten the other work and go a long way 
towards insuring its successful completion. 

I have frequently given instructions for building an 
induction coil and placed particular stress upon winding 
the secondary, only to have such suggestions ignored in 
the anxious endeavor of boys to finish the coil as soon as 
possible. In every such instance the coil has been a 
failure. 

.. figure:: images/Image181.jpg
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   :alt: Fig. 162.—Simple Winding Device for winding the Secondary. 
   
   Fig. 162.—Simple Winding Device for winding the Secondary. 

The illustration shows a simple form of winder, with 
which the operation of winding the secondary is a very 
slow one, but, on the other hand, it is possible to do very 
accurate, careful winding with the aid of such a device. 
The parts may all be made from wood. 

The chucks fit tightly over the ends of the core so that 
when the handle is turned, the coil will revolve also. The 
spring serves to keep the chucks snugly against the coil 
ends, so that they will not slip. 

From one-half to five-eighths of a pound of wire will be 
required to wind the coil. A large number of strips of thin 
paraffined or waxed paper must be cut five inches wide. 
The inside terminal, or "beginning" end of the wire is 
tied around the insulating tube near the left-hand end. 
The spool of wire must be placed in a position where it will 
revolve freely without strain on the wire. No. 36 is very 
fine and easily broken, so use the utmost care to guard 
against this mishap. 

.. figure:: images/Image182.jpg
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   :alt: Fig. 163.—Completed Secondary Winding. 
   
   Fig. 163.—Completed Secondary Winding. 

Wind on a smooth, even layer of wire, permitting each 
turn to touch the other, but none to lap over. Carry the 
winding to within one-half inch of the ends of the insulating 
tube and then 
wind on two 
layers of the 
waxed paper. 

The paper 
must be put on 
smoothly and 
evenly, so as to 
afford a firm 
foundation for 
the next layer. The wire is wrapped around with the 
paper, so that the next layer starts one-half inch from 
the edge. A second layer is then wound on very carefully, 
stopping when it comes one-half inch from the edge. Two 
more layers of paper are put on, and the process repeated, 
alternately winding on paper and wire until the stated 
quantity of the latter has been used up. The layers of 
wire may occasionally be given a coating of shellac. This 
is a good insulator, and will serve to hold them together 
and prevent the wire from becoming loose. 

In winding the coil, remember that if at any point you 
allow the winding to become irregular or uneven, the 
irregularity will be much exaggerated on the succeeding layers. 
For this reason, do not allow any to occur. If the wire tends 
to go on unevenly, wrap an extra layer of thick paper 
around underneath so as to offer a smooth foundation, and 
you will find the difficulty remedied. 

.. figure:: images/Image183.jpg
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   :alt: Fig. 164.—Interrupter Parts. 
   
   Fig. 164.—Interrupter Parts. 

An efficient vibrator for a coil cannot be easily made, and 
it is best to buy one which is already fitted with platinum 
points. The interrupter will play a very important part 
in the successful working of the coil, and its arrangement 
and construction are important. Interrupters like that 
shown in the illustration and used for automobile
will be found the best. 

The condenser may be home-made. It consists of 
alternate sheets of tinfoil and paraffined paper, arranged in 
a pile as shown in the illustration. The following table 
gives the proper sizes for condensers for three different 
coils. 

+--------------------+-----------------------------------+
|                    |            TINFOIL                |
|                    +----------------+------------------+
| SIZE OF SPARK-COIL |  NO. SHEETS    |  SIZE OF SHEETS  |
+--------------------+----------------+------------------+
|  1/2 inch          |       50       |      2 x 2       |
+--------------------+----------------+------------------+
|  1 inch            |       100      |      7 x 5       |
+--------------------+----------------+------------------+
|  1 1/2 inch        |       100      |      8 x 6       |
+--------------------+----------------+------------------+

The paper must be about one-half inch larger all the 
way around, so as to leave a good margin. The alternate 
sheets of tinfoil, that is, all on one side and all on the other, 
are connected. 

The condenser is 
connected directly across the 
terminals of the 
interrupter. 

.. figure:: images/Image184.jpg
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   :alt: Fig. 165.—Condenser.
   
   Fig. 165.—Condenser.

There are various methods of 
mounting a coil, the most common being to place it in a 
box with the interrupter at one end. Perhaps, however, 
one of the neatest and also the simplest methods is to mount 
it in the manner shown in the illustration. 

The end-pieces are cut out of wood. No specific 
dimensions can be given, because the diameter of the coils will 
vary somewhat according to who winds them and how 
tightly they are made. The coil is enclosed in a tube made 
by rolling up a strip of cardboard and then giving it a coat 
of shellac. The tube may be covered by a strip of black 
cloth, so as to improve its appearance. 

.. figure:: images/Image185.jpg
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   :alt: Fig. 166.—Completed Coil.
   
   Fig. 166.—Completed Coil.

The vibrator is mounted on the end. The core projects 
through a hole in the wood near the end of the vibrator 
spring so that the latter will be drawn in by the magnetism 
of the core when the current flows. The condenser may 
be placed in the hollow box which forms the base of the 
coil. 

The secondary terminals of the coil are mounted on a 
small strip of wood bridging the two coil ends. 

One terminal of the primary is connected to a 
binding-post mounted on the base, and the other led to the vibrator 
spring. The vibrator yoke is connected to a second 
binding-post on the base. One terminal of the condenser is 
connected to the spring, and the other to the yoke. 

Four cells of dry battery should be sufficient to run the 
coil and cause it to give a good one-half-inch spark if built 
according to the directions here given. The vibrator or 
interrupter will require adjusting and a position of the 
adjusting screw will soon be found where the coil works 
best. 

Experiments with a Spark-Coil
############################# 

**Electrical Hands.** Many extraordinary and interesting 
experiments may be performed with the aid of a spark-coil. 

The following experiment never fails to amuse a party 
of friends, and is mystifying and weird to the ordinary 
person, unacquainted with the secret of its operation. 

Figure 167 shows the arrangement of the apparatus. 
The primary of an ordinary one-inch spark induction coil 
is connected in series with a twelve-volt battery and 
telephone transmitter. A small switch is included in the 
circuit to break the current and prevent needless waste of the 
battery when the apparatus is not in immediate use. The 
secondary terminals of the induction coil are led by means 
of an insulated wire to the adjoining room where they 
terminate in a pair of scissors, or some other small metallic 
object which may be clasped in the hand. 

Each of two persons, wearing dry shoes or rubber-soled 
slippers, grasps the terminal of one wire in one hand. The 
other hand is placed flat against the ear of a third person, 
with a piece of dry linen paper intervening between the 
hands and the head. If a fourth person, in the room where 
the induction coil is located, then closes the small switch 
and speaks into the telephone transmitter, the person 
against whose ears the hands are being held will hear the 
speech very distinctly. The ticking of a watch held against 
the mouthpiece of the transmitter will be heard with 
startling clearness. 

.. figure:: images/Image186.jpg
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   :alt: Fig. 167.—Diagram showing how to connect the Apparatus for the "Electric Hands" Experiment.
   
   Fig. 167.—Diagram showing how to connect the Apparatus for the "Electric Hands" Experiment.

The principle governing the operation of the apparatus 
is very simple. Almost every experimenter is familiar with 
the ordinary electrical condenser, which consists of alternate 
sheets of paraffined paper and tinfoil. When this is 
connected to a source of electricity of high potential, but not 
enough so as to puncture the paper dielectric, the alternate 
sheets of tinfoil will become oppositely charged and attract 
each other. If the circuit is then broken the sheets will 
lose their charge and also their attraction for one another. 
If the tinfoil sheets and paper are not pressed tightly 
together, there will be a slight movement of the tinfoil and 
paper which will correspond in frequency to any 
fluctuations of the charging current which may take place. 

The head of the third person and the hands held against 
his head act like three tinfoil sheets of a condenser, 
separated by two sheets of paper. The words spoken in the 
transmitter cause the current to fluctuate and the induction 
coil raises the potential of the current sufficiently to charge 
the condenser and cause a slight vibration of the paper 
dielectric. The vibrations correspond in strength and 
speed to those of the voice, and so the words spoken in the 
transmitter are audible to the person over whose ears the 
paper is pressed. 

Everything about the apparatus must be as dry as 
possible, to insure its successful operation. The people holding 
the wires in their hands should stand on a carpeted floor. 
Always be very careful to tighten the adjusting screw and 
block the interrupter on the coil, so that by no means can 
it possibly commence to operate, or the person listening, 
instead of "hearing things" will become the victim of a 
rather painful, practical joke. 

Geissler Tubes
##############

The most beautiful and surprising effects may be 
obtained by lighting Geissler tubes with a coil. The tubes 
are made in intricate and varied patterns of special glass, 
containing fluorescent minerals and salts, and are filled with 
different rarefied gases. When the tubes are connected 
to the secondary of a spark-coil by means of a wire fastened 
to the little rings at the end, and the coil is set in operation, 
they light up in the most wonderful way imaginable. The 
rarefied gases and minerals in the glass throw out beautiful 
iridescent colors, lighting up a dark room with a weird 
flickering light. Every tube is usually of a different pattern 
and has a combination of different colors. The most 
beautiful tubes are those provided with a double wall 
containing a fluorescent liquid, which heightens the color 
effects when the tube is lighted. 

.. figure:: images/Image187.jpg
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   :alt: Fig. 168.—Geissler Tubes.
   
   Fig. 168.—Geissler Tubes.

Eight to ten tubes may be lighted at once on an ordinary 
coil by connecting them in series. 

Ghost Light
########### 

If you grasp the bulb of an old incandescent electric lamp 
in one hand and touch the base to one side of the secondary 
when the coil is in operation the bulb will emit a peculiar 
greenish light in the dark. 

Puncturing Paper
################ 

If you place a piece of heavy paper or cardboard between 
two sharp wires connected to the secondary of a spark-coil 
and start the coil working, the paper will be pierced. 

A Practical Joke
################ 

This action of the coil may be made the basis of an 
amusing joke. Offer a friend who may smoke cigarettes 
some cigarette paper which has been prepared in the following way. 

.. figure:: images/Image188.jpg
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   :alt: Fig. 169.—The Bulb will emit a Peculiar Greenish Light. 
   
   Fig. 169.—The Bulb will emit a Peculiar Greenish Light. 

Place several sheets of the paper on a piece of 
sheet-metal which is connected to one side of the secondary. By 
means of an insulated handle so that you will not get a 
shock, move the other wire all over the surface of the 
cigarette paper. The paper will be pierced with numerous 
fine holes which are so fine that they can hardly be seen. 

If your friend uses any of the paper in making a cigarette 
and tries to light it he will waste a box of matches without 
being able to get one good puff, because the little invisible 
holes in the paper will spoil the draft. Perhaps he may 
quit smoking altogether. 

An Electric Garbage-can
####################### 

.. figure:: images/Image189.jpg
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   :alt: Fig. 170.—An Electrified Garbage-can. 
   
   Fig. 170.—An Electrified Garbage-can. 

If there are any dogs in your neighborhood that have a 
habit of extracting things from your ash-barrel or 
garbage-can, place the latter on a piece of dry wood. Lead a well 
insulated wire from one secondary terminal of your coil 
to the can. Ground the other secondary terminal. If you 
see a dog with his nose in the can press your key and start 
the coil working. It will not hurt the dog, but he will get 
the surprise of his life. He will go for home as fast as he 
can travel and will not touch that particular can again, 
even if it should contain some of the choicest canine delicacies. 

Photographing an Electric Discharge 
###################################

The following experiment must be conducted in a dark 
room with the aid of a ruby photographic lamp, as 
otherwise the plates used would become lightstruck and 
spoiled. 

.. figure:: images/Image190.jpg
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   :alt: Fig. 171.—Jacob's Ladder.
   
   Fig. 171.—Jacob's Ladder.

Placed an ordinary photographic plate on a piece of 
sheet-metal with the coated side of the plate upwards. Connect 
one of the secondary terminals of the spark-coil to the piece 
of sheet-metal. 

Then sift a thin film of dry starch powder, sulphur, or 
talcum through a piece of fine gauze on the plate. Lead a 
sharp-pointed wire from the other secondary terminal of 
the coil to the center of the plate and then push the key 
just long enough to make one spark. 

Wipe the powder off the plate and develop it in the usual 
manner of films and plates. If you cannot do developing 
yourself, place the plate back in its box and send it to some 
friend, or to a photographer. 

The result will be a negative showing a peculiar electric 
discharge, somewhat like sea-moss in appearance. No two 
such photographs will be alike and the greatest variety 
of new designs, etc., imaginable may be produced in this 
manner. 

Jacob’s Ladder
##############

Take two pieces of bare copper wire about eight inches 
long and bend them at right angles. Place them in the 
secondary terminals of a spark-coil as in Figure 171. Bend 
them so that the vertical portions are about one-half of an 
inch apart at the bottom and one inch apart at the top. 
Start the coil working, and the sparks will run up the wires 
from the bottom to the top and appear very much like the 
rungs in a ladder. 

X-Rays
###### 

Most young experimenters are unaware what a 
wonderful and interesting field is open to the possessor of a small 
X-ray tube. 

Small X-ray tubes which will operate satisfactorily on 
an inch and one-half spark-coil may be obtained from 
several electrical supply houses. They usually cost about 
four dollars and a half. With such a tube and a *fluoroscope* 
it is possible to see the bones in the human hand, the 
contents of a closed purse, etc. 

The tube is made of glass and contains a very high 
vacuum. The long end of the tube contains a platinum 
electrode called the *cathode*. The short end contains two 
electrodes called *anodes*, one 
perpendicular to the tube and the 
other diagonal. 

The tube is usually clamped in 
a wooden holder called an X-ray 
tube stand. The tube should be 
so adjusted that the X-rays which 
are reflected from the diagonal 
anode will pass off in the direction 
shown by the dotted lines in 
Figure 174. 

The fluoroscope is a cone-shaped 
wooden box fitted with a screen 
composed of a sheet of paper 
covered with crystals of a chemical 
called platinum-barium-cyanide. 

.. figure:: images/Image191.jpg
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   :alt: Fig. 172.—An X-Ray Tube. 
   
   Fig. 172.—An X-Ray Tube. 

The opposite end of the box is 
fitted with a covering of felt or 
velvet which shuts off the light 
around the eyes and nose when you look into the 
fluoroscope and hold it tightly against the face. 

A fluoroscope may be purchased complete, or the 
platinum-barium-cyanide screen purchased separately and mounted 
on a box as shown in Figure 173. 

The two anodes of the tube should be connected, and 
led to one terminal of a spark-coil capable of giving a spark 
at least one and one-half inches long. Another wire should 
be led from the cathode of the tube to the other terminal 
of the coil. 

.. figure:: images/Image192.jpg
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   :alt: Fig. 173.—Fluoroscope.  
   
   Fig. 173.—Fluoroscope. 

When it is desired to inspect any object, such as the hand, 
it must be held close to the screen of the fluoroscope and 
placed between the latter and the tube in the path of the 
X-rays. The X-rays are thrown forth from the tube at an 
angle of 45 degrees from the diagonal anode. 

Look into the fluoroscope and it should appear to be 
filled with a green light. If not, the battery terminals 
connected to the primary of the coil should be reversed, so as 
to send the current through in the opposite direction. 

The X-rays will cause the chemicals on the screen to light 
up and give forth a peculiar green light. If the hand is 
held against the screen, between the screen and the tube, 
the X-rays will pass through the hand and cast a shadow 
on the screen. They do not pass through the bones as 
easily as they do through the flesh and so will cast a shadow 
of the bones in the hand on the screen, and if you look 
closely you will be able to see the various joints, etc. 
The interrupter on the coil should be carefully adjusted 
so that the light does not flicker too much. 

.. figure:: images/Image193.jpg
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   :alt: Fig. 174.—How to connect an X-Ray Tube to a Spark-Coil. 
   
   Fig. 174.—How to connect an X-Ray Tube to a Spark-Coil. 

If it is desired to take X-ray pictures, a fluoroscope is 
unnecessary. 

Turn the tube around so that the X-rays point downward. 

Shut the battery current off so that the tube is not in 
operation until everything else is ready. 

Place an ordinary photographic plate, contained in an 
ordinary plate-holder, directly under the tube with the 
gelatin side of the plate upwards. 

Place the hand flat on the plate and lower the tube until 
it is only about three inches above the hand. Then start 
the coil working so that the tube lights up and permit it to 
run for about fifteen minutes without removing the hand. 
Then turn the current off and develop the plate in a dark 
room. 

It is possible to obtain a very good X-ray photograph of 
the hand in this manner. Photographs showing the skeleton 
of a mouse, nails in a board, coins in a purse, a bullet in a 
piece of wood, etc., are a few of the other objects which 
make interesting pictures. 

.. figure:: images/Image194.jpg
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   :alt: An X-Ray Photograph of the hand taken with the Outfit shown in Figure 174. 
   
   An X-Ray Photograph of the hand taken with the Outfit shown in Figure 174. The arrows point to injuries to the bone of the third finger near the middle Joint Resulting in a Stiff Joint. 

.. figure:: images/Image195.jpg
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   :alt: TRANSFORMERS

CHAPTER XIII TRANSFORMERS
`````````````````````````

In most towns and cities where electricity for light and 
power is carried over long distances, it will be noticed that 
small iron boxes are fastened to the poles at frequent 
intervals, usually wherever there is a group of houses or 
buildings supplied with the current. Many boys know that 
the boxes contain "transformers," but do not quite 
understand exactly what their purpose is, and how they are 
constructed. 

When it is desired to convey electrical energy to a distance, 
for the purpose of producing either light or power, one of 
the chief problems to be faced is, how to reduce to a 
minimum any possible waste or loss of energy during its 
transmission. Furthermore, since wires and cables of large size 
are very costly, it is desirable that they be as small as 
possible and yet still be able to carry the current without 
undue losses. 

It has already been explained that wires offer resistance 
to an electrical current, and that some of the energy is lost 
in passing through a wire because of this resistance. Small 
wires possess more resistance than large ones, and if small 
wires are to be used, in order to save on the cost of the 
transmission line, the loss of energy will be greater, 
necessitating some method of partially reducing or overcoming 
this fault. 

In order to explain clearly how the problem is solved, the 
electric current may for the moment be compared to a 
stream of water flowing through a pipe. 

.. figure:: images/Image196.jpg
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   :alt: Fig. 175.—Comparison between Electric Current and Flow of Water. 
   
   Fig. 175.—Comparison between Electric Current and Flow of Water. 
   
The illustration shows two pipes, a small one and a large 
one, each supposed to be connected to the same tank, so 
that the pressure in each is equal, and it is clearly apparent 
that more water will 
flow out of the large 
one than out of the 
small one. If ten 
gallons of water flow out 
of the large pipe in one 
minute, it may be 
possible that the comparative sizes of the pipes 
are such that only one gallon of water will flow out of the 
small one in the same length of time. 

But in case it should be necessary or desirable to get ten 
gallons of water a minute out of a small pipe such as *B*, 
what could be done to accomplish it? 

The pressure could be increased. The water would 
then be able better to overcome the resistance of the small 
pipe. 

This is exactly what is done in the distribution of electric 
currents for power and lighting. The pressure or 
potential is increased to a value where it can overcome the 
resistance of the small wires. 

But unfortunately it rarely happens that electrical power 
can be utilized at high pressure for ordinary purposes. For 
instance, 110 volts is usually the maximum pressure 
required by incandescent lamps, whereas the pressure on the 
line wires issuing from the power-house is generally 2,200 
volts or more. 

Such a high voltage is hard to insulate, and would kill 
most people coming into contact with the lines, and is 
otherwise dangerous. 

Before the current enters a house, therefore, some 
apparatus is necessary, which is capable of reducing this 
high pressure to a value where it may be safely employed. 

This is the duty performed by the "transformer" 
enclosed in the black iron box fastened on the top of the 
electric light poles about the streets. 

If a transformer were to be defined it might be said to be 
a device for changing the voltage and current of an 
*Alternating* circuit in pressure and amount. 

The word, *alternating*, has been placed in italics because 
it is only upon alternating currents that a transformer may 
be successfully employed. Therein, also, lies the reason 
why alternating current is supplied in some cases instead 
of direct current. It makes possible the use of transformers 
for lowering the voltage at the point of service. 

Many boys possessing electrical toys and apparatus 
operating upon direct current only, have bemoaned the 
fact that the lighting system in their town furnished 
alternating current. Very often in the case of small cities or 
towns one power-house furnishes the current for several 
communities and the energy has to be carried a 
considerable distance. Alternating current is then usually 
employed. 

.. figure:: images/Image197.jpg
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   :alt: Fig. 176.—Alternating Current System for Light and Power. 
   
   Fig. 176.—Alternating Current System for Light and Power. 
   
The illustration shows the general method of arranging 
such a system. A large dynamo located at the power-house 
generates alternating current. The alternating current 
passes into a "step-up" transformer which raises the 
potential to 2,200 volts (approximately). It is then 
possible to use much smaller line wires, and to transmit the 
energy with smaller loss than if the current were sent out 
at the ordinary dynamo voltage. The current passes over 
the wires at this high voltage, but wherever connection is 
established with a house or other building, the "service" 
wires which supply the house are not connected directly to 
the line wires, but to a a "step-down" transformer which 
lowers the potential of the current flowing into the house 
to about 110 volts. 

In larger cities where the demand for current in a given 
area is much greater than that in a small town, a somewhat 
different method of distributing the energy is employed. 

.. figure:: images/Image198.jpg
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   :alt: Fig. 177.—Motor Generator Set for changing Alternating Current to Direct Current.  
   
   Fig. 177.—Motor Generator Set for changing Alternating Current to Direct Current. 
   
The alternating current generated by the huge dynamos 
at the "central" station is passed into a set of transformers 
which in some cases raise the potential as high as five or 
six thousand volts.
The current is then 
sent out over cables 
or "feeders" to 
various "sub" stations, 
or "converter" 
stations, located in various parts of the city. 
Here the current is first sent through a set 
of step-down transformers which reduce the potential to the approximate 
value originally generated by the dynamos. It then passes 
into the "rotary converters" which change the alternating 
current into direct current after which it is sent by 
underground cables direct to the consumers in the neighborhood. 

A transformer in its simplest form consists of two 
independent coils of wire wound upon an iron ring. When an 
alternating current is passed through one of the coils, known 
as the primary, it produces a magnetic field which induces 
a current of electricity in the other, or secondary, coil. 

The potential or voltage of the current in the secondary 
is in nearly the same ratio to the potential of the current 
passed into the primary as the number of turns in the 
secondary is to the number of turns in the primary. 

.. figure:: images/Image199.jpg
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   :alt: Fig. 178.—Step-Up Transformer. 
   
   Fig. 178.—Step-Up Transformer. 
   
Knowing this, it is very easy to arrange a transformer to 
"step" the potential up or down as desired. The 
transformer in Figure 178 
represents a "step-up" transformer 
having ten turns of wire on 
the primary and twenty turns 
on the secondary. If an 
alternating current of 10 volts 
and 2 amperes is passed 
into the primary, the 
secondary winding will double the 
potential, since it has twice as 
many turns as the primary 
and the current delivered 
by the secondary will be approximately 20 volts and 1 
ampere. 

The action may be very easily reversed and a 
"step-down" transformer arranged by placing twenty turns of 
wire on the primary and ten turns on the secondary. If a 
current of 20 volts and 1 ampere is passed into the primary, 
the secondary will deliver a current of only 10 volts and 2 
amperes, since it contains only half as many turns. 

A circular ring of iron wire wound with two coils would 
in many respects be somewhat difficult to construct, and 
so the iron core is usually built in the form of a hollow 
rectangle and formed of sheets of iron. 

.. figure:: images/Image200.jpg
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   :alt: Fig. 179.—Step-Down Transformer. 
   
   Fig. 179.—Step-Down Transformer. 
   
It is often desirable to have at hand an alternating 
current of low voltage for experimental purposes. Such a 
current may be used for operating induction coils, motors, 
lamps, toy railways, etc., and is quite as satisfactory as 
direct current for many purposes, with the possible 
exception of electro-plating and 
storage-battery charging, for 
which it cannot be used. 

When the supply is drawn 
from the 110-volt lighting circuit and passed through a 
small "step-down" transformer, the alternating 
current is not only cheaper but
more convenient. A transformer of about 100 watts capacity, capable of delivering 
a current of 10 volts and 10 amperes from the secondary 
will not draw more than approximately one ampere from 
the 110-volt circuit. This current is only equal to that 
consumed by two ordinary 16-candle-power lamps or one 
of 32 candle-power, making it possible to operate the 
transformer to its full capacity for about one cent an 
hour. A further advantage is the fact that a "step-down" 
transformer enables the small boy to use the lighting 
current for operating electrical toys without danger of receiving 
a shock. 

.. figure:: images/Image201.jpg
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   :alt: Fig. 180.—Core Dimensions. 
   
   Fig. 180.—Core Dimensions. 
   
The transformer described in the following pages can 
be easily built by any boy at all familiar with tools, and 
should make a valuable addition to his electrical 
equipment, provided 
that the directions are carefully 
followed and 
pains are taken to 
make the insulation perfect. 

The capacity 
of the transformer 
is approximately 
100 watts. The 
dimensions and 
details of construction described and 
illustrated are those of a transformer intended for use upon a 
lighting current of 110 volts and 60-cycles frequency. 
The frequency of most alternating current systems is 25, 
60, or 120 cycles. The most common frequency is 60. 
Dimensions and particulars of transformers for 25 and 
120 cycles will be found in the form of a table farther 
on. 

The frequency of your light circuit may be ascertained 
by inquiring of the company supplying the power. 

.. figure:: images/Image202.jpg
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   :alt: Fig. 181.—The Core, Assembled and Taped. 
   
   Fig. 181.—The Core, Assembled and Taped. 
   
The first part to be considered in the construction of a 
transformer is the core. The core is made up of thin 
sheet-iron strips of the dimensions shown in Figure 180. The iron 
may be secured from almost any hardware store or 
plumbing shop by ordering "stove-pipe iron." Have the iron cut 
into strips 1 1/4 inches wide and 24 inches long. Then, 
using a pair of tinner’s shears, cut the long strips into pieces 
3 inches and 4 3/4 inches long until you have enough to make 
a pile of each 2 1/2 inches high when they are stacked up 
neatly and compressed. The long strips are used to form 
the "legs" of the core, and the short ones the "yokes." 

.. figure:: images/Image203.jpg
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   :alt: Fig. 182.—Transformer Leg. 
   
   Fig. 182.—Transformer Leg. 
   
The strips are assembled according to the diagram shown 
in Figure 180. The alternate ends overlap and form a 
hollow rectangle 4 1/4 x 6 inches. The core should be pressed 
tightly together and the 
legs bound with three or 
four layers of insulating 
tape preparatory to 
winding on the primary. 
After the legs are bound, the yoke pieces may be pulled 
out, leaving the legs intact. 

Four fiber heads, 2 1/2 inches square and 1/8 of an inch 
thick, are made as shown in Figure 183. A square hole 
1 1/4 x 1 1/4 inches is cut in the center. Two of these are 
placed on each of the assembled legs as shown in Figure 184. 

.. figure:: images/Image204.jpg
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   :alt: Fig. 183.—Fiber Head.
   
   Fig. 183.—Fiber Head.
   
The primary winding consists of one thousand turns of 
No. 20 B. & S. gauge single-cotton-covered magnet wire. 
Five hundred turns are wound on each leg of the transformer. 
The wire should be wound on very 
smoothly and evenly with a layer 
of shellacked paper between each 
layer of wire. 

The two legs should be connected in series. The terminals 
are protected and insulated by 
covering with some insulating tape 
rolled up in the form of a tube. 

The secondary winding consists 
of one hundred turns of No. 10 B. & S. gauge 
double-cotton-covered wire. Fifty turns are wound on each leg, 
over the primary, several layers of paper being placed 
between the two. 

.. figure:: images/Image205.jpg
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   :alt: Fig. 184.—Leg with Heads in Position for Winding.
   
   Fig. 184.—Leg with Heads in Position for Winding.
   
A "tap" is brought out at every ten turns. The taps 
are made by soldering a narrow strip of sheet-copper to the 
wire at proper intervals. Care must be 
taken to insulate each 
joint and tap with a 
small strip of insulating 
tape so that there is no 
danger of a short circuit 
being formed between 
adjacent turns. 

After the winding is completed the transformer is ready 
for assembling. The yoke pieces of the core should be 
slipped into position and the whole carefully lined up. The 
transformer itself 
is now ready for 
mounting. 

.. figure:: images/Image206.jpg
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   :alt: Fig. 185.—How to make a Tap in the Primary by soldering a Copper Strip to the Wire.
   
   Fig. 185.—How to make a Tap in the Primary by soldering a Copper Strip to the Wire.
   
The base-board 
measures 11 x 7 3/4 
x 7/8 inches. It is shown in Figure 192.

The transformer rests upon two wooden strips, *A* and *B*, 
4 1/4 inches long, 1 1/4 inches wide, and 3/4 of an inch high. 
The strips are nailed to the base so that they will come 
under the ends of the core outside of the fiber heads. 

The transformer is held to the base by two tie-rods 
passing through a strip, *C*, 6 inches long, one-half of an inch thick 
and three-quarters of an inch wide. The strip rests on 
the ends of the core. The tie-rods are fastened on the 
under side of the base by means of a nut and washer on the 
ends. When the nuts are screwed up tightly, the 
cross-piece will pull the transformer firmly down to the 
base. 

.. figure:: images/Image207.jpg
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   :alt: Fig. 186.—The Transformer completely Wound and ready for Assembling.
   
   Fig. 186.—The Transformer completely Wound and ready for Assembling.
   
The regulating switches, 
two in number, are mounted 
on the lower part of the 
base. The contact points 
and the arm are cut out of 
sheet-brass, one-eighth of 
an inch thick. It is 
unnecessary to go into the 
details of their construction, 
because the dimensions are plainly shown in Figure 188.

The contacts are drilled out and countersunk so that 
they may be fastened to the base with small flat-headed 
wood screws. 

Each switch-arm is fitted with a small rubber knob to 
serve as a handle. The arm works on a small piece of 
brass of exactly the same thickness as the switch-points. 
Care must be taken that the points and this washer are all 
exactly in line, so that the arm will make good contact with 
each point. There are five points to each switch, as shown 
in Figure 190. 

.. figure:: images/Image208.jpg
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   :alt: Fig. 187.—Wooden Strips for mounting the Transformer on the Base.
   
   Fig. 187.—Wooden Strips for mounting the Transformer on the Base.
   
The switch, *D*, is arranged so that each step cuts in or 
out twenty turns of the secondary, the first point being 
connected with the end 
of the winding. The 
second point connects 
with the first tap, the 
third contact with the 
second tap, the fourth 
contact with the third 
tap, and the fifth contact with the fourth tap. 

.. figure:: images/Image209.jpg
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   :alt: Fig. 188.—Details of the Switch Parts. 
   
   Fig. 188.—Details of the Switch Parts. 
   
The switch, *E*, is arranged so that each step cuts in or 
out five turns. The contacts on this switch are numbered 
in the reverse direction. The 
fifth contact of switch *D*, and 
the fifth contact of switch *E*, 
are connected together. The 
fourth contact is connected to 
the fifth tap, the third contact 
to the sixth tap, the second 
contact to the seventh, and the first contact to the end of the winding.

This arrangement makes it possible to secure any 
voltage from one-half to ten in one-half-volt steps from the 
secondary of the machine. Each step on the switch, *D*, 
will give two volts, while those on *E* will each give one-half 
of a volt. 

.. figure:: images/Image210.jpg
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   :alt: Fig. 189.—The Complete Switch.
   
   Fig. 189.—The Complete Switch.
   
Two binding-posts 
(marked *P* and *P* in 
the drawing) mounted 
in the upper corners 
of the base are connected to the terminals of the primary 
winding. The two posts in the lower 
corners (marked *S* and *S* in the drawing) are connected to the 
switch levers, and are the posts from which the secondary or 
low voltage is obtained. 

.. figure:: images/Image211.jpg
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   :alt: Fig. 190.—Diagram of Connections. 
   
   Fig. 190.—Diagram of Connections. 
   
The transformer may be 
connected to the 110-V. 
alternating current circuit 
by means of an attachment 
plug and cord. One end 
of the cord is placed in 
each of the primary 
binding-posts. The other end 
of the cord is connected to 
the attachment plug so that 
the latter may be screwed 
into any convenient electric-light socket. 

.. figure:: images/Image212.jpg
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   :alt: Fig. 191.—Top View of the Transformer. 
   
   Fig. 191.—Top View of the Transformer. 
   
The transformer must not be connected directly to the 
line. An instrument such as this is not designed for 
continuous service and is intended to be disconnected as soon 
as you are through using it. 

.. figure:: images/Image213.jpg
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   :alt: Fig. 192.—Side View of the Transformer. 
   
   Fig. 192.—Side View of the Transformer. 
   
It will be found a great convenience in operating many 
of the electrical devices described, wherever direct current 
is not essential. 

.. figure:: images/Image214.jpg
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   :alt: WIRELESS TELEGRAPHY

CHAPTER VIV WIRELESS TELEGRAPHY
```````````````````````````````

Probably no branch of electrical science ever appealed 
more to the imagination of the experimenter than that 
coming under the heading of wireless telegraphy. Wherever you go,
you are likely to see the ear-marks of *amateur* wireless telegraph stations in the 
aerials and masts set up in trees and on house-tops. It is 
estimated that there are nearly a quarter of a million such 
stations in the United States. 

There is really no great mystery about this wonderful art 
which made possible the instantaneous transmission of 
messages over immense distances without any apparent physical 
connection save that of the earth, air, or water. 

Did you ever throw a stone in a pool of water? As soon 
as the stone struck, little waves spread out from the spot 
in gradually enlarging circles until they reached the shore or 
died away. 

By throwing several stones in succession, with varying 
intervals of time between them, it would be possible so to 
arrange a set of signals, that they would convey a meaning 
to a second person standing on the opposite shore of the pool. 

Wireless telegraphy is based upon the principle of *creating 
and detecting* waves in a great *pool* of ether. 

Modern scientists suppose that all space is filled with an 
"imaginary" substance called *ether*. The ether is invisible, 
odorless, and practically weightless. This ether, however,
bears no relation to the anaesthetic of that name which is 
used in surgical operations. 

It surrounds and penetrates all substances and all space. 

.. figure:: images/Image215.jpg
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   :alt: Fig. 193.—Little Waves spread out from the Spot. 
   
   Fig. 193.—Little Waves spread out from the Spot. 
   
It exists in a vacuum and in solid rocks. Since the ether 
does not make itself apparent to any of our physical senses, 
some of these statements may seem contradictory. Its 
definite existence cannot be proved except by reasoning, but by 
accepting and imagining its reality, it is possible to 
understand and explain many scientific puzzles. 

A good instance is offered by the sun. Light and heat 
can be shown to consist of extremely rapid vibrations. That 
fact can be proved. The sun is over 90,000,000 miles away 
from our earth and yet light and heat come streaming down 
to us through a space that is devoid even of air. Something 
must exist as a medium to transmit these vibrations; it is 
the ether. 

Let us consider again the pool of water. The waves or 
ripples caused by throwing in the stone are vibrations of 
the water. The distance between two adjacent ripples is 
called the *wave length*. 

The distances between two vibrations of light can also be 
*measured*. They are so small, however, that they may be 
spoken of only in *thousandths* of an inch. The waves created 
in the ether by wireless telegraph apparatus are the same 
as those of light except that their length usually varies 
from 75 to 9,000 *feet* instead of a fraction of a thousandth 
of an inch. 

.. figure:: images/Image218.jpg
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   :alt: Fig. 194.—A Simple Transmitter. 
   
   Fig. 194.—A Simple Transmitter. 
   
**A Simple Transmitter** is illustrated in Figure 194. A 
telegraph key is connected in series with a set of cells 
and the *primary* of an induction coil, which, it will be 
remembered, is simply a coil consisting of a few turns of wire. 
This induces a high voltage in a second coil consisting of a 
larger number of turns and called the *secondary*. 

The terminals of the secondary are led to a 
spark-gap—an arrangement composed of two polished brass balls, separated 
by a small air-gap. One of the balls, in turn, is connected 
to a metal plate buried in the earth, and the other to a 
network of wires suspended high in the air and insulated from 
all surrounding objects. 

When the key at the transmitter is pressed, the battery 
current flows through the primary of the induction coil and 
generates in the secondary a current of very high voltage, 
20,000 volts or more, which is able to jump an air-gap in 
the shape of a spark at the secondary terminals. The latter 
are connected to the earth and aerial, as explained above. 
The high potential currents are therefore enabled to charge 
the aerial. The charge in the aerial exerts a great tendency 
to pass into the ground, but is prevented from doing so by 
the small air-gap between the spark-balls until the charge 
becomes so great that the air-gap is punctured and the charge 
passes across and flows down into the ground. The passage 
of the charge is made evident by the spark between the two 
spark-balls. 

The electrical charges flowing up and down the aerial 
disturb the ether, strike it a blow, as it were. The effect of 
the blow is to cause the ether to vibrate and to send out 
waves in all directions. It may be likened to the pond of 
water which is suddenly struck a blow by throwing a stone 
into it, so that ripples are immediately sent out in widening 
circles. 

**These Waves in the Ether** are called electro-magnetic or 
*Hertzian* waves, after their discoverer, Hertz. The distance 
over which they pass is dependent upon the power of the 
transmitting station. The waves can be made to 
correspond to the dots and dashes of the telegraphic code by so 
pressing the key. If some means of detecting the waves is 
employed we may readily see how it is possible to send 
wireless messages. 

**The Action of the Receiving Station** is just the opposite 
of that of the transmitter. When the waves pass out 
through the ether, some of them strike the aerial of the 
receiving station and generate a charge of electricity in it 
which tends to pass down into the earth. If the transmitting 
and receiving stations are very close together and the former 
is very powerful, it is possible to make a very small gap in 
the receiving aerial across which the charge will jump in the 
shape of sparks. Thus the action of the receptor simply 
takes place in a reversed order from that of the transmitter. 

If the stations are any considerable distance apart, it is 
impossible for the currents induced in the receiving aerial to 
produce sparks, and so some more sensitive means of 
detecting the waves from the transmitter is necessary, 
preferably one which makes itself evident to the sense of hearing. 

The telephone receiver is an extremely sensitive 
instrument, and it only requires a very weak current to operate 
it and produce a sound. The currents or *oscillations* 
generated in the aerial, however, are alternating currents (see 
pages 97-99) of *high frequency*, that is, they flow in one 
direction and then reverse and flow in the other several thousand 
times a second. Such a current cannot be made to pass 
through a telephone receiver, and in order to do so the 
nature of the current must be changed by converting it into 
direct current flowing in one direction only. 

Certain minerals and crystals possess the remarkable ability 
to do this, *silicon*, galena, and iron pyrites are among
the best.

.. figure:: images/Image219.jpg
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   :alt: Fig. 195.—A Simple Receptor.
   
   Fig. 195.—A Simple Receptor.
   
The diagram in Figure 195 shows the arrangement of a 
simple receiving outfit. The *detector* consists of a sensitive 
mineral placed between two contacts and connected so that 
the aerial currents must pass through it on their way to the 
ground. A telephone receiver is connected to the detector 
so that the *rectified currents* (currents which have been 
changed into direct current) pass into it and produce a 
sound. By varying the periods during which the key is 
pressed at the transmitting station, according to a 
prearranged code, the sounds in the receiver may be made to 
assume an intelligible meaning. 

HOW TO BUILD WIRELESS INSTRUMENTS
################################# 

**The Aerial**

Every wireless station is provided with a system of wires elevated high in the air, above all surrounding objects, the purpose of which is to radiate or intercept the electromagnetic waves, accordingly as the station is transmitting or receiving. This system of wires is, as already has been stated, called the *aerial* or *antenna*.

The arrangement of the aerial will greatly determine the efficiency and range of the apparatus.

The aerial should be as long as it is reasonably possible to make it, that is from 50 to 150 feet.

It will be necessary for most amateurs to put up their aerial in some one certain place, regardless of what else may be in the vicinity, but whenever possible the site selected should preferably be such that the aerial will not be in the immediate neighborhood of any tall objects, such as trees, smoke-stacks, telephone wires, etc., because such objects will interfere with the aerial and noticeably decrease the range of the station, both when transmitting and receiving.

Bare copper wire makes the best aerials. Aluminum wire is very often used and on account of its light weight causes very little strain on the poles or cross arms. Iron wire should never be used for an aerial, even if galvanized or tinned, because it tends to choke the currents which must flow up and down the aerial when the station is in operation.

.. figure:: images/Image220.jpg
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   :alt: Fig. 196.—Molded Aerial Insulator
   
   Fig. 196.—Molded Aerial Insulator
   
The aerial must be very carefully insulated from its supports and all surrounding objects. The insulation must be strong enough to hold the weight of the aerial and able to withstand any strain caused by storms.

Special aerial insulators made of molded insulating material and having an iron ring imbedded in each end are the best.

.. figure:: images/Image221.jpg
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   :alt: Fig. 197.—A Porcelain Cleat will make a Good Insulator for Small Aerials. 
   
   Fig. 197.—A Porcelain Cleat will make a Good Insulator for Small Aerials. 
   
Ordinary porcelain cleats may be used on small aerials where the strain is light.

One insulator should be placed at each end of each wire close to the spreader or spar.

Most aerials are made up of four wires. The wires should be placed as far apart as possible.

There are several different forms of aerials, the principal ones of which are shown in Figure 199. They are known as the grid, “V," inverted “L,” and “T” types.

Most amateurs support their aerials from a pole placed on the top of the house, in a tree, or erected in the yard. Many use two supports, since such an aerial has many advantages. The facilities to be had for supporting the aerial will largely determine which form to use.

.. figure:: images/Image221a.jpg
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   :alt: Fig. 198.—Method of Arranging the Wires and Insulating them from the Cross Arm or Spreader.
   
   Fig. 198.—Method of Arranging the Wires and Insulating them from the Cross Arm or Spreader.
   
The grid aerial has no particular advantages or disadvantages.

The “V” aerial receives waves much better when they come from a direction opposite to that in which the free end points. The "free" end of the aerial is the one not leading into the station.

The inverted “L” aerial possesses the same characteristics as the “V” type.

The “T” aerial is the best “all around" and is to be recommended whenever it is possible to put up an aerial of this sort.

Much of the detail of actually putting up an aerial or antenna must be omitted, because each experimenter will usually meet different conditions.

It should be remembered, however, that the success of the whole undertaking will rest largely upon the construction of a proper aerial. The most excellent instruments will not give very good results if connected to a poor aerial, while, on the other hand, inferior instruments will often give fair results when connected to a good aerial.

.. figure:: images/Image221b.jpg
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   :alt: Fig. 199.—Various Types of Aerials.
   
   Fig. 199.—Various Types of Aerials.
   
The aerial should be at least thirty feet high. 

The wire should not be smaller than No. 14 B. & S. 

The masts which support the aerial should be of wood and provided with pulleys so that the wires may be lowered any time it may be necessary. The mast should be thoroughly braced with stays or guys so as to counteract the strain of the aerial.

The aerial should not be hoisted up perfectly tight, but should be allowed to hang somewhat loose, as it will then put less strain on the ropes and poles that support it.

When an aerial is to be fastened in a tree, it is best to attach it to a pole placed in the top of the tree, so that it will come well above any possible interference from the branches.

The wires leading from the aerial to the instruments should be very carefully insulated throughout their length. This part of the aerial is called the "rat-tail" or lead-in.

The illustrations in Figure 199 show the proper place to attach the “lead-in" form of aerial. The wires should gradually converge.

.. figure:: images/Image222.jpg
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   :alt: Fig. 200.—A Ground Clamp for Pipes.
   
   Fig. 200.—A Ground Clamp for Pipes.
   
It is very important that a good ground connection be secured for wireless instruments. A good ground is absolutely necessary for the proper working of the apparatus. Amateur experimenters usually use the water or gas-pipes for a ground, and fasten the wires by means of a ground clamp such as shown in Figure 200. In the country, where such pipes are not available, it is necessary to bury a sheet of copper, three or four feet square, in a moist spot in the earth and connect a wire to it.

**The Receiving Apparatus**

The receiving instruments form the most interesting part of a wireless station and usually receive first attention from the amateurs. They are the ears of the wireless station and are wondrously sensitive, yet are very simple and easy of construction.

The instruments necessary for receiving are: 

| A Detector, 

| A Tuning Coil or a Loose Coupler, 

| A Fixed Condenser, 

| A Telephone Receiver. 

Other devices, such as a test buzzer, variable condenser, etc., may be added and will improve the outfit.

After the aerial has been properly erected, the first instrument necessary to construct will be either a tuning coil or a loose coupler. It is a good plan to make a tuning coil first, and a loose coupler after you have had a little experience with your apparatus.

**A Tuning Coil** is a very simple arrangement making 
it possible to receive messages from greater distances, and 
also somewhat to eliminate any messages not desirable and 
to listen without confusion to the one wanted. 

A tuning coil consists of a single layer of wire wound upon 
a cylinder and arranged so that connection may be had with 
any part of it by means of sliding contacts. 

The cylinder upon which the wire is wound is a cardboard 
tube six and three-quarters inches long and two and 
seven-eighths inches in diameter outside. It should be given two 
or three coats of shellac both inside and out so that it is 
thoroughly impregnated, and then laid away until dry. 
This treatment will prevent the wire from becoming loose 
after the tube is wound, due to shrinkage of the cardboard. 

.. figure:: images/Image223.jpg
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   :alt: Fig. 201.—Details of the Tuning Coil.
   
   Fig. 201.—Details of the Tuning Coil.
   
After having become dry, the tube is wound with a single 
layer of No. 25 B. & S. gauge green silk or cotton-covered 
magnet wire. The wire must be wound on very smoothly 
and tightly, stopping and starting one-quarter of an inch 
back from each end. The ends of the wire are fastened by 
weaving back and forth through two small holes punched in 
the cardboard tube with a pin. 

The winding should be given a single coat of clear varnish 
or white shellac and allowed to dry. 

The coil heads or end pieces are cut from one-half-inch 
wood according to the plan and dimensions shown in the 
accompanying illustration.

The top corners are beveled and notched to receive the 
slider-rods. A circular piece of wood two and five-eighths 
inches in diameter and three-eighths of an inch thick is 
nailed to the inside of each of the coil heads to support the 
ends of the cylinder. 

The wooden parts should be stained mahogany or some 
other dark color and finished with a coat of shellac or varnish. 

The slider-rods are square brass 3-16 x 3-16 inches and 
seven and three-quarters inches long. A small hole is bored 
near the ends of each, one-quarter of an inch from the edge, 
to receive a round-headed brass wood screw which holds the 
rod to the tuner end. 

The sliders may be made according to the plan shown in 
Figure 201. 

The slider is made from a small piece of brass tubing, 
three-sixteenths of an inch square. An 8-32 flat-headed 
brass screw is soldered to one face, in the center. A small 
strip of phosphor bronze sheet or spring copper soldered to 
the bottom of the slider forms a contact for making 
connection to the wire on the cylinder. A small "electrose" knob 
screwed to the slider makes a neat and efficient handle. 

Two sliders are required, one for each rod. 

The tuning coil is assembled as shown in Figure 203. The 
cardboard tube is held in place by several small brass nails 
driven through it into the circular pieces on the coil heads. 

A slider is placed on each of the slider-rods and the rods 
fastened in the slots in the coil ends by a small 
round-headed brass screw, passing through the holes bored near 
the ends for that purpose. 

.. figure:: images/Image224.jpg
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   :alt: Fig. 202.—Side and End Views of the Tuning Coil. 
   
   Fig. 202.—Side and End Views of the Tuning Coil. 
   
Two binding-posts are mounted on one of the coil ends.
One should be connected to each of the slider-rods. A third 
binding-post is placed below in the center of the head and 
connected to one end of the wire wound around the cylinder. 

A small, narrow path along the coil, directly underneath 
each slider and to which the copper strip can make contact, 
must be formed by scraping the insulation off the wire with 
a sharp knife. The sliders should make contact with each 
one of the wires as they pass over, and should slide smoothly 
without damaging or disarranging any of the wires. 

.. figure:: images/Image225.jpg
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   :alt: Fig. 203.—Complete Double-Slider Tuning Coil. 
   
   Fig. 203.—Complete Double-Slider Tuning Coil. 
   
When scraping the insulation, be very careful not to loosen 
the wires or remove the insulation from between them, so 
that they are liable to short-circuit between adjacent turns. 

**A Loose Coupler** is a much more efficient tuning device 
than a double-slider tuner, and sooner or later most amateur 
wireless operators install one in their station.

The loose coupler shown in the figure given on the next 
page is a very simple one and is both easy and inexpensive 
to build. Its simplicity is a disadvantage in one respect, 
however. Owing to its construction, it is impossible to 
move the slider on the secondary when the latter is inside 
the primary. The reason that I have chosen this sort of
loose coupler to describe is to acquaint my young readers 
with the methods of making a loose coupler.

The "Junior" loose coupler described farther on is a 
more elaborate instrument of greater efficiency, but much 
harder to build.

.. figure:: images/Image226.jpg
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   :alt: Fig. 204.—A Simple Loose Coupler.
   
   Fig. 204.—A Simple Loose Coupler.
   
The base of the loose coupler is of wood and measures 
twelve by four inches. The head supporting the primary 
is of the same size as those used on the "Junior" 
double-slide tuning coil just described. It may be made in the
same manner, and fitted with a circular block to 
support the tube. The primary tube is of the same diameter as 
that on the tuning coil but is only four inches long. It is 
fastened to the primary head with glue and then secured 
with a number of small tacks. One or two coats of shellac 
liberally applied will render it non-shrinkable, so that 
the wire will not be apt to loosen after the loose coupler has 
been in use a while.

The secondary is of the same length as the primary, but
of smaller diameter, so that it will easily slip inside. It 
also is treated with shellac.

The primary should be wound with a single layer of 
No. 22 single-silk-covered magnet wire. The secondary is 
wound with No. 29 single-silk.

The head supporting the secondary is smaller than that 
used for the same purpose on the primary. The round 
boss to which the tube is fastened, however, is much thicker.

The secondary slides on a "guide-rod" supported at 
one end by passing through the primary head and at the 
other by a brass upright. The upright may also be made of wood.

If the secondary is "offset," that is, placed out of center 
slightly to one side, it will leave room so that the secondary 
slider will possibly pass inside of the primary without striking.

Both the primary and the secondary must be fitted with 
"sliders" to make contact with the various turns of wire.

The method of constructing a slider has already been described.

The ends of the slider-rods are bent at right angles and 
fastened to the coil heads by two small screws passing 
through holes bored near the ends. A small narrow path 
must be scraped in the insulation under each so that the 
slider will make contact with each turn. The secondary 
head may be provided with a small electrose handle to 
facilitate sliding it back and forth.

Two binding-posts are mounted on each of the coil heads.

One post on each is connected to the end of the coil farthest 
from the head, and the other posts are each connected to the slider-rods.

Figure 220 shows how to connect the loose coupler in 
the receiving set.

In order to tune with a loose coupler, first adjust the 
slider on the primary until the signals are the clearest. 
Then set the secondary slider in the best place and move 
the secondary in and out of the primary until the signals 
are clearest.

**How to Build the Junior Loose Coupler**

A loose coupler of the sort just described is simple and 
quite easily constructed, but will not be found to work as 
well as one in which the secondary may be varied by means 
of a switch while it is inside of the primary.

The base of the instrument measures twelve by three 
and five-eighths inches. The primary is composed of a 
single layer of No. 24 B. & S. gauge single-silk-covered 
wire wound on a cardboard tube two and three-quarter 
inches in diameter and three and three-quarter inches long. 
The winding is laid on in a single layer and should comprise 
about 150 turns. After winding on tightly, it should be 
given a coat of clean white shellac and allowed to dry. 
The shellac is for the purpose of fastening the wire down tightly 
to the tube so that it will not loosen when the slider is moved 
back and forth.

The primary is mounted between two heads, the details
of which are shown in Figure 205. One of the heads, *B*,
has a flanged hole two and three-quarter inches in diameter 
cut through the center so as to receive the end of the tube 
and permit the secondary to pass inside.

.. figure:: images/Image227.jpg
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   :alt: Fig. 205.—Details of the Wooden Parts.
   
   Fig. 205.—Details of the Wooden Parts.
   
The secondary winding is composed of a single layer of 
No. 28 B. & S. gauge silk-covered wire and divided into 
six equal sections. The secondary is supported by two 
circular wooden pieces, *C* and *F*, and slides back and forth 
on two guide-rods. The guide-rods should be brass. Iron or
steel rods running through the center of a loose coupler 
will seriously weaken the signals, and such practice must 
by all means be avoided.

.. figure:: images/Image228a.jpg
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   :alt: Fig. 206.—Side View of the Loose Coupler.
   
   Fig. 206.—Side View of the Loose Coupler.
   
.. figure:: images/Image228b.jpg
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   :alt: Fig. 207.—Top View of the Loose Coupler.
   
   Fig. 207.—Top View of the Loose Coupler.
   
The secondary sections are connected to six contacts 
and a switch-arm mounted on the end of the secondary so 
that by turning the switch either one, two, three, four, five, 
or six sections of the winding may be connected.

.. figure:: images/Image228c.jpg
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   :alt: Fig. 208.—End Views of the Loose Coupler.
   
   Fig. 208.—End Views of the Loose Coupler.
   
.. figure:: images/Image228d.jpg
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   :alt: Fig. 209.—Complete Loose Coupler.
   
   Fig. 209.—Complete Loose Coupler.
   
The two binding-posts near the secondary end of the 
coupler are connected to the terminals of the secondary 
winding by means of two flexible wires. They have not
been shown in several of the illustrations because they 
would be liable to confuse the drawing.

The primary is provided with a slider moving back and 
forth over a narrow path scraped through the insulation 
so that it may make contact with each wire independently.

**Detectors**

Detectors are very simple devices and consist merely of 
an arrangement for holding a small piece of certain minerals 
and making a contact against the surface. 

The crystal detector shown in Figure 210 is a very 
efficient form that may be easily and quickly made. When 
finished, it will make a valuable addition to almost any 
amateur experimenter's wireless equipment.

.. figure:: images/Image228e.jpg
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   :alt: Fig. 210.—A Crystal Detector.
   
   Fig. 210.—A Crystal Detector.
   
The bracket is bent out of a piece of strip brass about one-eighth of an inch thick and
five-eighths of an inch wide, according to the shape shown in the illustration. The 
bracket is mounted on a circular  wooden base about three inches 
in diameter. The circular wooden blocks used by electricians in 
putting up chandeliers, called “fixture blocks,” will make a 
satisfactory base. An electrose knob of the typewriter type may
be purchased from any good dealer in wireless supplies. It 
should be fitted with a threaded shank which will screw into 
a hole in the upper part of the bracket.

The mineral is contained in a small brass cup mounted on the base below the end of the knob.

Contact with the mineral in the cup is made by means of 
a fine wire spring soldered to the end of the adjusting screw. 

Moving the screw up or down will vary the pressure of
the spring on the mineral and permit the most sensitive 
adjustment to be secured. The bracket is connected to 
one of the binding-posts and the cup to the other.

.. figure:: images/Image228f.jpg
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   :alt: Fig. 211.—Details of the Crystal Detector.
   
   Fig. 211.—Details of the Crystal Detector.
   
The detector shown in Figure 212 is of the type often
termed a "cat-whisker," because of the long, fine wire 
resting on the mineral. 

It consists of a small clip, formed by bending a strip of 
sheet-brass, which grips a piece of galena. 

.. figure:: images/Image228g.jpg
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   :alt: A Double Slider Tuning Coil.
   
   A Double Slider Tuning Coil.
   
.. figure:: images/Image228h.jpg
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   :alt: A Junior Loose Coupler.
   
   A Junior Loose Coupler.
   
.. figure:: images/Image228i.jpg
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   :alt: Crystal Detectors.
   
.. figure:: images/Image228j.jpg
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   :alt: Crystal Detectors.
   
   Crystal Detectors.
   
Galena may be obtained from any dealer in radio supplies. 
A piece of No. 30 phosphor bronze wire is soldered to the 
end of a short length of brass 
rod supported by a binding 
post. The other end of the 
rod is fitted with an electrose 
knob. This part of the 
detector is called the "feeler." 

.. figure:: images/Image229.jpg
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   :alt: Fig. 212 Details of the "Cat Whisker" Detector.
   
   Fig. 212 Details of the "Cat Whisker" Detector.
   
The detector is fitted with 
binding posts and may be 
mounted upon any suitable small base. The mineral clip is 
connected to one post and the binding-post supporting the 
"feeler" to the other. The tension or pressure of the end 
of the fine wire upon the mineral may be regulated by 
twisting the electrose knob so as to twist the rod. The different 
portions of the crystal may be "searched" for the most 
sensitive spot by sliding the rod back and forth. 

A somewhat similar form of cat-whisker detector is shown 
in Figure 213. It is provided with a cup to hold the mineral 
in place of a clip. 

The detector shown in Figure 214 is more elaborate than 
any of the others described so far. 

.. figure:: images/Image230.jpg
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   :alt: Fig. 213.—Another Form of the "Cat-Whisker" Detector. 
   
   Fig. 213.—Another Form of the "Cat-Whisker" Detector. 

.. figure:: images/Image231.jpg
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   :alt: Fig. 214.—"Cat-Whisker" Detector. 
   
   Fig. 214.—"Cat-Whisker" Detector. 

The base is a wooden block, three and one-half by one 
and three-quarters inches by one-half inch. The 
binding-posts are of the type commonly used on electrical 
instruments. One of the posts is pivoted so that it will swing 
from side to side. A short 
piece of brass rod fitted with 
a rubber or fiber knob passes 
through the wire hole in the 
post. A piece of 
No. 30 B. & S. gauge bronze wire is soldered to the end of the rod. 
A small brass cup contains the mineral, which may be either 
*galena*, or *silicon*. By twisting the post and sliding the rod 
back and forth, any portions of the mineral surface may be 
selected. 

**Fixed Condenser.**

The construction of the condenser is 
illustrated in Figure 205. Take twenty-four sheets of thin 
typewriter paper, three by four inches, and twenty-three 
sheets of tinfoil, two by four inches. Pile them up, using 
first a sheet of paper then a sheet of tinfoil, then paper, and 
so on, so that every two sheets of tinfoil are separated by a 
sheet of paper. Each sheet of tinfoil must, however, 
project out beyond the edge of the paper. Connect all the 
tinfoil projections on one end of the condenser together and 
and attach a small wire. Connect all 
those on the opposite side in a similar 
manner. Then fasten a couple of 
rubber bands around the condenser 
to hold it together. 

.. figure:: images/Image232.jpg
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   :alt: Fig. 215.—Building up a Fixed Condenser. 
   
   Fig. 215.—Building up a Fixed Condenser. 

.. figure:: images/Image233.jpg
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   :alt: Fig. 216.—A Fixed Condenser enclosed in a Brass Case made from a Piece of Tubing fitted with Wooden Ends. 
   
   Fig. 216.—A Fixed Condenser enclosed in a Brass Case made from a Piece of Tubing fitted with Wooden Ends. 

If it is desired to give the condenser 
a finished appearance, it may be 
placed in a brass tube fitted with two 
wooden or fiber ends. The ends are 
provided with binding-posts to which 
the terminals of the condenser are 
connected. 

**Telephone Receivers** for use with wireless instruments 
must be purchased. Their construction is such that they 
cannot be made by the experimenter. 

.. figure:: images/Image234.jpg
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   :alt: Fig. 217.—A Telephone Head Set. 
   
   Fig. 217.—A Telephone Head Set. 

A seventy-five ohm, double-pole telephone receiver will do 
for stations not wishing to receive farther than fifty miles. 

In order to secure the best results from wireless 
instruments, it is necessary to have receivers especially made for 
wireless. Each receiver should have 1000 ohms resistance. 
Some boys may find it necessary to purchase one receiver 
at a time. Two receivers, a double headband, and a double 
cord, forming a complete head set as shown in Figure 217, 
should be secured as soon as possible. 

.. figure:: images/Image235.jpg
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   :alt: Fig. 218.—A Circuit showing how to connect a Double-Slider Tuning Coil. 
   
   Fig. 218.—A Circuit showing how to connect a Double-Slider Tuning Coil. 

**Connecting the Receiving Apparatus**

Figure 218 shows how to connect a double-slide tuner, a 
detector, a fixed condenser and a pair of telephones to the 
aerial and ground. The same instruments with a loose coupler in place of the double-slide tuner are shown in Figure 219.

The diagrams in Figure 220 are the same circuits as those shown in Figures 218 and 219, but show different instruments.

.. figure:: images/Image236a.jpg
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   :alt: Fig. 219.—Circuit showing how to connect a Loose Coupler. 
   
   Fig. 219.—Circuit showing how to connect a Loose Coupler. 

.. figure:: images/Image236b.jpg
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   :alt: Fig. 220.—A Diagram showing how to connect some of the Instruments described in this Chapter.
   
   Fig. 220.—A Diagram showing how to connect some of the Instruments described in this Chapter.

After the instruments are connected, place a piece of 
galena or silicon in the cup of the detector and bring the 
wire down on it. Then move the sliders on the tuning coil 
or loose coupler and adjust the detector until you can
hear a message buzzing in the telephones. It may require a little patience and practice, but if you persist you will soon learn how to adjust the apparatus so as to receive the signals loudly and clearly with very little trouble.

**The Transmitting Apparatus**

Spark coils have already been described in Chapter XII. They may be used to transmit wireless messages simply by connecting to a spark-gap and a key.

Spark coils which are especially made for wireless telegraphy will usually send farther than an ordinary spark coil used for experimental purposes.

.. figure:: images/Image236c.jpg
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   :alt: Fig. 221.—A Wireless Spark Coil.
   
   Fig. 221.—A Wireless Spark Coil.

A good one-inch coil costs from $4.50 to $5.00 and will send from three to five miles if used with a fair aerial.

A spark coil requires considerable current for its successful operation and will give the best results if operated on storage cells, dry cells, or bichromate cells. If dry cells are used, it is a good plan to connect them in series multiple as shown in Figure 69.

Spark-gaps may be made by mounting two double binding-posts on a wooden base as shown in Figure 222.

Zinc possesses some peculiar property which makes it very efficient for a spark-gap, and for this reason the electrodes of a spark-gap are usually zinc.

.. figure:: images/Image236d.jpg
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   :alt: Fig. 222.—Small Spark Gaps.
   
   Fig. 222.—Small Spark Gaps.

The figure shows two different forms of electrodes. In one, they are made of zinc rods and provided with “electrose” handles. In the other gap, the zinc electrodes are in the shape of "tips" fitted on the ends of two short brass rods.

A one-inch spark coil will give very good results by connecting the spark-gap directly across the secondary of the coils. The aerial is connected to one side of the gap and the ground to the other.

The transmitter may be "tuned" and the range sometimes increased by using a condenser and a helix.

A condenser is most easily made by coating the inside and outside of a test-tube with tinfoil so as to form a miniature Leyden jar. The end of the tube is closed with a cork through which passes a brass rod connecting to the inner coating of tinfoil.

.. figure:: images/Image236e.jpg
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   :alt: Fig. 223.—Diagram showing how to connect a Simple Transmitter.
   
   Fig. 223.—Diagram showing how to connect a Simple Transmitter.

If such a condenser is connected directly across the spark-gap, the spark will become very white and crackling.

Several tubes may be arranged in a rack as shown in Figure 225.

A helix consists of a spiral of brass ribbon set in a wooden frame. The two strips composing the frame are each nine inches long. The spiral consists of eight turns of brass ribbon, three-eighths of an inch wide, set in saw-cuts made in the frame. A binding-post is connected to the outside end of the ribbon.

Figure 228 shows how to connect a helix and a condenser to a coil and a spark-gap.

The two clips are made by bending a strip of sheet brass and connecting a piece of flexible wire to one end.

.. figure:: images/Image236f.jpg
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   :alt: Fig. 224.—A Test-Tube Leyden Jar.
   
   Fig. 224.—A Test-Tube Leyden Jar.

In large stations, the best position for the clips is found by placing a "hot-wire ammeter" in the aerial circuit and then moving the clips until the meter shows the highest reading.

The young experimenter will have to tune his set by moving the helix clips about until the best results are obtained in sending.

.. figure:: images/Image236g.jpg
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   :alt: Fig. 225.—Eight Test-Tube Leyden Jars mounted in a Wooden Rack.
   
   Fig. 225.—Eight Test-Tube Leyden Jars mounted in a Wooden Rack.

If the spark coil is a good one and capable of giving a good hot spark, it may be possible to tell when the set is in proper tune by placing a small miniature tungsten lamp in series with the aerial and changing the clips, the condenser, and the length of the spark-gap until the lamp lights the brightest. 

An *oscillation transformer* is sometimes used to replace an ordinary helix when it is desirable to tune a station very closely so that its messages shall not be liable to be confused with those of another station when both are working at the same time.

.. figure:: images/Image236h.jpg
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   :alt: Fig. 226.—A Helix and Clip.
   
   Fig. 226.—A Helix and Clip.

An oscillation transformer consists of two helixes arranged so that one acts as a primary and the other as a secondary. An oscillation helix may be made by making two sets of helix frames similar to that in Secondary Figure 226.

.. figure:: images/Image236i.jpg
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   :alt: Fig. 227.—An Oscillation Transformer.
   
   Fig. 227.—An Oscillation Transformer.

The primary should be provided with eight turns of brass ribbon and the secondary with twelve. The primary supports a stiff brass rod upon which the secondary is mounted. The secondary should slide up and down on the rod but move very stiffly so that it will stay where it is put.

.. figure:: images/Image236j.jpg
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   :alt: AN OSCILLATION HELIX.
   
   AN OSCILLATION HELIX.

.. figure:: images/Image236k.jpg
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   :alt: AN OSCILLATION CONDENSER.
   
   AN OSCILLATION CONDENSER.

An ordinary double-throw, double-pole knife switch having a porcelain base will make a very good aerial switch in a small station. It is used to connect the aerial and ground to either the transmitting or receiving apparatus at will. Such a switch is shown in Figure 230.

.. figure:: images/Image236l.jpg
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   :alt: Fig 228.—Circuit showing how to connect a Helix and a Condenser.
   
   Fig 228.—Circuit showing how to connect a Helix and a Condenser.

The aerial should be connected to the post *A* and the ground to *B*. The posts *E* and *F* lead to the transmitter, and *C* and *D* to the receptor, or vice-versa according to which is the more convenient from the location of the apparatus on the table or operating bench.

A suitable table should be arranged to place the wireless instruments upon so that they may be permanently connected together.

.. figure:: images/Image236m.jpg
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   :alt: Fig 229.—Circuit showing how to connect an Oscillation Transformer and a Condenser.
   
   Fig 229.—Circuit showing how to connect an Oscillation Transformer and a Condenser.

The Continental Code is the one usually employed in 
wireless telegraphy. It differs slightly from Morse as it 
contains no space letters. It will be found easy to learn 
and somewhat easier to handle than Morse. 

.. figure:: images/Image236n.jpg
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   :alt: Fig 230.—An Aerial Switch.
   
   Fig 230.—An Aerial Switch.

Two or three months’ steady practice with a chum should 
enable the young experimenter to become a very fair 
wireless telegraph operator. Then by listening for some of the 
high power wireless stations which send out the press news to ships at sea during the evening it 
should be possible to become very proficient. The press news is sent more slowly 
than ordinary commercial wireless messages, and is therefore easy to read and 
a good starting point for the beginner learning to read.

.. figure:: images/Image236o.jpg
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   :alt: Fig 231.—A Complete Wiring Diagram for both the Transmitter and the Receptor.
   
   Fig 231.—A Complete Wiring Diagram for both the Transmitter and the Receptor.

.. figure:: images/Image237.jpg
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   :alt: Fig. 232.—The Continental Alphabet.
   
   Fig. 232.—The Continental Alphabet.

**A Coherer Outfit**

**A Coherer Outfit** is usually capable of only 
receiving messages coming from a distance of under one 
mile. In spite of this fact, however, it is an exceedingly 
interesting apparatus to construct and experiment with, and 
for this reason is found fully described below. 

A coherer set will ring a bell or work a sounder for short 
distances and therefore is the best sort of an arrangement 
for demonstrating the workings of your wireless apparatus 
to your friends. 

The first thing that you need for a coherer is a pair of 
double binding-posts. Mount these about an inch and 
three-quarters apart on a wooden base, six inches long and 
four inches wide as shown in Figure 233. 

.. figure:: images/Image238.jpg
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   :alt: Fig. 233.—A Coherer and a Decoherer.
   
   Fig. 233.—A Coherer and a Decoherer.

Get a piece of glass tubing about an inch and one-half 
long and about one-eighth of an inch inside diameter. You 
will also need some brass rod which will just slide into the 
tube tightly. Cut off two pieces of the brass rod each one 
and three-quarters inches long and slip these through the 
upper holes in the binding-posts and into the glass tube as 
shown in Figure 234. Before putting the second rod in 
place, however, you must put some nickel and silver filings 
in the tube, so that when the rods are pushed almost 
together, with only a distance of about one-sixteenth of an 
inch between them, the filings will about half fill the space. 

The filings must be very carefully prepared, and in order 
to make them, first use a coarse-grained file on the edge of a 
five-cent piece. Do not use the fine dust and powder, but 
only the fairly coarse filings. Mix a few silver filings from 
a ten-cent piece with the nickel in such proportion that the 
mixture is 90% nickel and 10% silver. 

.. figure:: images/Image239.jpg
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   :alt: Fig. 234.—Details of the Coherer. 
   
   Fig. 234.—Details of the Coherer. 

You will have to experiment considerably to find out 
just the right 
amount of filings to place in 
the tube, and 
how far apart 
to place the 
brass rods or 
plugs. 

Remove the gong from an old electric bell and mount the bell on the 
base as shown in Figure 233. It should be in such a 
position that the bell hammer will touch the coherer very lightly 
when the bell is ringing. 

The two binding-posts, tube rods, and filings constitute 
the *coherer*. The bell is the *decoherer*. 

The next thing required in order to complete the 
apparatus is a relay. You may use the relay described in Chapter 
X or build one according to the plan shown in Figure 235. 
This relay consists of a single electro-magnet mounted on a 
wooden base, two inches wide and four inches long. The 
armature is a piece of soft iron rod one-quarter of an inch 
in diameter and one-eighth of an inch long, riveted to the 
end of a thin piece of spring brass, about No. 34 B. & S. 
gauge in thickness. 

.. figure:: images/Image240.jpg
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   :alt: Fig. 235.—The Relay.
   
   Fig. 235.—The Relay.

The other end of the spring is fitted to a bracket and 
provided with a thumbscrew to adjust the 
tension of the 
spring. 

The under side of 
the armature and 
the upper side of 
the magnet core are 
each fitted with a 
small silver contact. 

The contacts should 
meet squarely when 
the armature is drawn down on to the core by a current of 
electricity passing through the electro-magnet. 

By turning the adjusting screw, the armature can be 
raised or lowered. It should be adjusted so that it almost 
touches the core and is only just far enough away to slip 
a piece of thick paper under. 

The terminals of the magnet are connected to the two 
binding-posts on the base marked *S* and *S*. One of the 
binding-posts, *P*, is connected to the brass upright, and the 
other is connected to the core of the magnet. 

Figure 236 shows how to connect up the outfit. It will 
require some very nice adjusting before you will be able to 
get it to working properly. 

.. figure:: images/Image241.jpg
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   :alt: Fig. 236.—The Complete Coherer Outfit. 
   
   Fig. 236.—The Complete Coherer Outfit. 

If you wish to use the outfit for demonstration purposes 
or for sending messages for very short distances, as for 
instance across a room, you do not need an aerial but 
merely a pair of "catch-wires." 

The "catch-wires" are two pieces of stiff copper wire, 
about two feet long, placed in the lower holes in the double 
binding-posts forming part of the coherer. 

In order to set the apparatus for operation, raise the 
adjusting screw of the relay until the armature is quite far 
away from the core. Then push the armature down against 
the contact on the core. The decoherer should then 
immediately operate and begin to tap the coherer. Then turn 
the thumbscrew until the armature is brought down to the 
core in such a position that it is as close as it is possible to 
get it without ringing the bell. 

The transmitter should consist of a spark coil, battery, 
key, and a spark-gap. The gap should be 
connected to the secondary of the coil and adjusted so that 
the electrodes are only about one-eighth of an inch apart. 
The key is placed in series with the primary of the coil and 
the battery, so that pressing the key will send a stream of 
sparks across the gap. Fit the spark-gap with two 
catch-wires similar to those on the coherer and place the 
transmitter about four or five feet away from the coherer outfit. 

You are now likely to find that if you press the key of the 
transmitter, the decoherer will ring. It is possible that it 
will continue to ring after you have stopped pressing the 
key. If such is the case, it will be necessary to turn the 
adjusting screw on the relay so as to move the armature 
upward a short distance away from the core. 

If the decoherer will not operate each time when you press 
the key, the brass plugs in the coherer need adjusting. You 
must not be discouraged if you have some difficulty in 
making the apparatus work at first. After you learn how to 
adjust it properly, you will find that you can move the 
transmitter quite a distance away from the coherer and it 
will still operate very nicely. 

After you manage that, you can place the apparatus in 
separate rooms and find it possible to work it just the same, 
because ordinary walls will not make any difference to 
wireless waves. 

Bear in mind that the nearer the coherer plugs are to each 
other, the more sensitive the coherer will be, but that if too 
close, the decoherer will not be able to shake the filings 
properly and will not stop when you stop pressing the key. 

The operation of the apparatus depends upon the fact 
that when properly adjusted the resistance of the filings 
between the two brass plugs is too great to allow sufficient 
battery current to flow to attract the armature of the relay. 
As soon as any wireless waves from the transmitter strike 
the catch-wires of the coherer, they cause the filings to cling 
together or cohere. When in this state, they have a low 
resistance and permit the current to flow in the relay circuit 
and draw down the armature. The armature closes the 
second circuit and sets the decoherer into operation. The 
decoherer shakes the filings and causes them to decohere or 
fall apart and so makes them ready again for the next signal. 

A coherer set of this sort may be used on an aerial and 
ground by substituting the coherer for the detector, but 
otherwise following any of the receiving circuits which have 
already been shown.

.. figure:: images/Image270.jpg
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   :alt: A WIRELESS TELEPHONE

CHAPTER XV A WIRELESS TELEPHONE 
```````````````````````````````

Probably many readers of the "Boy Electrician" are 
amateur wireless operators and have constructed their own 
apparatus with which they are able to pick up commercial 
messages or communicate with other experimenters in the 
neighborhood, but not many have ever built a wireless telephone.

The device described in the following pages is easy to 
make and arrange, and will serve for some very interesting 
experiments. 

It is of no practical value as a commercial wireless 
telephone, because the distance over which it will transmit 
speech is limited to from 250 to 300 feet. If you have a 
chum who lives across the street and within the distance 
named above, it is possible for you to construct a simple 
wireless telephone which will enable you to remain in 
your own rooms and talk to each other without any 
connecting wires. 

The instruments operate by magnetic induction. It has 
already been explained how it is possible for the current 
in the primary of an induction coil to induce a current in 
the secondary coil, even though the two are not 
electrically connected. This type of wireless telephone really 
consists of an induction coil in which the two windings are 
widely separated. 

Suppose that two coils of wire are connected as in 
Figure 237. The illustration shows that one coil, *A*, is connected 
in series with a set of batteries and a telegraph key. The 
terminals of the other coil, *B*, are connected to a telephone 
receiver. The coils are placed parallel to each other and a 
few inches apart. If the key is pressed so that the battery 
current may flow through the coil, *A*, it will create a 
magnetic field, and lines of force will be set up in the immediate 
vicinity. The lines of force will pass through the coil, *B*, 
and induce in it a current of electricity which will cause a 
sound like a click to be heard in the telephone receiver. 

.. figure:: images/Image271.jpg
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   :alt: Fig. 237.—A Simple Arrangement showing the Inductive Action between two Coils. 
   
   Fig. 237.—A Simple Arrangement showing the Inductive Action between two Coils. 

If a telephone transmitter is substituted for the key and 
words are spoken into it, the current passing through the 
coil from the battery will vary with each vibration of the 
voice and the words will be distinctly repeated by the 
receiver connected to *B*. 

This experiment may be tried by any boy with the 
equipment he probably has already around his shop. 
Twenty-five to thirty turns of wire wound around a 
cardboard tube five or six inches in diameter will serve as a 
coil. Two such coils, an ordinary telephone transmitter, a 
telephone receiver and a couple of dry cells are all that is 
required. 

.. figure:: images/Image272.jpg
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   :alt: Fig. 238.—A Simple Wireless Telephone. 
   
   Fig. 238.—A Simple Wireless Telephone. Speech directed into the Transmitter can be heard in the Receiver, although there is no direct electrical connection between the two. 

The diagram in the accompanying illustration shows 
how the apparatus is arranged. The coils may be used 
several inches apart and the voice will be clearly heard in 
the receiver. 

Such an outfit is, however, only experimental, and if it 
is desired to make a practical set, the coils, etc., must be 
much larger in diameter and contain a greater number of 
turns. 

Larger coils are made by first drawing a circle four feet 
in diameter on the floor of the "shop" or attic. Then 
drive a number of small nails around the circumference, 
about four inches apart. 

Procure two and one-half pounds of No. 20 B. & S. gauge 
cotton-covered magnet wire and wind it around the 
circumference of the circle. The wire should form at least 
sixty complete turns. About one foot should be left at 
each end to establish connections with. After winding, the 
coil should be tied about every six inches with a small piece 
of string so that it will retain its shape and not come apart. 
The nails are then pulled out so that the coil may be 
removed. 

The coil may be used just as it is for experimental 
purposes, but if it is intended for any amount of handling 
it is wise to procure a large hoop such as girls use for 
rolling along the sidewalk, and make the coil the same 
diameter as the hoop so that upon completion they may 
be firmly bound together with some insulating tape. Two 
binding-posts may then be fastened to the hoop and the 
terminals of the coil connected to them. 

Two such coils are required for a complete wireless 
telephone system, one to be located at each station. 

It is also necessary to make a double-contact strap-key. 
Such a key is easily built out of a few screws and some 
sheet-brass. The illustration shows the various parts and 
construction so clearly that no detailed explanation is 
necessary. 

.. figure:: images/Image273.jpg
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   :alt: Fig. 239.—A Double-Contact Strap-Key.
   
   Fig. 239.—A Double-Contact Strap-Key. The Dotted Lines show how the Binding-Posts are connected. 

The telephone transmitter and the telephone receiver 
required for this experiment must be very sensitive, and 
it is hardly possible for the young experimenter to build 
one which will be satisfactory. They can be secured from a 
second-hand telephone or purchased at an electrical supply 
house. The transmitter should be of the "long distance" 
type. An 80-ohm receiver will serve the purpose, but if 
you also have a wireless station, use the same 1000-ohm 
receivers belonging to your wireless set and you will secure 
very good results. 

A battery capable of delivering about 10 volts and a good 
constant current is required. 

The apparatus is connected as shown in Figure 240. 

When the key is pressed, the coil is connected to the 
battery and the telephone transmitter. If words are then 
spoken into the transmitter they will vary the amount of 
current flowing and the magnetic field which is set up in 
the neighborhood of the coil will induce currents in the coil 
at the other station, provided that it is not too far away, 
and cause the words to be reproduced in the telephone 
receiver. 

When the key is released it will connect with the upper 
contact and place the telephone receiver in the circuit for 
receiving, so that your chum at the other station can 
answer your message by pressing his key and talking into 
his transmitter. 

.. figure:: images/Image274.jpg
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   :alt: Fig. 240.—The Circuit of the Wireless Telephone.
   
   Fig. 240.—The Circuit of the Wireless Telephone. When the Key is up, the Receiver is ready for Action. When the Key is pressed, the Transmitter and Battery are thrown into the Circuit. 

The best plan is to mount each of the coils upon a tripod 
and experiment by placing them close together at first 
and gradually moving them apart until the maximum 
distance that the apparatus will work is discovered. 

Be very careful to keep the two coils exactly parallel. 

Much depends upon the battery. Be certain that it is 
capable of delivering a good strong current. Do not hold 
the key down any longer than is absolutely necessary, or 
the telephone transmitter will become hot. 

By making the coils six feet in diameter and placing from 
200 to 400 turns of wire in each coil you can make a set 
which is capable of transmitting speech 300 feet or more. 

.. figure:: images/Image275.jpg
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   :alt: Fig. 241.—A Complete Wireless Telephone and Telegraph Station for Amateurs.
   
   Fig. 241.—A Complete Wireless Telephone and Telegraph Station for Amateurs. 1. The Telephone Coil. 2. The Telephone Transmitter. 3. Double-Contact Strap-Key. 4. The Battery. 5. Spark Coil. 6. Key. 7. Spark-Gap. 8. Aerial Switch. 9. Loose Coupler. 10. Detector, 11. Fixed Condenser. 12. Code Chart. 13. Amateur License. 14. Aerial. 15. Telephone Receivers. 

The coil may be mounted on the wall of your shop in a 
position where it will be parallel to one located in your 
chum’s house. 

The success of a wireless telephone system of this sort 
lies in making the coils of large diameter and many turns, 
in keeping the coils parallel, using a sensitive transmitter 
and receiver, and in employing a good strong battery. 
Storage cells are the best for the purpose. 

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   :alt: ELECTRIC MOTORS

CHAPTER XVI ELECTRIC MOTORS
```````````````````````````

The first American patentee and builder of an electric 
motor was Thomas Davenport. The father of Davenport 
died when his son was only ten years old. This resulted in 
the young inventor being apprenticed to the blacksmith’s 
trade at the age of fourteen. 

Some years later, after having thoroughly learned his 
trade, he married a beautiful girl of seventeen, named Emily 
Goss, and settled in the town of Brandon, Vermont, as an 
independent working blacksmith. 

About this time Joseph Henry invented the 
electro-magnet. Davenport heard of this wonderful "galvanic 
magnet" which it was rumored would lift a blacksmith’s 
anvil. This was his undoing, for never again was he to 
know peace of mind but was destined to always be a 
seeker after some elusive scientific "will-o’-the-wisp." 
Although many times he needed iron for his shop, the 
greater part of his money was spent in making 
electro-magnets and batteries. 

In those days insulated wire could not be purchased, and 
any one wishing insulated wire had to buy bare wire and 
insulate it himself. It was then supposed by scientists that 
silk was the only suitable material for insulating wire and 
so Davenport’s brave young wife cut her silk wedding gown 
into narrow strips and with them wound the coils of the 
first electric motor. 

Continuing his experiments in spite of almost 
insurmountable difficulties and making many sacrifices which 
were equally shared by his family, he was enabled to make 
a trip to Washington in 1835 for the purpose of taking out 
a patent. His errand was fruitless, however, and he was 
obliged to return home penniless. 

Nothing daunted, he made the second and third trip and 
finally secured his memorable patent, the first of the long 
line of electric-motor patents that have made possible both 
the electric locomotive that hauls its long train so swiftly 
and silently, and the whirring little fan which stirs up a 
breeze during the hot and sultry days. 

These are a few of the reasons why a modest country 
blacksmith, in turn an inventor and an editor, through 
perseverance in struggling against adversity and poverty 
succeeded in placing his name on the list which will be 
deservedly immortal among the scientists and engineers 
of the world. 

**A Simple Electric Motor** can be made in fifteen minutes 
by following the plan shown in Figure 242. 

The armature is made by sticking a pin in each end of a 
long cork. The pins should be as nearly central as it is 
possible to make them, so that when the cork is revolved 
upon them it will not wabble. The pins form the shaft or 
spindle of the motor. Then take about ten feet of fine 
magnet wire (Nos. 28-32 B. & S. gauge) and wind it on as 
shown in the illustration, winding an equal number of turns 
on each side of the two pins. 

.. figure:: images/Image277.jpg
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   :alt: Fig. 242.—A Simple Electric Motor which may be made in Fifteen Minutes. 
   
   Fig. 242.—A Simple Electric Motor which may be made in Fifteen Minutes. 

When this is finished, fasten the wire securely to the 
cork by binding it with thread. 

Bend the two free ends (the starting and the finishing 
end) down at right angles and parallel to the shaft so as to 
form two commutator sections as shown in the upper left 
hand corner of Figure 242. Cut them off so that they only 
project about three-eighths of an inch. Bare the ends of 
the wire and clean them with a piece of fine emery paper 
or sandpaper. 

The bearings are made by driving two pins into a couple 
of corks so that the pins cross each other as shown in the 
upper right-hand corner of Figure 242. 

They must not be at too sharp an angle, or when the 
armature is placed in position, the friction of the shaft will 
be so great that it may not revolve. 

The motor is assembled by placing the armature in the 
bearings and then mounting two bar magnets on either 
side of the armature. The magnets may be laid on small 
blocks of wood and should be so close to the armature that 
the latter just clears when it is spun around by hand. 
The north pole of one magnet should be next to the 
armature and the south pole of the other, opposite. 

Connect two wires about one foot long and No. 26 B. & S. 
gauge in diameter to a dry cell. Bare the ends of the wires 
for about an inch and one half. 

Take the ends of the two wires between the forefinger 
and thumb and bend them out, so that when the armature 
is revolved they can be made just to touch the ends of the 
wire on the armature, or the "commutator sections," as 
they are marked in the drawing. 

Give the armature a twist so as to start it spinning, and 
hold the long wires in the hand so that they make contact 
with the commutator as it revolves. 

Very light pressure should be used. If you press too 
hard, you will prevent the armature from revolving, while, 
on the other hand, if you do not press hard enough, the 
wires will not make good contact. 

The armature will run in only one direction, and so try 
both ways. If you start it in the right direction and hold 
the wires properly, it will continue to revolve at a high rate 
of speed. 

If carefully made, this little motor will reward its maker 
by running very nicely. Although it is of the utmost 
simplicity it demonstrates the same fundamental principles 
which are employed in real electric motors. 

**The Simplex Motor** is an interesting little toy which can 
be made in a couple of hours, and when finished it will make 
an instructive 
model. 

.. figure:: images/Image278.jpg
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   :alt: Fig. 243.—Details of the Armature of the Simplex Motor. 
   
   Fig. 243.—Details of the Armature of the Simplex Motor. 

As a motor 
itself, it is not 
very efficient, 
for the amount 
of iron used in 
its construction 
is necessarily small. The advantage of this particular type 
of motor and the method of making it is that it 
demonstrates the actual principle and the method of application 
that is used in larger machines. 

The field of the motor is of the type known as the 
"simplex" while the armature is the "Siemens H" or two-pole 
type. The field and the armature are cut from ordinary 
tin-plated iron such as is used in the manufacture of tin 
cans and cracker-boxes. 

The simplest method of securing good flat material is to 
get some old scrap from a plumbing shop. An old cocoa tin 
or baking-powder can may, however, be cut up and 
flattened and will then serve the purpose almost as well. 

.. figure:: images/Image279.jpg
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   :alt: Fig. 244.—The Armature. 
   
   Fig. 244.—The Armature. 

**The Armature.** Two strips of tin, three-eighths of an 
inch by one and one-half 
inches, are cut to form the 
armature. They are slightly 
longer than will actually be 
necessary, but are cut to 
length after the finish of 
the bending operations. 
Mark a line carefully across the center of each strip. 
Then, taking care to keep the shape symmetrical so that 
both pieces are exactly alike, bend them into the shape 
shown in Figure 243. The small bend in the center is 
most easily made by bending the strip over a 
knitting-needle and then bending it back to the required extent. 

.. figure:: images/Image280.jpg
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   :alt: Fig. 245.—The Field. 
   
   Fig. 245.—The Field.  

A piece of knitting-needle one and one-half inches long is 
required for the shaft. Bind the two halves of the 
armature together in the position shown in Figure 249. Bind 
them with a piece of iron wire and solder them together. 
The wire should be removed after they are soldered. 

.. figure:: images/Image281.jpg
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   :alt: Fig. 246.—The Field and Commutator. 
   
   Fig. 246.—The Field and Commutator. 

**The Field Magnet** is made by first cutting out a strip of 
tin one-half by 
four and then 
bending it into 
the shape shown 
in Figure 245. 

The easiest way 
of doing this with 
accuracy is to cut 
out a piece of 
wood as a form, and bend the tin over the form. The 
dimensions shown in Figure 245 should be used as a guide 
for the form. 

.. figure:: images/Image282.jpg
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   :alt: Fig. 247.—The Bearings. 
   
   Fig. 247.—The Bearings. 

Two small holes should be bored in the feet of the field 
magnet to receive No. 3 wood 
screws, which fasten the field 
to the base. 

**The Bearings** are shown in 
detail in Figure 247. They are 
easily made by cutting from sheet-tin. Two small washers, 
serving as collars, should be soldered to the shaft as 
shown in Figure 243. 

**The Commutator Core** is formed by cutting a strip of 
paper five-sixteenths of an inch wide and about five inches 
long. It should be given a coat of shellac on one side and 
allowed to get sticky. The strip is then wrapped around 
the shaft until its diameter is three-sixteenths of an inch. 

**The Base** is cut from any ordinary piece of wood and 
is in the form of a block about two by one and one-half 
by one-half inch. 

.. figure:: images/Image283.jpg
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   :alt: Fig. 248.—The Complete Motor.
   
   Fig. 248.—The Complete Motor.

**Assembling the Motor.** The parts must be carefully 
prepared for winding by covering with paper. Cut a 
strip of paper 
one-half inch 
wide and one 
and one-eighth 
of an inch long 
and give it a 
coat of shellac 
on one side. As 
soon as it 
becomes sticky, 
wrap it around 
the top bar of 
the field magnet. The armature is insulated in exactly 
the same way, taking care that the paper covers the entire 
flat portion. 

The field and armature are now ready for winding. It 
is necessary to take proper precautions to prevent the 
first turn from slipping out of place. 

This is accomplished by looping a small piece of tape or 
cord over it. The next two turns are then taken over the 
ends of the loop so as to embed them. Wind on three layers 
of wire and when in the middle of the fourth layer embed 
the ends of another loop, which may be used at the end 
of the fourth layer to fasten the end so that it will not 
unwind. After the winding is finished, give it a coat of shellac. 

The winding of the armature is somewhat more difficult. 

The wire used for winding both the armature and the 
field should be No. 25 or No. 26 B. & S. gauge 
double-cotton-covered. 

In order to wind the armature, cut off about five feet of 
wire and double it back to find the center. Then place the 
wire diagonally across the center of the armature so that 
there is an equal length on both sides. Place a piece of 
paper under the wire at the crossing point to insulate it. 
Then, using one end of the wire, wind four layers on half 
of the armature. Tie the end down with a piece of thread 
and wind on the other half. 

The ends of the wire are cut and scraped to form the 
commutator segments. Figure 246 shows how this is done. 

Bend the wires as shown so that they will fit closely to 
the paper core. Bind them tightly into position with 
some silk thread. Use care so that the two wires do not 
touch each other. Cut the free ends of the wires off close 
to the core. 

When finished, the relative positions of the armature 
and the commutator should be as shown in Figure 248. 

The brushes are made by flattening a piece of wire by a 
few light hammer blows. 

The brushes are fastened under a small clamp formed 
by a strip of tin held down at each end with a wood screw. 
They can be adjusted to the best advantage only under 
actual working conditions when the current is passing 
through the motor. One or two dry cells should be sufficient 
to operate the motor. 

.. figure:: images/Image284.jpg
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   :alt: Fig. 249.—Details of the Motor. 
   
   Fig. 249.—Details of the Motor. 

One end of the field winding is connected to one of the 
brushes. The other brush and the other end of the field 
form the terminals to which the battery is connected. 

The motor, being of the two-pole armature type, must 
be started when the current is turned on by giving it a 
twist with the fingers. 

**A Larger Motor** may be built in somewhat the same 
manner as the one just described by cutting armature and 
field out of sheet tin. It will be more substantial if it is 
built up out of laminations and not bent into shape, as in 
the case of the other. 

Lay out an armature disk and a field lamination on a 
sheet of tin in accordance with the dimensions and pattern 
shown in Figure 249. These pieces are used as patterns 
for laying out the rest of the laminations. 

.. figure:: images/Image285.jpg
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   :alt: Fig. 250.—Complete Motor.  
   
   Fig. 250.—Complete Motor. 

Place them on some thin sheet-iron and trace the outline 
with a sharp-pointed needle. Then cut a sufficient number 
of pieces of each pattern to form a pile three-quarters of an 
inch thick. 

Four laminations for the field should be cut with 
extensions shown by the dotted lines. They are bent out at 
right angles for mounting the motor and holding it 
upright. 

Assemble the armature and field by piling the pieces on 
top of each other and truing them up. Enough laminations 
should be used to form a pile three-quarters of an inch 
thick when piled up and clamped tightly. 

File off any burrs and rough edges and then bind the 
laminations together with some string to hold them until 
wound. 

Wrap a couple of layers of paper around those portions 
of the armature and field which are liable to come into 
contact with the iron. Five or six layers of No. 18 B. & S. 
gauge double-cotton-covered magnet wire are sufficient to 
form the field coil. 

The armature is wound with three or four layers of wire 
of the same size. 

The commutator is made out of a circular piece of hard 
wood or fiber, fitted with segments cut out of thin 
sheet-copper. The segments may be fastened to the core with 
thick shellac or some melted sealing-wax. The ends may 
be bound down tightly by wrapping with silk thread. 

The brushes are cut out of thin sheet-copper similar 
to that used for the commutator segments. 

The bearings are strips of heavy sheet-brass bent into the 
shape shown. They are mounted by passing a nail through 
the holes in the ends and through the holes, A and B, in 
the field and then riveting the ends over. 

Assemble the motor as shown in Figure 255. If desirable, 
a small pulley may be fitted to the shaft and the motor used 
to run small mechanical toys. If it is properly constructed, 
two or three dry cells will furnish sufficient current to run 
the motor at high speed. 

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   :alt: DYNAMOS

CHAPTER XVII DYNAMOS
````````````````````

There is perhaps no other electrical device entering into 
the young experimenter’s domain requiring the careful 
workmanship and tool facilities that the dynamo does. 
In order to construct a practical working dynamo it would 
be necessary to have at hand a lathe for turning the 
castings. 

Rather than describe a machine which comparatively 
few of my readers would be able to build, I have explained 
below how it is possible to so alter an old telephone 
magneto that it may be made to serve as a small dynamo. 
Telephone magnetos, also sometimes called hand 
generators, are used in many telephone systems to supply the 
current which rings the telephone bell at the other end. 
The magneto is placed in a small box on the telephone, 
only the handle being exposed. In order to make a call 
the handle is given several brisk turns before raising the 
receiver. When the handle is turned the moving parts 
of the generator revolve and produce a current of 
electricity which goes forth over the line and rings the bell at the 
other end. 

.. figure:: images/Image287.jpg
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   :alt: Fig. 251—A Telephone Magneto.
   
   Fig. 251—A Telephone Magneto.

Telephone magnetos are gradually being discarded in all 
the large telephone systems, a method known as "central 
energy," in which the current for ringing bells is supplied 
from the central office, taking their place. For that reason, 
there are a great many telephone magnetos to be found in 
second-hand shops and at electrical houses, where they 
can be purchased for a fractional part of the original cost. 
Fifty cents will buy a first-class second-hand telephone 
magneto. The author saw a pile of telephones as large as 
a haystack, each telephone containing a magneto, in the 
back yard of a second-hand shop, and the owner would have 
been glad to sell the complete instruments for fifty cents 
each. 

Before explaining how to reconstruct such a machine, it is 
best to impress upon the reader that a careful study of the 
principles of the dynamo is well worth the time spent. 

Almost any book on physics or electricity, or even the 
encyclopedia, will be found to contain a description of this 
wonderful machine that supplies the power for running the 
trolley cars, electric lights, etc., in fact all of the electricity 
in use to-day with the exception of that generated by 
batteries for telegraph and telephone lines. 

It will be remembered that if a bar magnet is suddenly 
plunged into a hollow coil of wire, a momentary electric 
current will be generated in the coil. The current is easily 
detected by means of an instrument called a galvanometer. 
The space in the vicinity of a magnet is filled with a 
peculiar invisible force called magnetism. The magnetism flows 
along a certain path, passing through the magnet itself and 
then spreading out in curved lines. If a sheet of paper is laid 
over a magnet and a few iron filings are sprinkled on the 
paper, they will follow the magnetic lines of force. 

When the magnet is plunged into the hollow coil, the 
lines of force flow through the turns of wire, or are said to 
cut them. Whenever lines of force cut a coil of wire and 
they are in motion, electricity is produced. It does not 
matter whether the coil is slipped over the magnet or the 
magnet is plunged into the coil, a current will be produced 
as long as they are in motion. As soon as the magnet or 
the coil stops moving the current stops. 

By arranging a coil of wire between the poles of a 
horse-shoe magnet so that it can be made to revolve, the motion 
can be made continuous and the current of electricity 
maintained. 

Figure 252 shows such an arrangement. Some means of 
connection with the coil of wire must be established so that 
the current can be led off. If two metal rings are 
connected to the ends of the coil, connection can be made by 
little strips of metal called brushes rubbing against the 
rings. This scheme is the principle of the telephone 
magneto and the basis of all dynamos. 

.. figure:: images/Image288.jpg
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   :alt: Fig. 252.—The Principle of the Alternator and the Direct-Current Dynamo.
   
   Fig. 252.—The Principle of the Alternator and the Direct-Current Dynamo.

In the telephone magneto, more than one horseshoe 
magnet is usually provided. The coil of wire revolves between 
the poles of the magnets. The coil is wound around an 
iron frame and together they are called the armature. The 
end of the armature shaft is fitted with a small spur gear 
meshing with a larger gear bearing a crank, so that when 
the crank is turned the motion is multiplied and the 
armature is caused to revolve rapidly. One end of the coil or 
armature winding is connected to a small brass pin. This 
pin connects with a second pin set in the end of the shaft 
in an insulating brush of hard rubber. The other terminal 
of the coil is connected to the armature itself. Thus 
connection can be had to the coil by connecting a wire to the 
frame of the machine and to the insulated pin. 

.. figure:: images/Image289.jpg
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   :alt: Fig. 253.—Details of the Armature, Commutator, and Brushes. 
   
   Fig. 253.—Details of the Armature, Commutator, and Brushes. 

The armature of a magneto is usually wound with a very 
fine silk insulated wire, about No. 36 B. & S. gauge in size. 
This should be carefully removed and wound upon a spool 
for future use. Replace the wire with some ordinary 
cotton-covered magnet wire, about No. 24 or 25 B. & S. gauge, 
winding it on very carefully and smoothly. Connect one 
end of the winding to the pin leading to the insulated pin 
by soldering it. This pin is the one at the end of the shaft 
opposite to that one to which the spur gear is fastened. 
Connect the other end of the wire to the pin at the same 
end of the shaft as the gear. This pin is grounded, that is, 
connected to the frame. 

An ordinary telephone magneto gives a very high voltage 
current. The voltage may vary from twenty-five to several 
hundred, depending upon how fast the machine is run. This 
is due to the fact that the armature winding is composed 
of a very large number of turns of wire. The more turns 
that are placed on the armature, the higher its voltage will 
be. The current or amperage of a large telephone magneto 
wound with a large number of turns of fine wire is very low. 
Too low in fact to be used for anything except ringing a 
bell or testing. Winding the armature with fewer turns 
of large wire reduces the voltage and increases the 
amperage so that the current will light a small lamp or may 
be used for other purposes. The winding does not change 
the principle of the magneto, it merely changes its 
amperage and voltage. 

The magneto may be mounted on a wooden base-board 
and screwed to a table, so that the handle may be turned 
without inconvenience. A small strip of copper, called a 
brush, should be fastened to the base with a screw and 
brought to bear against the end of the insulated pin. The 
brush should be connected to a binding-post with a piece 
of wire. A second wire leading to a binding-post should 
be connected to the frame of the magneto. When the 
handle is turned rapidly, currents may be drawn from the 
two binding-posts. 

The current is of the kind known as alternating, that is 
to say, it flows first in one direction, then reverses and flows 
in the other. 

In order to make the machine give direct current, it must 
be fitted with a commutator. This is somewhat difficult 
with some magnetos but the following plan may be carried 
out in most cases. Cut a small fiber circle or disk about 
one inch in diameter from sheet fiber three-sixteenths of 
an inch thick. Cut a small hole in the center, just large 
enough so that the fiber will slip very lightly over the end 
of the shaft from which the insulated pin projects. Two 
small commutator sections similar to that shown in Figure 
253 must be cut from sheet-brass or sheet-copper. The 
three long ears shown in the drawing are bent back around 
the fiber and squeezed down flat with a pair of pincers so 
that they grip the fiber very tightly and will not slip. One 
ear on one section should be bent over the back down to 
the hole, where it will connect with the shaft. The other 
section of the commutator is connected to the insulated 
pin by a drop of solder. In this manner, one end of the 
winding is connected to one section of the commutator and 
the other end to the other section. The commutator should 
fit tightly on the end of the shaft so that it will not twist. 
The dividing line between the section should be parallel 
to a line drawn to the axis of the actual armature coil. 
When the iron parts of the armature are nearest the poles 
of the horseshoe magnets in their revolution, the slot in 
the commutator should be horizontal. 

When the magnet is provided with a commutator, it 
may also be run as a motor by connecting it to a battery. 
In order to operate it either as a dynamo or a motor, 
however, it must first be fitted with a pair of brushes. They 
are shown in detail in Figure 253. They are made from 
two small strips of sheet-copper bent as shown and mounted 
on a small wooden block. They must be adjusted to bear 
against the commutator so that when the dividing line 
between the two sections is horizontal, the upper brush 
bears against the upper section and the lower brush against 
the lower section. The two brushes form the terminals of 
the machine. They should be connected to binding-posts. 

.. figure:: images/Image290.jpg
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   :alt: Fig. 254.—The Complete Generator. 
   
   Fig. 254.—The Complete Generator. 

In order to operate the dynamo properly and obtain 
sufficient current from it to operate a couple of small 
incandescent lamps, it will have to be provided with a pulley 
mounted on the end of the shaft after the gear wheel has 
been removed. The dynamo may then be driven at high 
speed by connecting it to a sewing-machine with a belt, 
or the back wheel of a bicycle from which the tire has been 
removed. 

The completed dynamo is shown in Figure 254. The 
voltage and amperage of the dynamo will depend upon the 
machine in question, not only upon the size of the wire 
but also upon the size of the machine, the speed at which 
it is run, and the strength of the horseshoe magnets. It 
is impossible to tell just what the current will be until it 
is tested and tried. 

A 10-Watt Dynamo
################ 

Probably few experimenters fully understand how almost 
impossible it is to construct a dynamo, worthy of the name 
as such, without resort to materials and methods employed 
in the commercial manufacture of such machines. 
Practical telegraph instruments, telephones, etc., can be 
constructed out of all sorts of odds and ends, but in order to 
make a real dynamo it is necessary to use certain materials 
for which nothing can be substituted. 

*The field magnets* must be soft gray cast-iron except in 
special instances. 

*The wire* used throughout must be of good quality and 
must be new. 

The necessity for good workmanship in even the smallest 
detail cannot be overestimated. Poor workmanship always 
results in inefficient working. No dynamo will give its 
stated output continuously and safely unless the materials 
and workmanship are up to a high standard. 

Since castings must be used as field magnets, a pattern 
is necessary to form the mould for the casting. Pattern 
work is something requiring skill and knowledge usually 
beyond the average experimenter. A lathe is necessary 
in order to bore or tunnel the space between the ends of the 
field magnet into which the armature fits. 

It may be possible for several boys to club together and 
have a pattern made by a pattern-maker for building a 
dynamo. Then by using the lathe in some convenient shop 
or manual training school secure a field magnet and 
armature for a really practical small dynamo. 

.. figure:: images/Image291.jpg
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   :alt: Fig. 255.—Details of the Field Casting. 
   
   Fig. 255.—Details of the Field Casting.

For these reasons, I have described below a small dynamo 
of about ten watts output, the castings for which can be 
purchased from many electrical dealers with all machine 
work done at an extremely low price. 

The field magnet shown in Figure 255 is drawn to scale 
and represents the best proportions for a small "overtype" 
dynamo of ten to fifteen' watts output. 

The dimensions are so clearly shown by the drawings 
that further comment in that respect is unnecessary. 

The armature is of the type known as the "Siemen’s H." 
It is the simplest type of armature it is possible to make, 
which is a feature of prime importance to the beginner at 
dynamo construction, although it is not the most efficient 
form from the electrical standpoint. The armature in 
this case is also a casting and therefore a pattern is 
required. 

.. figure:: images/Image292.jpg
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   :alt: Fig. 256.—Details of the Armature Casting. 
   
   Fig. 256.—Details of the Armature Casting.

The patterns for both 
the field and the 
armature are of the same 
size and shape as shown 
in Figures 255 and 256. 
They are made of wood, 
and are finished by 
rubbing with fine sandpaper 
until perfectly smooth 
and then given a coat of 
shellac. The parts are 
also given a slight "draft," that is, a taper toward one 
side, so that the pattern may be withdrawn from the 
mould. 

The patterns are turned over to a foundry, where they 
are carefully packed in a box, called a "flask," full of 
moulder’s sand. When the patterns are properly 
withdrawn, they will leave a perfect impression of themselves 
behind in the sand. The mould is then closed up and poured 
full of molten iron. When the iron has cooled the castings 
are finished except for cleaning and boring. 

The shaft is a piece of steel rod, three-sixteenths of an 
inch in diameter, and four and one-half inches in length. 

The portion of the field into which the armature fits is 
bored out to a diameter of one and five-sixteenth inches. 
Considerable care is necessary in performing this operation 
in order not to break the field magnet apart by taking too 
heavy a cut. 

.. figure:: images/Image293.jpg
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   :alt: Fig. 257.—Details of the Commutator. 
   
   Fig. 257.—Details of the Commutator. 

The armature should be turned down to a diameter of 
one and one-quarter inches or 
one-sixteenth of an inch smaller 
than the tunnel in which it 
revolves between the field 
magnets. The center of the 
armature is bored out to fit the shaft. 

Figure 257 shows a two-part 
commutator for fitting to an 
armature of the "Siemen’s H" type. It consists of a short 
piece of brass tubing fitted on a fiber core and split 
length-wise on two opposite sides, so that each part is insulated 
from the other. 

The fiber is drilled with a hole to fit tightly on the shaft. 
It is then placed in a lathe and turned down until a suitable 
piece of brass tube can be driven on easily. 

Two lines are then marked along the tube diametrically 
opposite. A short distance away from each of these lines, 
and on each side of them, bore two small holes to receive 
very small wood screws. The screws should be 
counter-sunk. It is very important that none of the screws should 
go into the fiber core far enough to touch the shaft. 

The commutator may then be split along each of the lines 
between the screws with a hacksaw. The saw-cut should 
be continued right through the brass and slightly into the 
insulating core. The space between the sections of the 
commutator should be fitted with well-fitting slips of fiber, 
glued in. 

The commutator should now be trued up and made perfectly smooth. 

.. figure:: images/Image294.jpg
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   :alt: Fig. 258.—Diagram showing how to connect the Armature Winding to the Commutator. 
   
   Fig. 258.—Diagram showing how to connect the Armature Winding to the Commutator. 

The commutator is provided with a small brass machine screw threaded into 
each section near the edge as 
shown in Figure 257. These 
screws are to receive the ends 
of the armature winding and 
so facilitate connections. 

The commutator, shaft and armature are assembled as 
shown in Figure 258. 

The armature may be held to the shaft by a small set 
screw or a pin. The commutator should fit on the shaft 
very tightly so that it will not slip or twist. 

Every part of the armature and shaft touched by the 
armature winding must be insulated with paper which has 
been soaked in shellac until soft. The armature must be 
left to dry before winding. 

The armature should next be wound with No. 20 B. & S. 
gauge single-cotton-covered magnet wire. Sufficient wire 
should be put on to fill up the winding space completely. 
Care should be taken, however, not to put on too much 
wire or it will interfere with the field magnets and the 
armature cannot revolve. After winding the armature, 
test it carefully to see that the wire is thoroughly insulated 
from the iron. 

.. figure:: images/Image295.jpg
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   :alt: Fig. 259.—Details of the Wooden Base.
   
   Fig. 259.—Details of the Wooden Base.

If the insulation is correct, paint the whole armature 
with thick shellac varnish and bake it in a warm oven to 
set the shellac. 

Figure 258 is a diagram showing how the winding is 
made and connected. It is wound about the armature, 
always in the same direction, just as if the armature were 
an ordinary electro-magnet. 

The ends of the winding are each connected to one of 
the commutator sections by scraping the wire and placing 
it under the screws. 

The winding space in the field magnet should be 
shellacked, and insulated with brown paper by wrapping the core 
with a strip of paper and covering the bobbin ends with 
circular pieces made in two halves. 

The field magnet is wound full of No. 20 B. & S. gauge 
single-cotton-covered wire. The wire should be put on in 
smooth, even layers and the winding space completely filled 
up. 

.. figure:: images/Image296.jpg
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   :scale: 85 %
   :alt: Fig. 260.—The Pulley and Bearings. 
   
   Fig. 260.—The Pulley and Bearings. 

The base for the dynamo is a piece of hard wood, five 
inches long, four inches wide, and five-eighths of an inch 
thick. 

The bearings are small brass castings of the dimensions 
shown in Figure 260. It is necessary first to make a wooden 
pattern and send it to the foundry for the castings. 

The bearings are fastened to the projecting arms on the 
field casting by means of machine screws 
eight-thirty-seconds of an inch in thickness. 

The field magnet should not be screwed down on to the 
base until the armature runs easily and truly in the tunnel. 

The brushes are made from thin gauge sheet-copper 
according to the shape and dimensions shown in Figure 261. 

They are bent at right angles and mounted on the base 
on either side of the commutator with small round-headed 
wood screws. 

The completed dynamo is shown in Figure 262. One 
end of the shaft is provided with a small pulley 
to accommodate a small 
leather belt. 

.. figure:: images/Image297.jpg
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   :alt: Fig. 261.—The Brushes. 
   
   Fig. 261.—The Brushes. 

The dynamo is connected as a "shunt" 
machine, that is, one terminal 
of the field magnet is connected to one of the brushes, and the other terminal to 
the other brush. 

A wire is then led from each of the brushes to a 
binding-post. 

A shunt dynamo will only generate when run in a certain 
direction. In order to make it generate when run in the 
opposite direction, it is necessary to reverse the field 
connections. 

The dynamo just described should have an output of 
from 10 to 15 watts and deliver about 6 volts and 1 3/4 to 
2 1/2 amperes. 

In order to secure current from the dynamo it will first 
be necessary to magnetize the field by connecting it to 
several batteries. 

.. figure:: images/Image298.jpg
   :align: center
   :scale: 85 %
   :alt: THE JUNIOR DYNAMO MOUNTED ON A LONG WOODEN BASE 
   
   THE JUNIOR DYNAMO MOUNTED ON A LONG WOODEN BASE AND BELTED TO A GROOVED WHEEL FITTED WITH A CRANK SO THAT THE DYNAMO CAN BE RUN AT HIGH SPEED BY HAND POWER. THE ILLUSTRATION ALSO SHOWS A SMALL INCANDESCENT LAMP CONNECTED TO THE DYNAMO SO THAT WHEN THE CRANK IS TURNED THE LAMP WILL LIGHT.

It will be found that the dynamo will also operate as a 
very efficient little motor, but that on account of having a 
two-pole armature it must be started by giving the shaft a 
twist. 

.. figure:: images/Image299.jpg
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   :alt: Fig. 262.—Complete Dynamo. 

   Fig. 262.—Complete Dynamo. 

It can be used as a generator for lighting small lamps, 
electro-plating, etc., but cannot be used for recharging 
storage cells on account of having a two-pole armature. 

The dynamo may be driven with a small water motor or 
from the driving-wheel of a sewing-machine. 

Before the machine will generate as a dynamo, it must 
be connected to a battery and run as a motor. This will 
give the field the "residual magnetism" which is 
necessary before it can produce current itself. 

.. figure:: images/Image300.jpg
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   :alt: AN ELECTRIC RAILWAY 

CHAPTER XVIII AN ELECTRIC RAILWAY
`````````````````````````````````

No toys loom up before the mind of the average boy with 
more appeal to his love of adventure than do railway cars 
and trains. In England, the construction and operation 
of miniature railways is the hobby not only of boys but 
of grown men, and on a scale that is hardly appreciated 
in this country. 

The height of ambition of many boys is not only to own 
a miniature railway system but to build one. For some 
unknown reason, none of the boys’ papers or books have 
heretofore given any information on this interesting 
subject. The car shown in Figure 263 is such that it can be 
easily built by any boy willing to exercise the necessary 
care and patience in its construction. 

The first operation is to cut out the floor of the car. This 
is a rectangular piece of hard wood, eight inches long, three 
and one-quarter inches wide and one-half of an inch thick. 
Its exact shape and dimensions are shown in Figure 264. 

The rectangular hole cut in the floor permits the belt 
which drives the wheels to pass down from the 
counter-shaft to the axle. 

.. figure:: images/Image301.jpg
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   :alt: Fig. 263.—Complete Electric Railway operated by Dry Cells.
   
   Fig. 263.—Complete Electric Railway operated by Dry Cells. Note how the Wires from the Battery are connected to the Rails by means of the Wooden Conductors illustrated in Figure 277.  

The two pieces forming the wheel-bearings are cut out 
of sheet-brass according to the shape and dimensions shown 
in Figure 265. The brass should be one-sixteenth of an 
inch thick. The two projecting pieces at the top are bent 
over at right angles so that they can be mounted on the 
under side of the car floor by small screws passing through 
the holes. The holes which form the bearings for the ends 
of the axles upon which the wheels are mounted should 
be three inches apart. The bearings cannot be placed in 
position on the under side of the car floor until the wheels 
and axles are ready, but when this work is done, care 
should be taken to see that they line up and come exactly 
opposite to each other. 

.. figure:: images/Image302.jpg
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   :alt: Fig. 264.—Details of the Floor of the Car.
   
   Fig. 264.—Details of the Floor of the Car.

The wheels themselves cannot be made by the young 
experimenter unless he has a lathe. They are flanged 
wheels, one and one-eighth inches in diameter, and are 
turned from cast iron or brass. Such wheels can be 
purchased ready made, or it may be possible to obtain from 
some broken toy a set which will prove suitable. 

.. figure:: images/Image303.jpg
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   :alt: Fig. 265.—Details of the Bearing which supports the Wheel and Axle.
   
   Fig. 265.—Details of the Bearing which supports the Wheel and Axle.

Each shaft is composed of two pieces of "Bessemer" 
rod held together by a short piece of fiber rod having a 
hole in each end into which one end of each piece of iron 
rod is driven. The wheels fit tightly on the other end of 
each of these pieces. They should be spaced so as to run on 
rails two inches apart. 

.. figure:: images/Image304.jpg
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   :alt: Fig. 266.—The Wheels and Axle. 
   
   Fig. 266.—The Wheels and Axle. 

The purpose of the fiber rod is to insulate the halves of 
the axle from each other. The electric current which 
operates the car is carried by the two rails which form the 
track, and if the axles were made in one piece or the halves 
joined together so as to form an electrical connection, the 
battery furnishing the current would be short-circuited, 
because the current would pass along the two rails and 
across the axles instead of through the motor. 

One pair of wheels are fitted with a grooved pulley one 
inch in diameter. 

It is hardly necessary to say that the wheels and axles 
should be perfectly aligned, and should run true. 

.. figure:: images/Image305.jpg
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   :alt: Fig. 267.—The Motor. 
   
   Fig. 267.—The Motor.  

The motor used to drive the car will prove more 
satisfactory if purchased ready made. A self-starting 
three-pole motor similar to that shown 
in Figure 267 will serve very 
nicely. The wooden base should 
be removed and the motor 
screwed down firmly to the 
floor of the car as in Figure 268. 

One terminal of the motor is 
connected to one of the bearings, 
and the other terminal to the 
other bearing. 

The motor is belted to a 
countershaft so that it will have sufficient power to move 
the car. It cannot be directly connected or belted to the 
axle, because the speed of a small motor is so high that 
it has comparatively little turning power or *torque*. The 
speed must be reduced and the torque increased before it 
will drive the car. 

The countershaft consists of two grooved pulleys mounted 
upon an axle running in two bearings mounted upon the 
floor of the car. The bearings are made from a strip of 
heavy sheet-brass, bent at right angles and fastened to the 
car floor with small screws. The large pulley, *A*, is one 
inch and one half in diameter and the small pulley, *B*, is 
five-sixteenths of an inch in diameter. The countershaft 
is mounted in such a position that a belt may be run from 
the small pulley, *B*, to the pulley mounted on the axle of 
one pair of wheels. A belt is also run from the small pulley 
on the motor to the large pulley, *A*, on the countershaft. 
The pulleys must all be carefully lined up so that the belts 
will run in their grooves without danger of slipping out. 

.. figure:: images/Image306.jpg
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   :alt: Fig. 268.—The Complete Truck of the Car without the Body. 
   
   Fig. 268.—The Complete Truck of the Car without the Body. 

The shield on the platform at each end of the car is made 
of sheet-iron or tin. Two small projections on the bottom 
are bent over at right angles and used to secure the shields 
in position by driving a small tack through them into the 
floor of the car. 

The steps on either side of each platform are also made 
by bending strips of sheet-iron or tin and fastening them to 
the car with small nails or tacks. 

The coupler consists of a strip of tin having a small hook 
soldered to the end so that a trail car may be attached if 
desirable. 

.. figure:: images/Image307.jpg
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   :alt: Fig. 269.—Pattern for the Sides and Ends of the Car.
   
   Fig. 269.—Pattern for the Sides and Ends of the Car. 

The car is now ready for testing, and when held in the 
hand so that the wheels are free to run, two cells of dry 
battery should be found all that 
is necessary to drive them at a 
fair rate of speed. The two 
wires leading from the battery 
should be connected to the 
bearings, one wire leading to 
each bearing. It will require 
more than two cells, however, 
to drive the wheels properly 
when the car is on the track, 
All moving parts should run 
freely and smoothly. The car 
may be used just as it is, but if 
fitted with a body and a top it 
will present a much more realistic appearance. 

The sides and ends of the car 
body are made of sheet-iron or 
tin. Figure 269 shows the 
pattern and dimensions for these 
parts. They may be made from 
one piece of metal eighteen and 
one-half inches long and three 
and three-quarters inches wide. 
The doors and windows are cut 
out with a pair of tin-snips. 
The small projections along the 
top are bent down at right 
angles and the roof is fastened to them. The dotted lines 
indicate the places for bending these projections and also 
the sides and ends of the car. 

.. figure:: images/Image308.jpg
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   :alt: Fig. 270.—The Roof of the Car. 
   
   Fig. 270.—The Roof of the Car. 

The roof is made in two pieces. It also is sheet-iron or 
tin. The roof proper is eight inches long and four inches 
wide. It has a hole five and one-half inches long and one 
and three-quarters inches wide cut in the center. A 
number of small projections are left and bent upward to 
support the deck and to form imitation ventilators. The deck 
is six inches long and two and one-quarter inches in width. 
It is placed in position on the roof and fastened by 
soldering. The roof is fastened to the sides and ends of the 
car by soldering. It must be bent slightly to conform 
with the curve at the top of the front and the rear of the 
car. 

.. figure:: images/Image309.jpg
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   :alt: Fig. 271.—The Completed Car.
   
   Fig. 271.—The Completed Car.

The car when completed will appear as in Figure 271. 

The track is made of smooth spring steel, one-half inch 
wide and either No. 20 or No. 22 gauge in thickness. 

.. figure:: images/Image310.jpg
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   :alt: Fig. 272.–Details of a Wooden Tie.
   
   Fig. 272.–Details of a Wooden Tie.

The wooden ties are three and one-half inches long, 
three-quarters of an inch wide and three-eighths of an inch 
thick. Each tie has two saw-cuts, exactly two inches apart 
across the top face. This 
part of the work is best 
performed in a miter-box 
so that the cuts will be 
perfectly square across 
the ties. A saw should 
be used which will make 
a cut of such a size that 
the steel track will fit 
tightly into it. 

The distance between the two rails of the track, or the 
"gauge," as it is called, is two inches. 

.. figure:: images/Image311.jpg
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   :alt: Fig. 273.–Arrangement of Track.
   
   Fig. 273.–Arrangement of Track.

The track is assembled as in Figure 273. The spring steel 
is forced into the saw-cuts in the ties by tapping with a 
light wooden mallet. The ties should be spaced along the 
track about three inches apart. The work of laying the 
track must be very carefully done so that the car wheels 
will not bind at any spot. Curves should not be too sharp, 
or the car will not pass around. 

The track may be laid out in a number of different 
shapes, some of which are shown in Figure 274. 

.. figure:: images/Image312.jpg
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   :alt: Fig. 274.—Three Different Patterns for laying out the Track. 
   
   Fig. 274.—Three Different Patterns for laying out the Track. 

A circle is the easiest form of track to make. In 
laying out a circle or any sort of curved track, the outside 
rail must necessarily be made longer than the inside 
one. 

The oval shape is a very good form to give the track in 
a great many cases, especially where it is desirable for the 
car to have a longer path than that afforded by a circle. 

.. figure:: images/Image313.jpg
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   :alt: Fig. 275.—Details of the Base of the Cross-over.
   
   Fig. 275.—Details of the Base of the Cross-over.

In order to make a figure-eight out of the track, a 
crossing, or "cross-over," as it is sometimes called, will be 
required. This is shown in Figure 275. A cross-over permits 
two tracks to cross each other without interference. It 
consists of a wooden base, eight inches square and 
three-eighths of an inch thick. Four saw-cuts, each pair exactly 
parallel, and two inches apart, are made at right angles 
to each other across the top surface of the base, as shown 
in the illustration. 

The track used on the cross-over is semi-hard hoop-brass, 
one-half of an inch wide and of the same gauge as the steel 
track. The brass is more easily bent than the steel and is 
used for that reason, it being practically impossible to bend 
the steel track at right angles without snapping it. 

Four pieces of the brass, each five inches long, are bent 
at right angles exactly in the center. Four short pieces, 
each one and one-half inches long, will also be required. 

.. figure:: images/Image314.jpg
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   :alt: Fig. 276.—The Completed Cross-over.
   
   Fig. 276.—The Completed Cross-over.

The cross-over is assembled as shown in Figure 276. 
The strips marked *D* are strips of very thin sheet-brass or 
copper. The purpose of these strips is to connect the ends 
of the track on the cross-over to the ends of the track 
forming the figure-eight so that the cross-over will not be a 
"dead" section, that is, a section of track where the car 
cannot get any current. 

The long strips, bent at right angles to each other and 
marked *A*, *A*, *B*, *B*, in the illustration, are forced into the 
saw-cuts in the base over the strips marked *D*. 

The small pieces, *C*, *C*, *C*, *C*, are placed in between the 
long strips, leaving a space between so that the flanges of 
the car wheels can pass. The pieces, *C*, *C*, *C*, *C*, should 
form a square open at the corners. The two long strips, 
*A*, *A*, should be at opposite corners diagonally across the 
square. *B* and *B* should occupy the same relative position 
at the other corners. *A* and *A* are connected together and 
*B* and *B* are connected together by wires passing on the 
under side of the base. 

The ends of the track forming the figure-eight are forced 
into the saw-cuts at the edges of the base so that they 
form a good electrical connection with the small strips 
marked *D*. 

It is quite necessary to use care in arranging a figure-eight 
track, or there will be danger of short-circuiting the 
batteries. The outside rails of the figure-eight, distinguished 
by the letter *B* in the illustration, should be connected 
together by the cross-over. The inside rails, marked *A*, 
should also be connected together by the cross-over. 

In order to make a good mechanical and electrical 
connection between the ends of the rails when two or more 
sections of track are used in laying out the system, it is 
necessary to either solder the rails together or else use a 
connector such as that shown in Figure 277. 

This consists of a small block of wood having a saw-cut 
across its upper face and a piece of thin sheet-brass set into 
the cut. The two rails are placed with their ends abutting 
and one of these connectors slipped up from beneath and 
forced on the rails. The piece of thin brass set into the 
wooden block serves to make an electrical connection 
between the two rails and also to hold them firmly in position. 
A small screw and a washer placed outside the track 
and passing through 
the brass strip will 
allow a battery wire to 
be conveniently 
attached. 

.. figure:: images/Image315.jpg
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   :alt: Fig. 277.—A Connector for joining the Ends of the Rails. 
   
   Fig. 277.—A Connector for joining the Ends of the Rails. 

The steel rails should 
be occasionally wiped 
with machine oil or 
vaseline to prevent rusting, and also to allow the car to 
run more freely wherever the flanges of the wheels rub 
against the rails in passing around a curve. 

Four dry cells or three cells of storage battery should be 
sufficient to operate the car properly. If it is desirable, a 
small rheostat may be included in the battery circuit, so 
that the speed of the car can be varied at will. The motor 
and the wheels should be carefully oiled so that they will 
run without friction. The belts should not be so tight that 
they cause friction or so loose that they allow the motor 
to slip, but should be so adjusted that the motor runs 
freely and transmits its power to the wheels. 

The car may be made reversible by fitting with a small 
current reverser, but unless the reverser is carefully made 
the danger of loss of power through poor contacts is 
quite considerable. If the car is fitted with a reverser the 
handle should be arranged to project from the car in a 
convenient place where it can be easily reached by the 
fingers and the car sent back or forth at will. 

A railway system 
such as this can be 
elaborated and 
extended by adding 
more than one car 
to the line or such 
features as bridges 
and stations. 

.. figure:: images/Image316.jpg
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   :alt: Fig. 278.—A Bumper for preventing the Car from leaving the Rails.  
   
   Fig. 278.—A Bumper for preventing the Car from leaving the Rails. 

The ends of a 
blind section of 
track, that is, a straight piece of 
track not part of a 
circle or curve so that the car can return, should be fitted 
with a track bumper, to prevent the car leaving the rails. 

.. figure:: images/Image317.jpg
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   :alt: Fig. 279.—A Design for a Railway Bridge.  
   
   Fig. 279.—A Design for a Railway Bridge. 

No dimensions are given in Figures 279 and 280, 
showing designs for a bridge and a station, because they are 
best left to be determined by the scale upon which the 
railway system is to be extended. 

.. figure:: images/Image318.jpg
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   :alt: Fig. 280.—A Design for a Railway Station.  
   
   Fig. 280.—A Design for a Railway Station.

Both the bridge and the station are very simple. The 
bridge is built entirely of wood, with the exception of the 
steel rails. 

The station may be made out of thin wood, such as 
cigar-box wood. The doors, windows, etc., may be painted on 
the walls. If this is carefully done, it will give a very 
realistic appearance to your station. 

.. figure:: images/Image319.jpg
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   :alt: MINIATURE LIGHTING
   
CHAPTER XIX MINIATURE LIGHTING
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Miniature lighting is a field of many interesting 
possibilities for the young experimenter. Any labor expended 
along this line will result in something far more useful from 
a practical standpoint than almost any of the other 
things described in this book. 

Miniature lights, operated from batteries, may be used 
in various ways; to light dark corners, hallways, or other 
places where a light is often temporarily wanted without 
the accompanying danger and nuisance of matches or 
kerosene lamps. 

Miniature lighting has only been made practical by the 
tungsten filament lamp. The filament, or wire inside 
the globe, which becomes hot and emits the light when 
the current is turned on, is made of *tungsten* in a tungsten 
lamp. In the earlier lamps, it was made of carbon. The 
carbon lamp is now seldom used and is highly inefficient 
when compared to the tungsten. 

**A Carbon Lamp** consumes about three and one-half 
watts of current for each candle-power of light, whereas 
a small tungsten lamp uses only about one watt per 
candle-power small tungsten lamp uses only about one watt per 
candle-power. The tungsten lamp is therefore three times as
efficient as a carbon lamp, and when used on a battery of 
equal voltage it is possible to obtain the same amount of 
light with one-third of the current that would be required 
by a carbon lamp.

.. figure:: images/Image319a.jpg
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   :alt: Fig. 281.—Miniature Carbon Battery Lamp.
   
   Fig. 281.—Miniature Carbon Battery Lamp.
   
Carbon lamps similar to that shown in Figure 281 are made in a number of different voltages. 
The lowest voltage that it is practically possible 
to make a carbon lamp for is three and one-half. A three-and-one-half volt carbon lamp is 
designed to be operated on small dry cells such 
as flashlight batteries. The E. M. F. of a dry 
cell is about one and one-half volts, but when
three small dry cells of the flashlight type 
are connected in series and used to operate
a lamp, their voltage "drops," and the 
available E. M. F. is only about three and one-half volts.

Four-volt carbon lamps are intended to be operated on 
large dry batteries or wet cells because they do not lose 
their voltage as quickly as small dry cells. The table 
below gives the voltage and candle-power of the various 
small carbon lamps which are carried in stock by most 
electrical dealers or supply houses:

**MINIATURE CARBON BATTERY LAMPS**

| 3.5 volts for flashlight batteries 

| 4 volts. 2 candle-power

| 5.5 volts for flashlight batteries 

| 6 volts. 2 candle-power 

| 6 volts. 4 candle-power 

| 8 volts. 4 candle-power 

| 10 volts. 6 candle-power

**Tungsten Lamps** are made for voltages as low as 
one and one-half, and will light on one cell of dry battery. 
The range of voltages is quite wide and varied. A few of 
the most common sizes are given below:

**MINIATURE TUNGSTEN BATTERY LAMPS**

| 1.5 volts. for one dry cell 

| 2.5 volts. for two-cell flashlight battery 

| 2.8 volts. for two-cell flashlight battery 

| 3.5 volts. for three-cell flashlight battery 

| 3.8 volts. for three-cell flashlight battery 

| 4 volts. 4 candle-power 

| 6 volts. 2 candle-power 

| 6 volts. 4 candle-power 

| 6 volts. 6 candle-power 

| 6 volts. 8-10-12-16-20-24 candle-power 

.. figure:: images/Image320.jpg
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   :alt: Fig. 282.—Miniature Tungsten Battery Lamp. 
   
   Fig. 282.—Miniature Tungsten Battery Lamp. 
   
To find the approximate amount of current drawn from 
a battery by a tungsten lamp, divide the 
candle-power by the voltage and the 
result will be the current in amperes. For 
example, a 6 v. 2 c. p. lamp will require, 
2 divided by 6, or one-third of an ampere. 

Six-volt tungsten lamp giving a light 
greater than six candle-power are only 
used on storage batteries and are 
employed principally for automobile lighting. 

The filament of a tungsten lamp is 
much longer than that of a carbon lamp 
and is usually in the form of a spiral or 
helix, as shown in Figure 282. 

The bases of battery lamps, the base being the lower 
portion of the lamp, which is made of brass and fits into a 
socket or receptacle, are made in three different styles: 
*miniature*, *candelabra*, and *Ediswan*. 

.. figure:: images/Image321.jpg
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   :alt: Fig. 283.—Lamps fitted respectively with Miniature, Candelabra, and Ediswan Bases. 
   
   Fig. 283.—Lamps fitted respectively with Miniature, Candelabra, and Ediswan Bases. 
   
The miniature and candelabra bases have a threaded 
brass shell on the outside and a small brass contact-button 
on the bottom. They are similar except in respect to size. 
The miniature base is smaller than the candelabra. The 
Ediswan base is a plain brass shell having two pins on the 
side and two contacts on the bottom. This type of base is 
only used in this country on automobiles. The miniature 
and the candelabra bases are standard for battery lighting. 
The miniature base has many advantages over the 
candelabra for the young experimenter, and should be adopted 
in making any of the apparatus described in this chapter. 
These three bases are shown in Figure 283. 

.. figure:: images/Image322.jpg
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   :alt: Fig. 284.—Miniature Flat-Base Porcelain Receptacle.
   
   Fig. 284.—Miniature Flat-Base Porcelain Receptacle.
   
In order to form a good electrical connection between the 
lamp and the power wires some sort of a receptacle or 
socket is necessary. The most common arrangement for 
this purpose is the miniature flat-base 
porcelain receptacle shown in 
Figure 284. This type of receptacle is used 
in places where it can be permanently 
fastened in position with two small screws.

.. figure:: images/Image323.jpg
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   :alt: Fig. 285.—Weather-proof and Pin-Sockets. 
   
   Fig. 285.—Weather-proof and Pin-Sockets. 
   
The devices shown in Figure 285
are known respectively as a porcelain 
weather-proof socket and a pin-socket. Sockets similar 
to the weather-proof socket are also made of wood. The 
weather-proof sockets are used in places where the light 
is to be exposed out-of-doors, as 
for instance on a porch. The small 
metal parts are sealed in the 
porcelain and entirely protected. 

The pin-sockets and the wooden 
sockets are used principally on 
Christmas trees or in decorative 
outfits where lamps are hung in 
festoons. The flat-base receptacle, 
the pin-socket, and also the wooden 
socket will be found very useful 
in making the apparatus 
described farther on in this chapter. 

**The Wires** used to carry the current in a miniature 
lighting system may be of the sort known as *annunciator* or 
*office* wire if the wires are to be run entirely indoors. The 
wire should not be smaller than No. 16 B. & S. gauge. 
When the wires are run outdoors, on a porch, or in some 
other place exposed to the weather, the wire used should 
be rubber-covered. Hanging lights or lights intended to 
be adjustable should be connected with "flexible 
conductor." This is made of a number of very fine wires 
braided together and insulated with silk. The wires used 
in a lighting system should not in any case be longer than 
it is necessary to have them. When a battery is connected 
to a system of wires it is found that the voltage at the end 
of the wires is much lower than at a point near the battery. 
This is called voltage "drop," and is much greater as the 
wires grow longer. A light placed at the end of two very 
long wires will not burn as brightly as it would if connected 
to the same battery by means of short wires. 

**Switches** can be made by following the suggestions given 
in Chapter VII. Suitable switches can be purchased for a 
few cents at a most any electrical house and will prove very 
much neater and efficient. They should preferably be of 
any of the types shown in Figure 286. 

**The Batteries** used for miniature lighting may be made 
up of storage cells, dry cells or carbon cylinder cells. 
Storage cells will prove the most satisfactory, provided that 
the experimenter has some convenient means of 
recharging them or of having them recharged. Storage cells will 
be found of especial value wherever it is desirable to operate 
several lights from one battery. 

Carbon cylinder cells are only suitable where one cell 
is to be operated at a time. If more than one is used, 
the battery is liable to become polarized and the lamps 
will not burn brightly. Carbon cylinder batteries are 
very inexpensive to renew, and will be found the cheapest 
method of lighting a small tungsten lamp. 

.. figure:: images/Image324.jpg
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   :alt: Fig. 286.—Types of Battery Switches suitable for Miniature Lighting. 
   
   Fig. 286.—Types of Battery Switches suitable for Miniature Lighting. 
   
If lamps requiring more than two amperes are to be 
operated on dry cells, the latter should be connected in 
series-multiple, as shown in Figure 69. Two sets of dry 
cells connected in series-multiple will give more than twice 
the service of a single set. 

Lamps may be connected either in multiple or in series, 
provided that the proper voltages of both battery and 
lamps are used. 

When they are to be connected in multiple, the voltage 
of the lamps should be the same as that of the battery. 
When they are to be used in series, the voltage of the lamps 
multiplied by the number used should equal the voltage of 
the battery. For example, suppose that you wish to use 
a number of six-volt lamps on a six-volt storage battery. 
In that case they must be connected in multiple. But if 
it should be that the lamps are only two-volt lamps and 
you wish to operate three of them on a six-volt battery 
you will have to place them in series. 

.. figure:: images/Image325.jpg
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   :alt: Fig. 287.—How Lamps are Connected in Multiple. 
   
   Fig. 287.—How Lamps are Connected in Multiple. 
   
.. figure:: images/Image326.jpg
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   :alt: Fig. 288.—How Lamps are Connected in Series. 
   
   Fig. 288.—How Lamps are Connected in Series. 

It is sometimes desirable to arrange a lamp and two 
switches so that it can be turned off or on from either 
switch independently of the other. This is called 
"three-way wiring," and is a very convenient method of arranging 
a light in a hallway. If one switch is placed at the top of a 
stairway and the other switch at the bottom, a person can 
pass upstairs or downstairs, light the lamp ahead, and turn 
it out as he passes the last switch, no matter in which 
direction the previous user of the light may have gone. 

The switches are two-point switches, and the circuit 
should be arranged as in Figure 289. 

The switch-levers should always rest on one of the 
contacts and never be left between, as shown in the drawing. 

.. figure:: images/Image327.jpg
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   :alt: Fig. 289.—Three-way Wiring Diagram. The Light may be turned off or on from either Switch. 
   
   Fig. 289.—Three-way Wiring Diagram. The Light may be turned off or on from either Switch. 

They are represented that way in the illustration in order 
not to conceal the contacts. 

Small brackets made of brass and similar to that shown 
in Figure 290 are for sale at many electrical supply houses, 
and will add a very realistic appearance to a miniature 
lighting plant. 

.. figure:: images/Image328.jpg
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   :alt: Fig. 290.—A Lamp Bracket for Miniature Lighting.  
   
   Fig. 290.—A Lamp Bracket for Miniature Lighting. 

Brackets may be constructed after the plan shown in 
Figure 291. A wooden socket or a pin-socket is mounted 
on the end of a small piece of brass tubing which has been 
bent into the shape shown in the illustration. The other 
end of the tube is set into a wooden block so that the 
bracket may be mounted on the wall. The wires from the 
socket lead through the brass tube and through the back 
or top of the block. 

.. figure:: images/Image329.jpg
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   :alt: Fig. 291.—A Home-made Bracket.  
   
   Fig. 291.—A Home-made Bracket. 

Hanging lights may be arranged by fitting a wooden 
socket and a lamp with a reflector as shown in Figure 296. 
The reflector consists of a circular piece of tin or 
sheet-aluminum having a hole in the center large enough to pass 
the base of a miniature lamp. The circle is then cut along 
a straight line from the circumference to the center. If 
the edges are pulled together and lapped the circular sheet 
of metal will take on a concave shape and form a shade 
or reflector which will throw the light downwards. The 
overlapping edges of the reflector should be soldered or 
riveted together. The reflector is slipped over the 
base of the lamp, a small 
rubber or felt washer having 
been placed over the base 
next to the glass bulb so that 
the reflector will not break 
the lamp. The lamp is then 
screwed into a socket and allowed to hang downwards 
from a flexible conductor. 

.. figure:: images/Image330.jpg
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   :alt: Fig. 292.—A Hanging Lamp. 
   
   Fig. 292.—A Hanging Lamp. 

A very pretty effect can be secured by drilling the edges 
of a reflector full of small holes about three-sixteenths of 
an inch apart and then hanging short strings of beads from 
the holes. The beads should form a hanging fringe around 
the edge of the reflector, and if they are of glass, a pleasing 
brilliancy is produced. Figure 293 shows how to make the 
reflector. 

.. figure:: images/Image331.jpg
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   :alt: Fig. 293.—How the Reflector is made. 
   
   Fig. 293.—How the Reflector is made.  

The batteries for a miniature lighting plant may be 
located in a closet, under a stairway, or in some other 
out-of-the-way place. Wires from there may be extended to 
various parts of the house, such as hallways, closets, the 
cellar stairs, over a shaving-mirror in the bath-room or in 
any dark corner where a 
light is often temporarily 
needed. The wires can 
be run behind 
picture-mouldings or along the 
surbase and be almost 
entirely concealed. 

.. figure:: images/Image332.jpg
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   :alt: Fig. 294.—A Three-Cell Dry Battery for use in Hand-Lanterns, etc. 
   
   Fig. 294.—A Three-Cell Dry Battery for use in Hand-Lanterns, etc. 

**Small Batteries** 
consisting of three small dry 
cells enclosed in 
cardboard box, as shown in 
Figure 294, are on the 
market, and may be 
bought at prices 
ranging from thirty to forty cents, depending upon the size 
and the maker. One of the most convenient and practical 
sizes of this type of battery has the dimensions shown in 
the illustration, and with its aid it is possible to construct a 
number of very useful electrical novelties and household 
articles in the shape of portable electric lamps, etc. These 
batteries are quite small and are only intended to operate 
very small lamps. Only one lamp should be used on each 
battery at a time, and it should not be allowed to burn 
long. Some of these batteries will give ten to fourteen 
hours of intermittent service but 
if allowed to burn continuously 
would only light the lamp for 
about five hours at the most. 
It is much the better plan to 
use them only for a few minutes 
at a time, and then turn the 
light off and allow the battery 
to recuperate. 

**An Electric Hand-Lantern** is a 
very convenient device which is 
quite simple to make. It 
consists of a wooden box large 
enough to receive a three-cell 
battery, such as that shown in Figure 295. The back of 
the box should open and close on hinges and be fastened 
with a hook so that the battery may be easily removed for 
renewal. 

.. figure:: images/Image333.jpg
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   :alt: Fig. 295.—An Electric Hand-Lantern.
   
   Fig. 295.—An Electric Hand-Lantern.

A three-and-one-half-volt tungsten lamp is mounted 
on the front of the lantern and connected with the battery 
and a switch so that the light can be turned on and off at 
will. The switch may be placed at the top of the box so 
that the fingers of the same hand used to carry the lantern 
may be used to turn the light on and off. The lantern is 
fitted with a leather strap at the top, to be convenient for 
carrying. 

**The Ruby Lantern** shown in Figure 296 is somewhat 
similar in arrangement to the lantern just described, 
which may be used both as 
a hand-lantern and a ruby 
light for developing 
photographs. 

.. figure:: images/Image334.jpg
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   :alt: Fig. 296.—An Electric Ruby Lantern. 
   
   Fig. 296.—An Electric Ruby Lantern. 

It consists of a wooden box 
to hold a three-cell dry battery, 
and is provided with a handle so 
that it may be easily carried. A 
switch by which to turn the 
lamp on and off is mounted on 
the side of the box. 

The light is furnished by a
three-and-one-half-volt tungsten 
lamp mounted on the front of an inclined wooden board 
arranged as shown in the illustration so as to throw the 
light downward. The sides and bottom of the box are 
grooved near the front edges so that a piece of ruby glass 
may be inserted. Ruby glass for this purpose may be 
purchased at almost any store dealing in photographers’ 
supplies. 

The top is provided with a shield which is fastened in 
position by means of four small hooks after the glass is in 
place. The shield is used in order to prevent any white 
light from escaping through the crack between the glass 
and the top of the box. A ruby lamp of this sort must be 
made absolutely "light-tight" so that the only light 
emitted is that which passes through the ruby glass. If any 
white light escapes it is liable to fog and spoil any 
pictures in process of development. 

.. figure:: images/Image335.jpg
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   :alt: Fig. 297.—The Electric Ruby Lamp with Glass and Shield Removed. 
   
   Fig. 297.—The Electric Ruby Lamp with Glass and Shield Removed. 

By removing the ruby glass and the shield, as shown in 
Figure 297, the light is changed into a hand-lantern. The 
back of the box should be made removable so that the 
battery can be replaced when worn out. 

**A Night-Light** arranged to shine on the face of the clock 
so that the time may be easily told during the night 
without inconvenience is shown in Figure 298. 

.. figure:: images/Image336.jpg
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   :alt: Fig. 298.—An Electric Night-Light for telling the Time during the Night. 
   
   Fig. 298.—An Electric Night-Light for telling the Time during the Night. 

It consists of a flat wooden box containing a three-cell 
dry battery and having a small three-and-one-half-volt 
tungsten lamp mounted on the top in the front with 
room for a clock to stand behind. The battery and the 
lamp are connected to a switch so that the light may be 
turned on and off. By attaching a long flexible wire and a 
push-button of the "pear-push" type it is possible to 
place the light on a table and run the wire with the 
push-button attached over to the bed so that one may see the 
time during the night without getting up. The bottom of 
the box should be made removable so that a new battery 
may be inserted when the old one is worn out. 

**The Watch-Light** is in many ways similar to the clock 
light just described—but is smaller. It consists of a box 
just large enough to receive a three-cell flashlight battery. 
A piece of brass rod is bent into the form of a hook or crane 
from which to suspend the watch. 

.. figure:: images/Image337.jpg
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   :alt: Fig. 299.—A Watch-Light. 
   
   Fig. 299.—A Watch-Light. 

The light is supplied by a three-and-one-half-volt tungsten 
flashlight bulb mounted on the top of the box in front of 
the watch. If desirable, the light may be fitted with a small 
shade or reflector so that it shines only on the dial and not 
in the eyes. The figures on the face of the timepiece can 
then be seen much more plainly. 

The lamp is mounted in a small wooden socket or a 
pin-socket passing through a hole in the top of the box, so that 
the wires are concealed. A small push-button is located 
in one of the forward corners of the box, so that when it 
is pressed the lamp will light. Two small binding-posts 
mounted at the lower right-hand corner of the box are 
connected directly across the terminals of the switch, so 
that a flexible wire and a push-button can be connected, 
and the light operated from a distance. 

**An Electric Scarf-Pin** can be made by almost any boy 
who is skillful with a pocket-knife. The material from 
which the pin is made may be a piece of bone, ivory, or 
meerschaum. It is carved into shape with the sharp point 
of a penknife and may be made to represent a skull, dog’s 
head, an owl, or some other simple figure. The inside is 
hollowed out to receive a "pea" lamp. Pea lamps with a 
cord and a plug attached as shown in Figure 300 may be 
purchased from almost any electrical supply house. The 
lamp is a miniature carbon bulb about one-eighth of an 
inch in diameter. The eyes, nose, and mouth of the figure 
are pierced with small holes, so that when the lamp is 
lighted the light will show through the holes. The figure 
should be carved down thin enough to be translucent and 
light up nicely. 

.. figure:: images/Image338.jpg
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   :alt: Fig. 300.—A "Pea" Lamp attached to a Flexible Wire and a Plug. 
   
   Fig. 300.—A "Pea" Lamp attached to a Flexible Wire and a Plug. 

A large pin is cemented or otherwise fastened to the back 
of the figure so that it can be placed on the necktie or the 
lapel of the coat. The lamp is removed from the socket 
of an electric flashlight and the plug attached to the pea 
lamp screwed into its place. The pea lamp is inserted 
inside the figure and bound in place with some silk thread. 
Then when the button is pressed on the flashlight case, the 
pin will light up and tiny beams of light will shoot out from 
the eyes, nose, and mouth of the figure. 

.. figure:: images/Image339.jpg
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   :alt: Fig. 301.—Four Steps in Carving a Skull Scarf-Pin. 1. The Bone. 2. Hole drilled in Base. 3. Roughed out. 4. Finished. 
   
   Fig. 301.—Four Steps in Carving a Skull Scarf-Pin. 1. The Bone. 2. Hole drilled in Base. 3. Roughed out. 4. Finished. 

The drawings in Figure 301 show how to carve a skull 
scarf-pin. It is made from a cylindrical piece of bone 
about five-eighths of an inch long and three-eighths of an 
inch in diameter. The first operation is to drill a hole 
three-eighths of an inch deep into the bottom. The hole 
should be large enough in diameter to pass the pea lamp. 

.. figure:: images/Image340.jpg
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   :alt: Fig. 302.—The Completed Pin ready to be connected to a Battery by removing the Lamp from a Flashlight and screwing the Plug into its Place. 
   
   Fig. 302.—The Completed Pin ready to be connected to a Battery by removing the Lamp from a Flashlight and screwing the Plug into its Place. 

Then carve the eyes and nose and teeth. The drawings 
will give a good idea of the steps in this part of the work. 
Next round off the top of the skull. Bore a small hole in 
the back to receive the pin. Put the light inside of the 
skull, and after it is bound in position the scarf-pin is 
finished. 

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   :alt: MISCELLANEOUS ELECTRICAL APPARATUS 

CHAPTER XX MISCELLANEOUS ELECTRICAL APPARATUS
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HOW ELECTRICITY MAY BE GENERATED FROM HEAT
##########################################

For the past century there has been on the part of many 
scientists and inventors a constant endeavor to "harness 
the sunlight." The power which streams down every day 
to our planet is incalculable. The energy consumed in the 
sun and thrown off in the form of heat is so great that it 
makes any earthly thing seem infinitesimal. We can only 
feel the heat from a large fire a few feet away, yet the 
scorching summer heat travels 90,000,000 miles before it 
reaches us, and even then our planet is receiving only the 
smallest fractional part of the total amount radiated. 

Dr. Langley of the Smithsonian Institute estimated that 
all the coal in the State of Pennsylvania would be used 
by the sun in a fraction of a second if it were sent up there 
to supply energy. 

Perhaps, some day in the future, electric locomotives 
will haul their steel cars swiftly from city to city by means 
of electricity, generated with "sun power." Perhaps 
energy from the same source will heat our dwellings and 
furnish us light and power. 

This is not an idle dream, but may some day be an 
actuality. It has already been carried out to some extent. 
A Massachusetts inventor has succeeded in making a 
device for generating electricity from sun energy. 

The apparatus consists of a large frame, in appearance 
very much like a window. The glass panes are made of 
violet glass, behind which are many hundred little metallic 
plugs. The sun’s heat, imprisoned by the violet glass, 
acts on the plugs to produce electricity. One of these 
generators exposed to the sun for ten hours will charge a 
storage battery and 
produce enough current to run 30 large 
tungsten lamps for 
three days. 

.. figure:: images/Image342.jpg
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   :alt: Fig. 303.—How the Copper Wires (C) and the Silver Wires (I) are twisted together in Pairs.
   
   Fig. 303.—How the Copper Wires (*C*) and the Silver Wires (*I*) are twisted together in Pairs.

The principle upon which the apparatus works was  
discovered by a scientist named Seebeck, in 1822. He succeeded in producing 
a current of electricity by heating the points of contact 
between two dissimilar metals. 

Any boy can make a similar apparatus, which, while not 
giving enough current for any practical purpose, will 
serve as an exceedingly interesting and instructive 
experiment. 

Cut forty or fifty pieces of No. 16 B. & S. gauge German 
silver wire into five-inch pieces. Cut an equal number of 
similar pieces of copper wire, and twist each German silver 
wire firmly together with one of copper so as to form a 
zig-zag arrangement as in Figure 303. 

.. figure:: images/Image343.jpg
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   :alt: Fig. 304.—Wooden Ring.
   
   Fig. 304.—Wooden Ring.

Next make two wooden 
rings about four inches in 
diameter by cutting them 
out of a pine board. Place 
the wires on one of the 
rings in the manner shown 
in Figure 305. Place the 
second ring on top and clamp it down by means of two or 
three screws. 

.. figure:: images/Image344.jpg
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   :alt: Fig. 305.—Complete Thermopile.
   
   Fig. 305.—Complete Thermopile. An Alcohol Lamp should be lighted and placed so that the Flame heats the Inside Ends of the Wires in the Center of the Wooden Ring. 

The inner junctures of the wires must not touch each 
other. The outer ends should be bent out straight and 
be spaced equidistantly. The ring should be supported by 
three iron rods or legs. The two terminals of the 
thermopile as the instrument is called, should be connected to 
binding-posts. 

Place a small alcohol lamp or Bunsen burner in the 
center, so that the flame will play on the inner junctures 
of the wires. A thermopile of the size and type just 
described will deliver a considerable amount of electrical 
energy when the inside terminals are good and hot and the 
outside terminals fairly good. 

The current may be very easily detected by connecting 
the terminals to a telephone receiver or galvanometer. 
By making several thermopiles and connecting them in 
parallel, sufficient current can be obtained to light a small 
lamp. 

HOW TO MAKE A REFLECTOSCOPE
###########################

A reflectoscope is a very simple form of a "magic 
lantern" with which it is possible to show pictures from 
post-cards, photographs, etc. The ordinary magic lantern 
requires a transparent lantern slide, but the reflectoscope 
will make pictures from almost anything. The picture 
post-cards or the photographs that you have collected 
during your vacation may be thrown on a screen and 
magnified to three or four feet in diameter. Illustrations 
clipped from a magazine or newspaper or an original 
sketch or painting will likewise show just as well. 
Everything is projected in its actual colors. If you put your 
watch in the back of the lantern, with the wheels and 
works exposed, it will show all the metallic colors and the 
parts in motion. 

.. figure:: images/Image345.jpg
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   :alt: Fig. 306.—A Reflectoscope.
   
   Fig. 306.—A Reflectoscope.

The reflectoscope, shown in Figure 306, consists of a 
rectangular box nine inches long, six inches wide, and six 
inches high outside. It may be built of sheet-iron or tin, 
but is most easily made from wood. Boards three-eighths 
of an inch thick are heavy enough. The methods of making 
an ordinary box are too simple to need description. The 
box or case in this instance, however, must be carefully 
made and be "light-tight," that is, as explained before, it 
must not contain any cracks or small holes which will allow 
light to escape if a lamp is placed inside. 

A round hole from two and one-half to three inches in 
diameter is cut in the center of one of the faces of the box. 

The exact diameter cannot be given here because it will 
be determined by the lens which the experimenter is able 
to secure for his reflectoscope. Only one lens is required. 
It must be of the "double-convex" variety, and be from 
two and one-half to three inches in diameter. A lens is 
very easily secured from an old bicycle lantern. It should 
be of clear glass. 

.. figure:: images/Image346.jpg
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   :alt: Fig. 307.—How the Lens is Arranged and Mounted. 
   
   Fig. 307.—How the Lens is Arranged and Mounted. 

A tube, six inches long and of the proper diameter to 
fit tightly around the lens, must be made by rolling up a 
piece of sheet-tin and soldering the edges together. This 
tube is the one labeled "movable tube" in the illustrations. 
A second tube, three inches long and of the proper diameter 
to just slip over the first tube, must also be made. A flat 
ring cut from stiff sheet-brass is soldered around the 
outside of this second tube, so that it may be fastened to the 
front of the case by three or four small screws in the 
manner shown. The hole in the front of the box should be only 
large enough to receive the tube. 

The lens is held in position near one end of the movable 
tube by two strong wire rings. These rings should be made 
of wire that is heavy and rather springy, so that they will 
tend to open against the sides of the tube. It is a good plan 
to solder one of them in position, so that it cannot move, 
and then put in the lens. After the lens is in position, the 
second ring should be put in and pushed down against the 
lens. Do not attempt to put the lens in, however, until 
you are sure that the metal has cooled again after soldering, 
or it will be liable to crack. 

.. figure:: images/Image347.jpg
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   :alt: Fig. 308.—A View of the Reflectoscope from the Rear, showing the Door, etc. 
   
   Fig. 308.—A View of the Reflectoscope from the Rear, showing the Door, etc. 

The back of the box contains a small hinged door about 
four inches high and five and one-half inches long. The 
pictures that it is desired to project on the screen are held 
against this door by two small brass clips, as shown in 
Figure 308. 

.. figure:: images/Image348.jpg
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   :alt: Fig. 309.—A View of the Reflectoscope with the Cover removed, showing the Arrangement of the Lamps, etc.  
   
   Fig. 309.—A View of the Reflectoscope with the Cover removed, showing the Arrangement of the Lamps, etc. 

The light for the reflectoscope is most conveniently made 
by two 16-candle-power electric incandescent lamps. Figure 
309 shows a view of the inside of the box with the cover 
removed, looking directly down. The 
lamps fit into ordinary flat-base 
porcelain receptacles, such as 
that shown in 
Figure 310. Two 
of these receptacles are 
required, one for 
each lamp. They 
cost about ten 
cents each. 

.. figure:: images/Image349.jpg
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   :alt: Fig. 310.—A Socket for holding the Lamp.  
   
   Fig. 310.—A Socket for holding the Lamp.

The reflectors are made of tin, bent as shown in Figure 311. They are fastened in 
position behind the lamps by four small tabs. 

It is possible to fit a 
reflectoscope with gas or oil 
lamp to supply the light, 
but in that case the box 
will have to be made much 
larger, and provided with 
chimneys to carry off the hot air. 

The interior of the reflectoscope must be painted a dead 
black by using a paint made by mixing lampblack and 
turpentine. The interior also includes the inside of the tin 
tubes. 

The electric current is led into the lamps with a piece of 
flexible lamp-cord passing through a small hole in the 
case. An attachment-plug is fitted to the other end of the 
cord, so that it may be screwed into 
any convenient lamp-socket. 

.. figure:: images/Image350.jpg
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   :alt: Fig. 311.—The Tin Reflector. 
   
   Fig. 311.—The Tin Reflector. 

The pictures should be shown in a 
dark room and projected on a smooth 
white sheet. They are placed under 
the spring clips on the little door and 
the door closed. The movable tube 
is then slid back and forth until the 
picture on the screen becomes clear 
and distinct. 

The lantern may be improved considerably by using tungsten lamps 
of 22 c. p. each in place of ordinary c. p. carbon filament lamps. 

If four small feet, one at each corner, are attached to the 
bottom of the case, its appearance will be much improved. 

Very large pictures will tend to appear a little blurred 
at the corners. This is due to the lens and cannot be easily 
remedied. 

HOW TO REDUCE THE 110-v. CURRENT SO THAT IT MAY BE USED FOR EXPERIMENTING
#########################################################################

Oftentimes it is desirable to operate small electrical 
devices from the 110-v. lighting or power circuits. 
Alternating current can be reduced to the proper voltage by 
means of a small step-down transformer, such as that 
described in Chapter XIII. Direct current may be reduced 
by means of a resistance. The most suitable form of 
resistance for the young experimenter to use is a "lamp 
bank." 

A lamp bank consists of a number of lamps connected 
in parallel, and arranged so that any device may be 
connected in series with it. 

The lamps are set in sockets of the type known as 
"flat-base porcelain receptacles," such as that shown in Figure 
310, mounted in a row upon a board and connected as 
shown in Fig. 312. 

The current from the power line enters through a switch 
and a fuse and then passes through the lamps before it 
reaches the device it is desired to operate. The switch 
is for the purpose of shutting the current on and off, while 
the fuse will "blow" in case too much current flows in 
the circuit. 

The amount of current that passes through the circuit 
may be accurately controlled by the size and number of 
lamps used in the bank. The lamps may be screwed in or 
out and the current altered by one-quarter of an ampere 
at a time if desirable. 

The lamps should be of the same voltage as the line 
upon which they are to be used. Each 8-candle-power, 
110-v. carbon lamp used will permit one-quarter of an 
ampere to pass. Each 16-candle-power, 110-v. lamp will 
pass approximately one-half an ampere. 
A 32-candle-power lamp of the same voltage will permit one ampere to 
flow in the circuit. 

.. figure:: images/Image351.jpg
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   :alt: Fig 312.—Top View of Lamp Bank, showing how the Circuit is arranged.
   
   Fig 312.—Top View of Lamp Bank, showing how the Circuit is arranged. A and B are the Posts to which should be connected any Device it‘s desirable to operate. 

AN INDUCTION MOTOR 
##################

**An Induction Motor** is a motor in which the currents in 
the armature windings are *induced*. An induction motor 
runs without any brushes, and the current from the power 
line is connected only to the field. The field might be 
likened to the primary of a transformer. The currents in 
the armature then constitute a secondary winding in which 
currents are induced in the same manner as in a transformer. 

An induction motor will operate only on alternating current. 

A small motor such as that shown in Figure 267, and 
having a three-pole armature, is the best type to use in 
making an experimental induction motor. 

Remove the brushes from the motor and bind a piece of 
bare copper wire around the commutator so that it 
short-circuits the segments. 

A source of alternating current should then be connected 
to the terminals of the field coil. If you have a step-down 
transformer, use it for this purpose, but otherwise connect 
it in series with a lamp bank such as that just described. 

Place a switch in the circuit so that the current may be 
turned on and off. Wind a string around the end of the 
armature shaft so that it may be revolved at high speed 
by pulling the string in somewhat the same manner that 
you would spin a top. When all is ready, give the string 
a sharp pull and immediately close the switch so that the 
alternating current flows into the field. 

If this is done properly, the motor will continue to run 
at high speed, and furnish power if desirable. 

Most of the alternating-current motors in every-day use 
for furnishing power for various purposes are induction 
motors. They are, however, self-starting, and provided 
with a hollow armature, which contains a centrifugal 
governor. When the motor is at rest or just starting, four 
brushes press against the commutator and divide the 
armature coils into four groups. After the motor has attained 
the proper speed, the governor is thrown out by 
centrifugal force and pushes the brushes away from the 
commutator, short-circuiting all the sections and making each 
coil a complete circuit of itself. 

ELECTRO-PLATING
############### 

Water containing chemicals such as sulphate of copper, 
sulphuric acid, nitrate of nickel, nitrate of silver, or other 
metallic salts is a good conductor of electricity. Such a 
liquid is known as an *electrolyte*. 

It has been explained in Chapter IV that chemical action 
may be used to produce electricity and that in the case of 
a cell such as that invented by Volta, the zinc electrode 
gradually wastes away and finally enters into solution in 
the sulphuric acid. 

It is possible exactly to reverse this action and to 
produce what is known as *electrolysis*. If an electrolyte in 
which a metal has been "dissolved" is properly arranged 
so that a current of electricity may be passed through the 
solution, the metal will "plate out," or appear again upon 
one of the electrodes. 

Electrolysis makes possible electro-plating and thousands 
of other exceedingly valuable and interesting chemical 
processes. 

More than one-half of all the copper produced in the 
world is produced *electrolytically*. 

Practically all plating with gold, silver, copper and 
nickel is accomplished with the aid of electricity. 

These operations are carried out on a very large scale 
in the various factories, but it is possible to reproduce them 
in any boy’s workshop or laboratory, with very simple 
equipment. 

The proper chemicals, a tank, and a battery are the only 
apparatus required. The current must be supplied by 
storage cells or a bichromate battery because the work will 
require five or six amperes for quite a long period. 

A small rectangular glass jar will make a first class tank 
to hold the electrolyte. 

The simplest electro-plating process, and the one that 
the experimenter should start with is copper-plating. 

Fill the tank three-quarters full of pure water and then 
drop in some crystals of copper-sulphate until the liquid 
has a deep blue color and will dissolve no more. 

Obtain two copper rods and lay them across the tank. 
Cut two pieces of sheet copper having a tongue at each 
of two corners so that they can be hung in the solution, as 
shown in Figure 313. Hang both of the sheets from one 
of the copper rods. Connect this rod to the *positive* pole 
of the battery. These sheets are known as the anodes. 

Then if a piece of carbon, or some metallic object is hung 
from the other rod and connected to the *negative* pole of 
the battery, the electro-plating will commence. The 
apparatus should be allowed to run for about half an hour 
and then the object hung from the rod connected to the 
negative pole of the battery should be lifted out and 
examined. It will be found thickly coated with copper. It 
is absolutely necessary to have the poles of the battery 
connected in the manner stated, or no deposit of copper 
will take place. 

Objects which are to be electro-plated must be free from 
all traces of oil or grease and absolutely clean in every 
respect, or the plating will not be uniform, because it will 
not stick to dirty spots. 

.. figure:: images/Image352.jpg
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   :alt: Fig. 313.—A Glass Jar arranged to serve as an Electro-Plating Tank. 
   
   Fig. 313.—A Glass Jar arranged to serve as an Electro-Plating Tank. 

Such articles as keys, key-rings, tools, etc., can be 
prevented from rusting by coating with nickel. 

Nickel-plating is very similar to copper-plating. Instead, 
however, of having two copper sheets suspended from the 
rod connected to the positive pole of the battery, they must 
be made of nickel. 

The electrolyte is composed of one part of 
nickel-sulphate dissolved in twenty parts of water to which one part 
of sodium-bisulphate is added. 

This mixture is placed in the tank instead of the 
copper-sulphate. The objects to be plated are hung from the 
copper rod connected to the negative pole of the battery. 

When the nickel-plated articles are removed from the 
bath they will have a dull, white color known as "white 
nickel." When white nickel is polished with a cloth wheel 
revolving at high speed, and known as a buffing-wheel, it 
will assume a high luster. 

HOW TO MAKE A RHEOSTAT
######################

It is often desirable to regulate the amount of current 
passing through a small lamp, motor, or other electrical 
device operated by a battery. 

This is accomplished by inserting resistance into the 
circuit. A rheostat is an arrangement for quickly altering 
the amount of resistance at will. 

A simple rheostat is easily made by fitting a five-point 
switch such as that shown in Figure 95 with several coils 
of German-silver resistance wire. German silver has much 
more resistance than copper wire, and is used, therefore, 
because less will be required, and it will occupy a smaller 
space. 

A five-point switch will serve satisfactorily in making a 
rheostat, but if a finer graduation of the resistance is 
desired it will be necessary to use one having more points. 

Two lines of small wire nails are driven around the 
outside of the points, and a German-silver wire of 
No. 24 B. & S. gauge wound in zig-zag fashion around the nails from 
one point to the other. 

.. figure:: images/Image353.jpg
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   :alt: Fig. 314.—A Rheostat.
   
   Fig. 314.—A Rheostat.

The rheostat is placed in series with any device it is 
desirable to control. When the handle is on the point to 
the extreme left, the rheostat offers no resistance to the 
current. When the lever is placed on the second point, 
the current has to traverse the first section of the 
German-silver wire and will be appreciably affected. Moving the 
handle to the right will increase the resistance. 

If the rheostat is connected to a motor, the speed can 
be increased or decreased by moving the lever back and 
forth. 

In the same manner, the light from a small incandescent 
lamp may be dimmed or increased. 

A CURRENT REVERSER OR POLE-CHANGING SWITCH
##########################################

A pole-changing or current reversing switch is useful to 
the experimenter. For example, if connected to a small 
motor, the motor can be made to run in either direction at 
will. A motor with a permanent magnet field can be reversed 
by merely changing the wires from the battery so that the 
current flows through the circuit in the opposite direction. 
If the motor is provided with a field winding, however, the 
only way that it can be made to run either way is by 
reversing the field. This is best accomplished with a pole-changing 
switch. 

Such a switch may be made by following the same 
general method of construction as that outlined on pages 107 
and 108, but making it according to the design shown in Figure 315.

Motors such as those illustrated can be made to reverse 
by connecting to a pole-changing switch in the proper manner.

The two outside points or contacts (marked *D* and *D*) 
should both be connected to one of the brushes on the motor. 
The middle contact, *C*, is connected to the other brush.

One terminal of the field is connected to the battery. 
The other terminal of the field is connected to the lever, *A*.
*B* connects to the other terminal of the battery.

.. figure:: images/Image354.jpg
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   :alt: Fig. 315.—A Pole-Changing Switch or Current Reverser.
   
   Fig. 315.—A Pole-Changing Switch or Current Reverser. The Connecting Strip is pivoted so that the Handle will operate both the Levers, A and B.

When the switch handle is pushed
to the left, the 
lever *A* should rest
on the left-hand
contact, *D*. The 
lever *B* should 
make contact with *C*. The motor will then run in one direction. If the
handle is pushed 
to the right so
that the levers *A* and *B* make contact respectively with *C* and *D* (right-hand), the motor 
will reverse and run in the opposite direction.

A COMPLETE WIRELESS RECEIVING SET
#################################

Many experimenters may wish to build a wireless 
receiving set which is permanently connected and in which the 
instruments are so mounted that they are readily portable 
and may be easily shifted from one place to another 
without having to disturb a number of wires.

The receiving set shown in Figure 316 is made up of some 
of the separate instruments described in Chapter XIV, 
and illustrates the general plan which may be followed 
in arranging an outfit in this manner.

.. figure:: images/Image355.jpg
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   :alt: COMPLETE RECEIVING SET, CONSISTING OF DOUBLE SLIDER TUNING COIL, DETECTOR AND FIXED CONDENSER.
   
   COMPLETE RECEIVING SET, CONSISTING OF DOUBLE SLIDER TUNING COIL, DETECTOR AND FIXED CONDENSER.

.. figure:: images/Image356.jpg
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   :alt: COMPLETE RECEIVING SET, CONSISTING OF A LOOSE COUPLER IN PLACE OF THE TUNING COIL, DETECTOR AND FIXED CONDENSER.
   
   COMPLETE RECEIVING SET, CONSISTING OF A LOOSE COUPLER IN PLACE OF THE TUNING COIL, DETECTOR AND FIXED CONDENSER.

The base is of wood, and is nine inches long, seven inches 
wide, and one-half of an inch thick.

A double-slider tuning coil, similar to that shown in 
Figure 203, is fastened to the back part of the base by two
small wood-screws passing upwards through the base into the tuner heads.

.. figure:: images/Image357.jpg
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   :alt: Fig. 316. A Complete Wireless Receiving Outfit.
   
   Fig. 316. A Complete Wireless Receiving Outfit.

The fixed condenser is enclosed in a rectangular wooden 
block which is hollowed out underneath to receive it and 
then screwed down to the base in the forward right-hand corner.

The detector is mounted in the forward left-hand part 
of the base, and in the illustration is shown as being similar 
to that in Figure 210. Any type of detector may, however, be substituted.

The tuning coil may be replaced by a loose coupler if 
desirable, but in that case the base will have to be made larger.

The telephone receivers are connected to two binding-posts 
mounted alongside the detector.

The circuit shown in Figure 218 is the one which should 
be followed in wiring the set. The wires which connect 
the various instruments should be passed through holes 
and along the under side of the base so that they are concealed.

HOW TO BUILD A TESLA HIGH-FREQUENCY COIL
######################################## 

A Tesla high-frequency coil or transformer opens a field 
of wonderful possibilities for the amateur experimenter. 
Innumerable weird and fascinating experiments can be 
performed with its aid. 

When a Leyden jar or a condenser discharges through a 
coil of wire, the spark which can be seen does not consist 
simply of a single spark passing in one direction, as it 
appears to the eye, but in reality is a number of separate 
sparks alternately passing in opposite directions. They 
move so rapidly that the eye is unable to distinguish them. 
The time during which the spark appears to pass may only 
be a fraction of a second, but during that short period the 
current may have oscillated back and forth several thousand 
times. 

If the discharge from such a Leyden jar or a condenser 
is passed through a coil of wire acting as a *primary*, and 
the primary is provided with a *secondary* coil containing a 
larger number of turns, the secondary will produce a 
peculiar current known as *high-frequency* electricity. 
High-frequency currents reverse their direction of flow or 
*alternate* from one hundred thousand to one million times a 
second. 

.. figure:: images/Image358.jpg
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   :alt: Fig. 317.—Illustrating the Principle of the Tesla Coil.
   
   Fig. 317.—Illustrating the Principle of the Tesla Coil. A Leyden Jar discharges through the Primary Coil and a High-Frequency Spark is produced at the Secondary. 

High-frequency currents possess many curious 
properties. They travel only on the surface of wires and 
conductors. A hollow tube is just as good a conductor for 
high-frequency currents as a solid rod of the same diameter. 
High-frequency currents do not produce a shock. If you 
hold a piece of metal in your hand you can take the shock 
from a high-frequency coil throwing a spark two or three 
feet long with scarcely any sensation save that of a slight 
warmth. 

The Tesla coil described below is of a size best adapted 
for use with a two-inch or three-inch spark coil, or a small 
high-potential wireless transformer. The purpose of the 
spark coil or the transformer is to charge the Leyden jars 
or condenser which discharge through the primary of the 
Tesla coil. 

.. figure:: images/Image359.jpg
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   :alt: Fig. 318.—Details of the Wooden Rings used as the Primary Heads. 
   
   Fig. 318.—Details of the Wooden Rings used as the Primary Heads. 

If the young experimenter wishes to make a Tesla coil 
which will be suited to a smaller spark coil, for instance, 
one capable of giving a one-inch spark, the dimensions of 
the Tesla coil herein described can be cut exactly in half. 
Instead of making the secondary twelve inches long and 
three inches in diameter, make it six inches long and one 
and one-half inches in diameter, etc. 

**The Primary** consists of eight turns of No. 10 B. & S. 
gauge copper wire wound around a drum. The heads of 
the drum are wooden rings, seven inches in diameter and 
one-half inch thick. A circular hole four and one-half 
inches in diameter is cut in the center of each of the heads. 

.. figure:: images/Image360.jpg
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   :alt: Fig. 319.—Details of the Cross Bars which support the Primary Winding. 
   
   Fig. 319.—Details of the Cross Bars which support the Primary Winding. 

The cross bars are two and 
one-half inches long, three-quarters of 
an inch thick and one-half of an 
inch wide. Six cross bars are 
required. They are spaced at equal 
distances around the rings and 
fastened by means of a *brass* screw 
passing through the ring. When 
the drum is completed it should 
resemble a "squirrel cage." 

Small grooves are cut in the 
cross bars to accommodate the 
wire. The wires should pass around the drum in the form 
of a spiral and be spaced about five-sixteenths of an inch 
apart. 

The ends of the wire should be fastened to binding-posts 
mounted on the heads. 

**The Secondary** is a single layer of No. 26 B. & S. silk- or 
cotton-covered wire wound over a cardboard tube, twelve 
inches long and three inches in diameter. 

The tube should be dried in an oven and then given a 
thick coat of shellac, both inside and out, before it is used. 
This treatment will prevent it from shrinkage and avoid 
the possibility of having to rewind the tube in case the 
wire should become loose. 

.. figure:: images/Image361.jpg
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   :alt: Fig. 320.—The Secondary Head. 
   
   Fig. 320.—The Secondary Head. 

The secondary is fitted with 
two circular wooden heads 
just large enough to fit 
tightly into the tube, 
having a half-inch flange, and 
an outside diameter of three 
and seven-eighths inches. 

**The Base** of the coil is 
fifteen inches long and six 
inches wide and is made of 
wood. 

The coil is assembled by 
placing the primary across 
the base and exactly in the 
center. Two long 
wood-screws passing through the 
base and into the primary 
heads will hold it firmly in 
position. 

The secondary is passed 
through the center of the 
primary and supported in that position by two hard rubber 
supports, four inches high, seven-eighths of an inch wide and 
one-half of an inch thick. A brass wood-screw is passed 
through the top part of each of the supports into the 
secondary heads so that a line drawn through the axis of the 
secondary will coincide with a similar line drawn through 
the axis of the primary. 

.. figure:: images/Image362.jpg
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   :alt: A COMPLETE COHERER OUTFIT AS DESCRIBED ON PAGE 274.
   
   A COMPLETE COHERER OUTFIT AS DESCRIBED ON PAGE 274.

.. figure:: images/Image363.jpg
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   :scale: 85 %
   :alt: THE TESLA HIGH FREQUENCY COIL.
   
   THE TESLA HIGH FREQUENCY COIL.

The supports are made of hard rubber instead of wood, 
because the rubber has a greater insulating value than the 
wood. High-frequency currents are very hard to insulate, 
and wood does not usually offer sufficient insulation. 

A brass rod, five inches long and having a small brass 
ball at one end, is mounted on the top of each of the 
hard-rubber supports. The ends of the secondary winding are 
connected to the brass rods. 

.. figure:: images/Image364.jpg
   :align: center
   :scale: 85 %
   :alt: Fig. 321.—End View of the Complete Tesla Coil. 
   
   Fig. 321.—End View of the Complete Tesla Coil. 

The lower end of each of the hard-rubber supports is 
fastened to the base by means of a screw passing through 
the base into the support. 

In order to operate the Tesla coil, the primary should be 
connected in series with a condenser and a spark-gap as 
shown in Figure 324. The condenser may consist of a 
number of Leyden jars or of several glass plates coated with 
tinfoil. It is impossible to determine the number required 
ahead of time, because the length of the connecting wires, 
the spark-gap, etc., will have considerable influence upon 
the amount of condenser required. The condenser is 
connected directly across the secondary terminals of the spark 
coil. 

When the spark coil is connected to a battery and set 
into operation, a snappy, white spark should jump across 
the spark-gap. 

If the hand is brought close to one of the secondary 
terminals of the Tesla coil, a small reddish-purple spark will 
jump out to meet the finger. 

.. figure:: images/Image365.jpg
   :align: center
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   :alt: Fig. 322.—The Complete Tesla Coil. 
   
   Fig. 322.—The Complete Tesla Coil. 

Adjusting the spark-gap by changing its length and also 
altering the number of Leyden jars of condenser plates 
will probably increase the length of the high-frequency 
spark. It may be possible also to lengthen the spark by 
disconnecting one of the wires from the primary 
binding-posts on the Tesla coil and connecting the wire directly 
to one of any one of the turns forming the primary. In 
this way the number of turns in the primary is changed 
and the circuit is *tuned* in the same way that wireless 
apparatus is tuned by changing the number of turns in the 
tuning coil or helix. 

.. figure:: images/Image366.jpg
   :align: center
   :scale: 85 %
   :alt: Fig 323.—Showing how a Glass-Plate Condenser is built up of Alternate Sheets of Tinfoil and Glass. 
   
   Fig 323.—Showing how a Glass-Plate Condenser is built up of Alternate Sheets of Tinfoil and Glass. 

The weird beauty of a Tesla coil is only evident when it 
is operated in the dark. The two wires leading from the 
secondary to the brass rods and the ball on the ends of the 
rods will give forth a peculiar *brush* discharge. 

If you take a piece of metal in your hand and hold it 
near one of the secondary terminals, the brushing will 
increase. If you hold your hand near enough, a spark will 
jump on to the metal and into your body without your 
feeling the slightest sensation. 

If one of the secondary terminals of the Tesla coil is 
*grounded* by means of a wire connecting it to the primary, 
the brushing at the other terminal will increase considerably. 

Make two rings out of copper wire. One of them should 
be six inches in diameter and the other one four inches in 
diameter. Place the small ring inside the large one and 
connect them to the secondary terminals. The two circles 
should be arranged so as to be *concentric*, that is, so that 
they have a common center. 

The space between the two coils will be filled with a 
pretty brush discharge when the coil is in operation. 

.. figure:: images/Image367.jpg
   :align: center
   :scale: 85 %
   :alt: Fig. 324.—A Diagram showing the Proper Method of Connecting a Tesla Coil.

   
   Fig. 324.—A Diagram showing the Proper Method of Connecting a Tesla Coil.

There are so many other experiments which may be 
performed with a Tesla coil that it is impossible even to 
think of describing them here, and the young experimenter 
wishing to continue the work further is advised to go to 
some library and consult the works of Nikola Tesla, wherein 
such experiments are fully explained. 

CONCLUSION
########## 

Unless the average boy has materially changed his 
habits, in recent years, it matters not what the preface of 
a book may contain, for it will be unceremoniously skipped 
with hardly more than a passing glance. With this in 
mind, the author has tried to "steal a march" on you, 
and instead of writing a longer preface, and including some 
material which might properly belong in that place, has 
added it here in the nature of a conclusion, thinking that 
you would be more likely to read it last than first. 

Some time ago, when in search for something that might 
be described in this book, I thought of some old boxes into 
which my things had been packed when I had dismantled 
my workshop before going away to college. They had been 
undisturbed for a number of years and I had almost 
forgotten where they had been put. At last a large box was 
unearthed from amongst a lot of dusty furniture put away 
in the attic. I pried the cover off and took the things out 
one by one and laid them on the floor. Here were 
galvanometers, microphones, switches, telegraph keys, 
sounders, relays, and other things too numerous to mention. 
They had all been constructed so long ago that I was 
considerably amused and interested in the manner in 
which bolts, screws, pieces of curtain rod, sheet-iron, brass, 
and other things had been taken to form various parts of 
the instruments. The binding-posts had almost in every 
case seen service as such on dry cells before they came 
into my hands. The only parts that it had been necessary 
to buy were a few round-headed brass screws and the wire 
which formed the magnets. In several instances, the 
latter were made so that they might be easily removed and 
mounted upon another instrument. The magnets on the 
telegraph sounder could be removed and fitted to form part 
of an electric engine or motor. 

One particular thing which struck me very forcibly was 
the lack of finish and the crudeness which most of the 
instruments showed. 

Of course it was impossible to avoid the clumsy 
appearance which the metal parts possessed, since they were not 
originally made for the part that they were playing, but I 
wished that I had taken a little more care to true up things 
properly or to smooth and varnish the wood, or that I had 
removed the tool-marks and dents from the metal work 
by a little filing. 

If I had done so, I should now be distinctly proud of my 
work. That is not to say that I am in the least ashamed 
of it, for my old traps certainly served their purpose well, 
even if they were not ornamental and were better back 
in their box. Perhaps I might be excused for failing in this 
part of the work through lack of proper tools, and also 
because at that time there were no magazines or books 
published which explained how to do such things, and 
when I built my first tuning coils and detectors nothing on 
that subject had ever been published. I had to work out 
such problems for myself, and gave more thought to the 
principles upon which the instruments operated than to 
their actual construction. 

The boys who read this book have the advantage of 
instructions showing how to build apparatus that has 
actually been built and tested. You know what size of 
wire to use and will not have to find it out for yourself. 
For that reason you ought to be able to give more time 
to the construction of such things. The purpose of this 
conclusion is simply a plea for better work. The 
American boy is usually careless in this regard. He often 
commences to build something and then, growing tired before 
it is finished, lays it aside only to forget it and undertake 
something else. *Finish whatever you undertake*. The 
principle is a good one. Remember also that care with the 
little details is what insures success in the whole. 

If in carrying out your work, you get an idea, do not 
hesitate to try it. A good idea never refused to be 
developed. It is not necessary to stick absolutely to the 
directions that I have given. They will insure success if 
followed, but if you think you can make an improvement, do 
so. 

Of course, such a book as this cannot, in the nature of 
things, be exhaustive, nor is it desirable, in one sense, that 
it should be. 

I have tried to write a book which, considered as a 
whole, would prove to be exhaustive only in that it 
treats of almost every phase of practical electricity. 

The principle in mind has been to produce a work which 
would stimulate the inventive faculties in boys, and to 
guide them until face to face with those practical 
emergencies in which no book can be of any assistance but 
which must be overcome by common sense and the exercise 
of personal ingenuity. 

The book is not as free from technical terms or phrases, 
as it lay in my power to make it, because certain of those 
terms have a value and an every-day use which are a 
benefit to the young experimenter who understands them. 

Any one subject treated in the various chapters of the 
"Boy Electrician" may be developed far beyond that 
point to which I have taken it. The railroad system could 
be fitted with electric signals, drawbridges, and a 
number of other devices. 

Many new ideas suggest themselves to the ready-witted 
American boy. I shall always be pleased to hear from any 
boy who builds any of the apparatus I have described, 
and, if possible, to receive photographs of the work. I 
should be glad to be of any assistance to such a lad, but 
remember that some of the drawings and text in this book 
required many hours even to complete a small portion, 
and therefore please do not write to ask how to build other 
apparatus not described herein. And, as the future years 
bring new inventions and discoveries, no one now knows 
but that, some day, perhaps I will write another "Boy 
Electrician." 

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THE END. 

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