pg 113

Musical Instruments.--Jack. Yes; let us take some sounds we know about and see what makes them. In the first place there is the bell. The hammer strikes it and makes it vibrate. It is just the same with a piano; the wire is struck and made to vibrate. A violin string vibrates. In an organ pipe or in a


Fig. 94 A glass plate vibrates when a fiddle bow is drawn across its edge so that the plate makes a sound. If you put a little clean dry sand on the plate, the sand will move so as to make patterns (as in the cut). By drawing the bow at different places you can get different patterns, especially if you lightly touch the plate with a lead pencil while the bow is moving. Some of the patterns are shown in the next picture.

trumpet the air vibrates. When you speak or sing a couple of elastic muscles in your throat vibrate. In a drum the parchment vibrates when the drumsticks strike. Something always vibrates first; that, whatever it is, sets the air to vibrating, and the vibration travels to where we happen to be and we hear a sound.

pg 114

Tom. How do you know that the bell vibrates?

Jack. The next time you are in the village go up in the clock tower when the clock is going to strike and hold a lead pencil against the bell. You can feel the bell vibrate.

Here is a curious thing to think of. First the bell vibrates and you can hear it for miles in every direction. Every


Fig. 95. Patterns made by Loose Sand on a Vibrating Plate. (See Fig. 94.)
After the patterns have been made they can be preserved by
carefully pouring varnish on the plate and letting it dry.


particle in a very large sphere of air is set in motion. We hear the sound at our house, miles away from the village. Now the air that is set in motion weighs hundreds of tons, and it is all moved by one stroke of the hammer on the bell.

pg 115

Tom. You can hear a locust chirping a quarter of a mile off. I suppose he sets the whole air in motion, too.

Jack. That is a very good example. A small insect moves tons and tons of air; and a violin string, vibrating so little that you can hardly see it move, stirs all the air in a great concert hall.


Fig. 96 A watch ticking in front of one mirror can be plainly heard through a tube placed in front of another. If you take the second mirror away, you cannot hear it at all. The first mirror acts as a speaking trumpet (a megaphone), and the second mirror acts as an ear trumpet.

Sometimes when the organ is playing a low note in church you can actually hear the air flutter and vibrate. The organ makes a noise then, not music.

Mary. What is the difference between noise and music, Jack?

pg 116

Jack. If the vibrations of a bell, a violin string, an organ pipe--anything--come at even intervals, then they make a musical note. If they come irregularly, the sound is usually a mere noise. Music is pleasant to hear, and noise is not. That is the real difference.


Fig. 97. Echoes An echo is made by the reflection of sound from a wall, a rock, etc.
The person who speaks must be at least 100 feet away from the wall to get a good echo.


Reflection of Sound.--Sound can be reflected somewhat as light is, as the following experiment shows.

Musical Notes.--Mary. Are the sounds from my piano regular?

Jack. Yes; perfectly regular. Each string vibrates regularly just so many times in a second, no more and no less. The middle C of your piano is a wire just long enough to vibrate 261 times every second, and all of its vibrations are alike.

The shorter a string is the quicker it vibrates, and you will notice that the highest notes of your piano come from the

pg 117

shortest strings. It is the same with drums; the small drums give the highest notes, the large drums the lowest.

The phonograph is a machine for recording the vibrations of the air that are made when a person speaks. He speaks into a tube (F in Fig. 98) and sets the air into vibration. At the small end of the tube is a little round thin metal plate that moves up and down (slightly) as the air vibrates. The motions of this little plate copy the vibrations of the air. On the lower side of this thin plate is a sharp needle point. (See Fig. 99.)


Fig. 98. The Phonograph

While the person is speaking the barrel (A), which is covered with tin foil, is turned by the crank, and the little needle makes marks on the tin foil. These marks are the record of the human voice. Every vibration of the voice has left its mark on the tin foil. If now we put a piece of tin foil so marked into the machine and turn the barrel, what will happen? As each one of the marks in the tin foil passes underneath the small needle the needle will move a little (if the mark in the tin foil is shallow) or moves a little more (if the mark in the tin foil is deep). The needle will move up and down for the tin foil just as it formerly moved up and down for the voice. As the

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needle moves, so must the thin plate move; and as the plate moves and vibrates, so must the air above it move and vibrate, and you will hear from the machine now the very words you spoke into it an hour ago, or a year ago, or twenty years ago, whenever the record on the tin foil was made. You can keep the tin foil and repeat the words whenever and wherever you like.

Tom. If the phonograph had been invented in Julius Caesar's time, we might be able to hear his voice now, or George Washington's, or Lincoln's.


Fig. 99

Jack. The records of the speeches of some of the great men of today actually have been preserved; and long after they are dead, so long as the little pieces of tin foil last, other people will know exactly how they spoke.

Mary. It would be a find thing for us to get Eleanor to sing into the phonograph now, so that when we go home after vacation we could still hear her!

Jack. A wise man in England (1) once suggested that there could be no worse punishment in the future life than to be forced perpetually to hear all the foolish things you had said in this life. It might not be a bad way to punish naughty boys and girls in this world to shut them up in a room with phonographs that would continually repeat the silly and foolish things they had said.

Agnes. I think it would be dreadful, Jack. Nothing could be worse.

Jack. Very well, my dear, you need not mind. The things you say are always nice to hear. I was only trying to frighten the boys.

(1) Charles Babbage (born 1792, died 1871).

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The children made some experiments in electricity which any one of you can make too, if you like. You will need a few things, most of which you can get at home or make for yourself. A few you will have to buy (they do not cost much). The principal things to get are: a couple of toy magnets, one straight, one shaped like a horseshoe; a piece of glass tube (or a glass rod) about half an inch in diameter and eight or ten inches long; a piece of sealing wax about half an inch square


Fig. 100

and about six inches long; a rubber comb; an old silk handkerchief; a piece of old flannel; an ounce of sulphuric acid in a bottle with a glass stopper (be careful not to get the acid on your hands, and be sure that the bottle is labeled Sulphuric Acid); an ounce of quicksilver in a bottle (be sure that the quicksilver is labeled; it is poisonous if swallowed); about twenty feet or so of insulated copper wire (No. 18 annunciator wire is the most handy to use); a piece of sheet copper about three sixteenths of an inch thick, one and one half inches wide, and five inches long; a piece of sheet zinc of the same size as the copper. Take the copper sheet and the zinc sheet to a plumber and have him solder a piece of copper wire (each piece about twelve inches long) at A and B. After this is done

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take a large tumbler, fill it two-thirds full of water, pour three tablespoonfuls of sulphuric acid in it (use an old kitchen spoon for this purpose), dip the zinc plate in it, and leave it there for a minute. Then take the zinc plate out, hold it over a china plate, pour quicksilver on it, and rub the quicksilver on to the surface of the zinc until it is all covered and shining. (Do not empty the water and acid from the tumbler; you will need it by and by; save it.) Now you have all the things you need for your experiments, but it is convenient to get two double connectors (so called) like Fig. 101.


Fig. 101 A double connector (so called) is a cylinder of brass with two holes in it and with two screws. It is used to connect the ends of two wires and saves the trouble of twisting the ends together. It is convenient, though not necessary.

Jack. Before we begin our experiments with the things you have collected, tell me what you already know about electricity. You have heard it talked about. Tell me what you have seen on your own account.

Agnes. Well, lightning is electricity, they say.

Mary. And electric bells ring by electricity, and some street railways go by electricity.

Fred. And then there is the electric telegraph.

Tom. Yes, and the telephone, and the electric light.

Jack. All these things have to do with electricity. Let us begin by making some lightning.

Agnes. Oh, Jack! make lightning? It would be dangerous.

Tom. Agnes thinks Jack can make anything--even a thunderstorm if he wants to.

Jack. Well, Agnes, the lightning we are going to make will not be dangerous; but I will put off making it for a little while and begin with something else.

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Here is a lot of small pieces of tissue paper--they are very light, you see--laid on the table. Now take the glass rod, Agnes, and hold it over them. What happens?


Fig. 102 Little pieces of tissue paper (or light grains of sawdust) are attracted by a glass rod rubbed with a silk handkerchief (or by a piece of sealing wax or a rubber comb rubbed with flannel).

Agnes. Nothing happens at all.

Jack. Try the rubber comb, Mary.

Mary. Well, nothing happens.

Jack. Now, Agnes, rub the glass rod briskly with the silk handkerchief; and you, Mary, rub the comb with the flannel; and try again; Agnes first.

Agnes. Why, Jack! the little pieces of paper rise up to meet the glass. (See Fig. 102.)

Jack. Take the glass rod away, Agnes; and now, Mary try with your comb.

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Mary. It is just the same thing; the little pieces of paper rise up to meet the comb--it is like magic.

Jack. We have learned something, anyway. What have we learned, Fred, so far?

Fred. We have learned that if you rub a glass rod with silk, the rod will attract pieces of paper as a magnet attracts pieces of iron.

Tom. And that if you rub a piece of rubber (1) with flannel, the same thing happens.

Jack. That is very good so far. Now, Agnes, rub the glass rod with the flannel, not the silk; and Mary, rub the comb (2) with the silk, and both of you try once more. What happens?

Agnes. Nothing happens now.

Mary. Nothing happens with I try with the comb either.

Jack. Well, we have learned that to lift the little pieces of paper with a glass rod you must rub the rod with silk, not flannel; and the comb with flannel, not silk. Glass rubbed with silk is made electric--electrified, as they call it; and rubber (or sealing wax) rubbed with flannel is electrified. When either glass or rubber is so electrified it will attract little pieces of paper, or light grains of sawdust.

I want you all to try this experiment, too. Electrify the glass rod and the comb and then hold them near your face. What happens?

Agnes. Why it tickles! it feels as if there were a cobweb on my cheek.

 (1) A piece of sealing wax rubbed with flannel will act just as the rubber comb acts. Try it.
(2) A piece of amber does the same thing. The Greek name for amber is elektron, and we get the name "electricity" that way.

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Jack. Tom, take the glass rod and rub it smartly with the silk. Now hold your knuckle close down to the tube. See there is a little spark.

Tom. I felt it and I heard it, too.

Jack. Agnes, that spark was lightning and the crackling noise was thunder, only they are not dangerous. Real lightning is just the same kind of thing as that little spark, and real thunder is just like the little noise that spark made. Perhaps you know that in 1752 Benjamin Franklin sent a kite up in the air during a thunderstorm and brought down some of the electricity that was in the clouds and proved that the lightning in the sky was exactly the same thing as the spark you have just seen.


Fig. 103. A Long Electric Spark between Two Electrified Balls--Lightning takes the shape of this spark.
It is never a zigzag bolt made up of straight lines, as it often seems to be.


Now you children have seen the kind of electricity that makes thunder and lightning. Let us make some of the kind that they use in the telegraph. I want to make a current of electricity that I can use to carry a message from New York to San Francisco.

Now we shall need our tumbler of water with the acid in it and the strips of copper and zinc. (See page 119.) Stand the

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two strips upright in the tumbler and put some strips of wood across the top of the tumbler to keep the zinc and copper apart. They must not touch anywhere. When you have arranged this all right so that everything will stay in place you have got a battery, and if you join the two wires (see the picture, Fig. 104) a current of the electricity will flow from the copper plate to the zinc. I am going to prove it to you.

Take the end of the wire from the copper and put it on one side of your tongue and put the wire from the zinc on the other side, and you'll feel a little current passing. The current goes from the copper through the wire, and through your tongue to the zinc. Your tongue connects the two wires. If you actually join the two wires, the current will be there just the same. Feeling it with your tongue proves that it is there, and that is what I wanted to prove. If you had two such batteries joined together, (1) you would have a current twice as strong. With many batteries joined together you would have a current strong enough to travel over a wire as long as from New York to Boston; and that is the kind of electricity they use in telegraphing.


Fig. 104 A glass jar containing dilute sulphuric acid with a plate of zinc and a plate of copper in it (they must not touch each other) is called an electric battery. If you join the copper (C) and the zinc (Z) plates by a wire (M), a current of electricity will flow from C to Z through the wire; no matter how long the wire is, the current will still flow. It would flow (with a strong current) from New York to Boston.

(1) The zinc of one battery to the copper of the next one.

pg 125

The Telegraph.--"I understand how you send a current of electricity from New York to Boston," said Tom; "you have a


Fig. 105. A Battery of Fifteen Cups Notice that the zinc of one cup is connected to the copper of the next one, and so on. At the ends there are two short wires marked + and--. If you join these to two telegraph wires reaching a distant town, a current of electricity will flow from + to the distant town and back from the town to--. There would be a continuous circuit of wire from + to the town, and back again to--. If you cut the circuit of wire anywhere and put the two ends of the wire to your tongue, you will feel the current. That is proof that the current is always there, in the wire. It is always flowing so long as the battery is joined to the long loop, or circuit of wire.

battery at New York and a loop of wire--what you call a circuit--going to Boston and returning to New York, this way:


Fig. 106 shows rectangle, with Boston at one end, Battery in New York at the other end, and wire going from New York to Boston on one side of the rectangle, and wire back again from Boston to New York on the other side of the rectangle.

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But I don't see how you make the signals. The current flows through the wires quietly; it makes no noise."

Fred. You have to put telegraph instruments--a key and so forth--on the wire, don't you?

Jack. Yes; we can improve Tom's drawing by putting them in, this way:


Fig. 107 (figure as in Fig. 6, but including a telegraph instrument listed on each end,
with New York and Boston respectively)


The battery in New York is all the while sending a current of electricity along the wire. It fills the whole of the wire from New York to Boston and back again. It flows along the wire and through the telegraph instruments at both places. When you wish to talk to Boston you move your New York key up and down, and the receiving instrument in Boston makes little sounds, one sound for each motion of the New York key. You can arrange an alphabet that way. For instance, three dots (. . .) might be C, one dot (.) might be E, and two dots (. .) might be I. You could spell ice, for example, this way: [ .., ..., . ].

Fred. And Boston could talk to New York by dotting with the Boston key, and New York would hear it.

Jack. That is exactly the way it is done. Go into a telegraph office sometime and listen, and you will hear the instruments clicking away. Sometimes they make dots and

pg 127


Fig. 108 This figure shows the way in which New York and Boston are connected by telegraph. It is more complicated than the way described before, but the idea is the same. The key at New York is marked K (the K on the right-hand side). If this key is tapped, a signal goes over the wire to Boston and is received on a sounder there. (See the picture of the sounder at the bottom of the cut.) In the same way signals made with the Boston key are heard on the New York sounder.

sometimes larger sounds called dashes, and sometimes very long dashes. The alphabet they use is:


Fig. 109 [table of Morse Code]

pg 128

Velocity of Electricity.--"You children must remember," said Jack, "that electricity travels at the same speed as light--it travels 186,000 miles in a second. They once sent a message from Cambridge through Canada to San Francisco, returning by Omaha and Chicago, and it took only four seconds for the message to go those 7000 miles. If the telegraph instruments had been perfect, the message would have gone instantaneously. As it was it did not take long."

Magnetism.--Jack. Suppose we stop talking about electricity for a while and learn something about magnets. There is a magnet in every telegraph sounder, in every telephone, and in every dynamo, and I want you to understand how they are used. But we will begin far off and come to these complicated machines by and by. In the first place, Fred, what is a magnet?


Fig. 110 A straight magnet held in the hand
will attract little pieces of iron and
will make each of them into magnets,
so that they will hold up other small pieces.


Fred. A magnet is a piece of iron or steel that attracts other pieces of iron.

Tom. Try your straight magnet on these iron filings, Fred.

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Jack. There is a special thing to notice. A bar magnet attracts iron filings, tacks, and so forth, at its ends, but not at its middle. It is just the same with a horseshoe magnet. Try it.

We have learned one thing. Magnets of all shapes attract iron filings, tacks, needles, and so forth, to their ends, not to their centers.


Fig. 111 A straight magnet--a bar magnet--attracts
iron filings to its ends, but not to its middle part.


Here are four little piles of sawdust, of copper filings, of sand, and of coat dust. Try to pick them up with your magnets.

Agnes. They do not move; magnets do not attract such things as sand.

Jack. No; magnets attract iron and steel and nothing else. If you take a pile of copper filings and iron filings mixed together, the magnet will pick up the iron filings and leave the copper. Try the experiment and see for yourself.


Fig. 112 A horseshoe magnet attracts iron filings to its ends;
but if you try the curved part of the magnet on the needle,
there is almost no attraction.



Fig. 113. A Horseshoe Magnet with an Iron Bar (an armature) across its Ends.

Tom. So it does. That is a way of sorting iron out of a pile. If some one told me to pick the iron filings out of this pile by hand, it would take all day to do it; but with a magnet I can do it in five minutes.

Jack. See what the magnet will do through a pane of glass. Lay a needle on a pane of glass held horizontally and

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put the magnet under the glass. You will see that the needle moves over the glass as you move the magnet around.

Tom. So it does; glass does not stop the attraction.


Fig. 114 Iron filings on a horizontal pane of glass will move into a certain set of curves when you hold a horseshoe magnet underneath the glass. (You must tap the glass gently with your finger tip.)

Jack. Try putting the needle on a sheet of writing paper or on a piece of silk.

Tom. It is just the same; the needle moves when I move the magnet.

Jack. So much is clear; a magnet is made of iron; it attracts iron and nothing else; it attracts it through silk, or paper, or glass--through anything.

These magnets that you have been using are manufactured. They were made. Let us make some more. Agnes, have you got any needles?

Agnes. Here are some.

Jack laid the needles on the table and rubbed them with the horseshoe magnet, as if he were stroking them with it. (1) He tried each needle on the pile of iron filings, and every one was able to lift up some filings just as the horseshoe magnet did. Then he took two of the needles and tied a bit of silk about each, near its middle, and hung the silk from

(1) Make all the strokes on all needles in one direction, so as to have the needle magnets all alike. Stroke all of them from eye to point, or from point to eye.

pg 131

two pencils (see. Fig. 115), so that he had two little magnets, like pendulums. Next he took the bar magnet--a straight magnet--and tried some experiments with needle No. I (the other needle was laid aside for the moment). The bar magnet had two ends of course; one was the point, and the other happened to be painted red.


Fig. 115


Fig. 116

By trials with needle No. I he found:

1. That the point of the bar magnet attracted the point end of needle No. I.
2. That the point of the bar magnet repelled the eye end of the needle No. I.
3. That the red end of the bar magnet repelled the point end of needle No. I.
4. That the red end of the bar magnet attracted the eye end of needle No. I.

Then he tried needle No. 2 and found just the same things for it also.

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5. The point of the bar magnet attracted the point end of needle No. 2;
6.--and repelled the eye end of No. 2.
7. The red end of the bar magnet repelled the point end of No. 2;
8.--and attracted the eye end of No. 2.

The next thing was to put aside the bar magnet and to try the two needles together. He found:

9. That the two points of the needles repelled each other.
10. That their two eye ends repelled each other.
11. and 12. That the point end of either needle attracted the eye end of the other. (1)

Tom. What is the explanation of all these experiments, Jack?

Jack. It is like this: just suppose there were two kinds of magnetism in the bar magnet. We might call them point-end magnetism and red-end magnetism, for want of better names. Now when we made magnets out of these needles we put the two kinds of magnetism into them. We put one kind into the point ends of both needles and another kind into their eye ends. Suppose we say that point-end magnetism, wherever it is found, will repel point-end magnetism; and that red-end magnetism, wherever found, will repel red-end magnetism; and that point-end magnetism will attract red-end magnetism, and vice versa, wherever they are found. Would not that explain all that we have seen?

Taking all the twelve cases one by one, the children found that the explanation was right. Magnetism of the same name repels; magnetism of different name attracts. It is not easy

(1) These experiments take some space to describe, but they are so interesting that they should be tried in the schoolroom.

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to explain in simple words why this is so; but any child who will pay attention and make these simple experiments can prove it.

Natural Magnets.--"These magnets are artificial; they are manufactured," said Jack; "but there are stones that are magnetic to begin with. They were first found in Magnesia, a town of Asia Minor, long ago, and the ancients therefore called them magnets."

Mary. In the Arabian Nights, in "Sinbad the Sailor," there is a story of a whole mountain made of magnets, so that when a ship came that way the mountain pulled all its iron nails out, and the ship broke to pieces and sank.

Agnes. That isn't true, is it Jack?

Jack. Certainly not, my dear; it is one of the big stories told by travelers. But don't you recollect how they got past the mountain with their ships?

Mary. They built their ships with wooden pins instead of nails and got safely past, so the story says.

Electro-Magnets.--Jack. There is another kind of magnet that I want you to know about. It is made by a current of electricity from a battery passing through a wire wrapped round a bar of soft iron. (See Fig. 117.)

You see now how a telegraph operator in New York can make a click on the sounder in Boston. The battery current is flowing all the time except just at the moment when the New York man lifts his key and breaks the circuit.


Fig. 117 If wire be wrapped in a spiral around a bar of iron, and if a current of electricity flow through the wire,
the bar becomes a magnet and stays so as long as the current is flowing, and no longer.


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The electro-magnet of the sounder in Boston is a magnet so long as the current flows, and stops being a magnet the instant the current stops.


Fig. 118 Electro-magnets are often made of a core of soft iron bent into the shape of a horseshoe, and wound with wire. The two ends of the wire go to the copper and zinc of a battery. So long as the current flows the iron core is a magnet. When the current stops it is no longer a magnet.

Whenever the New York man lifts his key the Boston sounder makes a click--a dot or a dash, just as he chooses. In that way the message is spelled out.


Fig. 119

Electric Bells.--"Now," said Jack, "it is easy to understand how electric bells work. It is like a telegraph. In the first place you must have a battery. We could make a battery by using

pg 135

several tumblers (like those described on page 124), but it is more satisfactory to buy one cell of "dry" battery, so called.


Fig. 120. The Telegraph Key


Fig. 121. A Repeating Sounder The coils of its magnets are vertical. The armature is fastened to the horizontal bar which moves as the armature moves and clicks against the point of the little screw about it.


Fig. 122. A Cell of Dry Battery

"We must run our wire along one station to another like this:"


Fig. 123


Fig. 124. A Push Button--It is like a very simple telegraph key. When you push it two ends of the wire are connected so that the current from the battery can flow to the bell and ring it. Until the button is pushed the circuit is broken and the current cannot flow. If you should take away the push button and join the ends of the wire where it now is, the battery current would flow continuously and the bell would ring all the time.


Fig. 125. An Electric Bell--When the push button is touched the current from the battery flows along the wire into the box and round the coils shown in the picture. So long as the current is flowing the soft iron inside the coils is a magnet and attracts the piece of iron which is the hammer (K) of the bell (T). But this piece is a vibrating spring and it keeps moving to and fro and sounding the bell. The moment that the push button is released the current stops flowing and the bell stops sounding.

pg 137


Fig. 126. An Electric-Bell Outfit Complete--It can be bought in this form with seventy-five feet of wire and staples to fasten the wire for about $2.75.


Fig. 127. The Telephone--F is a handle; turn it and the bell (G) will ring on your telephone and also at the other end of the line. The man you wish to talk to will hear it. He has another instrument just like yours. Take down your telephone (B) and put it to your ear. Speak into your transmitter (C) and he will hear you in his telephone. When he speaks into his transmitter you will hear him in your telephone.

pg 138

The Mariner's Compass.--"You know that a magnetized needle points north and south," said Jack. "A compass needle will point to the north no matter to what part of the earth you take it.


Fig. 128. The Telephone--One view shows the telephone as it really is; the other as it would look if it were split down the middle so as to show what is inside. 'A' is the long steel magnet wound with fine wire (B). The ends of the spool of wire (B) are connected to the outside posts (D,D). Close to the magnet A (Near B) there is a thin iron plate (CC) which vibrates so as to copy the voice of the person speaking to you. That person speaks into his transmitter. (See Fig. 127.) The vibrations of his voice make vibrations in the disk of his transmitter; these vibrations are sent along the telephone wire and come to your telephone; there they make the disk (EE) of your telephone vibrate just as his voice vibrated; the disk (EE) makes the air in your telephone vibrate like the speaker's voice, and you hear him speak.



The reason is that a current of electricity is flowing round and round the earth all the time and that any magnet will always arrange itself at right angles to a current, if it can.

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The fact is so, and I am going to prove it." So Jack took one of the little magnetized needles (Fig. 115) and let it swing freely. It swung so as to point to the north and rested in that direction, thus:


Fig. 130

Then Jack took the two ends of the wire from his battery and made them parallel to the needle, being careful not to touch the ends together, this way:


Fig. 131

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No current was flowing, and the needle remained as it was before. Then he joined the ends A and B. A current flowed through the wire, and immediately the needle moved round and pointed west and not north (Fig. 132).

"You see," said Jack, "the needle moves so as to be perpendicular to the direction of the current. A current is always flowing round and round the earth from east to west. The sun makes the current. The compass needle is always perpendicular to the direction of the current, and that is why the mariner's compass points to the north. It is a good thing for us that it does so.


Fig. 132

Sailors can make long voyages and always know which way is north whether the stars are shining or not. They do not need the north star any more."

The Electric Light.--The first electric light was made about a hundred years ago by using a battery of 3000 cells. (See Fig. 105.) The wires from the ends of this immense battery were brought close together, and the spark between the ends did not come and go as lightning does, but was steady, like our electric street lamps. The current from so many cells made a great heat as well as a brilliant light. The ends of the wires were melted off where the light was produced, and they

pg 141


Fig. 133. The Carbons of an Electric Street Lamp


Fig. 134. An Electric Street Lamp--Such lamps are as bright as 1000 candles.


Fig. 135. An Electric Light Like Those used in Houses--It is called an incandescent lamp, because the carbon filament (Lnn in the picture) does not waste away but simply glows. The glass globe has no air in it. The air is pumped out when the lamp is made, and the glass is then sealed tight. It screws into a socket in the chandelier by a screw at B. The light is turned off and on by a button (not shown in the picture) as gas is. Turning the button turns off the electric current. Such lamps give as much light as sixteen candles would do.

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were obliged to use carbon ends (round sticks of coal dust or coke) at the ends, just as we do to-day.

The Dynamo.--It is possible to make batteries of thousands of cells, like those shown in Fig. 105, so powerful as to do the work of electric lighting; but it is very troublesome and expensive. A much simpler and cheaper way to get the current that is needed is to use a dynamo driven by a steam engine.


Fig. 136. A Dynamo-Electric Machine--A belt from a steam engine is put on the wheel at the right of the picture and turns this wheel very rapidly. The central part of the dynamo is a large stationary electro-magnet. Fastened to the revolving wheel (and not visible in the picture) are a number of small electro-magnets. When these small electro-magnets are revolved very rapidly in front of the large magnet a strong current of electricity is made, and this current is carried off on wires to where we wish to use it. It will light lamps or drive a street car, etc.

The steam engine is used to turn a set of little electro-magnets in front of a larger magnet. When this is done a current of electricity flows through two wires leading from the machine, and these wires can be led to the place where we want to use the current--to a distant part of the city to light

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lamps, or to drive electric cars. Lamps are lighted by letting the current from the dynamo pass through them.

Electric Railways.--Street cars are driven in this way. Underneath each car is a dynamo (called a motor) fastened to the wheels.


Fig. 137. Part of the Front Truck of a Street Car (Showing the wheels and the motor between them)


Fig. 138 An Electric Street Railway--The power house with its dynamo (D) driven by a large steam engine is shown on the left-hand side. From this dynamo a current goes out on an overhead wire (A). A moving trolley (T) on each car takes the current to the motor. The motor turns the wheels whenever the motorman turns the current on, and stops turning them whenever he shuts the current off.

APPENDIX

Some of the experiments that were tried by the children are given here. Nearly all of them can be repeated in the schoolroom or by children at home who will take the trouble. It is well worth while to do it, because we learn so much more by really doing a thing than by merely talking or reading about it. The teacher can readily buy or make the simple apparatus described; and, once made, it will serve for successive classes. Nearly every child has a father, or an older brother, or a friend, who will help him to make these experiments at home if they cannot be seen at school.

What Kind of Things Bodies are.--We need a convenient name for solids, liquids, and gases; let us call them bodies, and say that a piece of iron is a solid body, a lake of water is a body of liquid, etc. When we think about any body of this sort--a nugget of gold, for instance--we always think of it as filling some space.

Extension.--All bodies are extended; they fill a space. Even a sponge fills a space; the holes in the sponge are full of air, and the air in a sponge fills a space and has a shape of its own.

Impenetrability.--Where one body is, another body cannot be at the same time. Putty is soft and can be molded into almost any shape, but where the putty is, nothing else can be at the same time. It completely fills its own space.

Divisibility.--Every body can be divided into two halves, and each of those halves into halves again, and so on. If you will get from the druggist a little piece of permanganate of potash (write the name down) and put it into a hogshead of water, you will find that the whole of the water has been colored red. Every drop of water that you take up in your hand is red, and there are millions of drops in the hogshead. That means that the little piece of permanganate of potash has been divided into millions of smaller pieces, and that every single drop of water has several of those small pieces in it; for it takes more than one piece to color a whole drop of water.

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If you put a piece of musk no larger than a green pea (you can buy musk from any druggist) in a room, it will scent the room and everything in it, and it will keep on doing so for years and years. Leave a towel in the room over night, and the next morning every thread of the towel will smell of musk. You could go on leaving towels in the room for a dozen years and taking them away after one night, and every thread of every towel would show that the musk had been near it. That means that every one of the threads of every one of the towels has several particles of musk on it; and it means that the original piece of musk (which seems hardly to grow any smaller) has been divided into millions of little pieces.

Cohesion.--If you take two bars of soap and press them together under a press, you can make one piece out of the two. That piece is held together by a force that we call cohesion. All solids are held together by such a force. One part of a lump of iron is held to the other parts by cohesion. It requires a good deal of pulling to pull one part of an iron rail away from the other parts (though it can be done). You can weld two pieces of iron together (by heating) so that they become one piece.

If you stretch a solid body (or compress it) and then take away the force that was stretching (or pressing) it, the body will usually spring back to its first shape. A piece of India rubber stretched (or compressed) flies back to its first shape as soon as you stop forcing it out of shape. A bent steel knitting needle flies back into shape very quickly. India rubber, steel, glass, and indeed most solid bodies, are elastic. If you strain them a certain amount, they will spring back into shape like the springs of a buggy. If you strain them too much, they sometimes lose their elasticity like the springs of a farm wagon that has been used to carry very heavy loads. Most solid bodies are elastic; all liquids are so.

Viscosity.--Did you ever see very cold molasses flowing from a spigot? It is viscous--a little like a solid and a little like a liquid at the same time. Warm it, and it becomes like a liquid. Tar that is very hot acts like a liquid; as it cools it is viscous; when it is perfectly cold it becomes a solid. Water is not viscous; it flows freely.

All Bodies are Heavy.--All Bodies--solids, liquids, and gases--have weight. A cubic inch of any solid is usually (not always) heavier than a cubic inch of any liquid. Iron will sink in water, but wood will float on it. Iron itself will float on quicksilver. The gases have weight. Air has weight, for instance, as the barometer proves. (See page 84.)

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Hardness.--By a little trouble any child can get pieces of soapstone (talc) (1), rock (2), fluor spar (4), fieldspar (6), quartz (7). The numbers 1, 2, 4, 6, 7 denote the degree of hardness of these stones. The very hardest stone is a diamond, whose hardness is 10. Rock salt (2) will scratch soapstone (1); feldspar (6) will scratch fluor spar (4); quartz (7) will scratch all of them and will scratch glass, too. You can write your name on glass with a piece of pure quartz. A diamond will scratch every stone. If you want to say how hard a stone is, you can give its hardness in a number. Topaz is 8; it will scratch quartz but not diamond.

Ductility.--You can draw some metals out into long fine wires. These are the ductile metals, like gold, silver, iron, copper, etc. Glass can be drawn out into fine threads by heating it. Gold can be hammered out into leaves so thin that 30,000 of them, piled one above another, would be only an inch high. If you were to press these leaves under a strong press, they would go back into a gold plate by cohesion. (See page 145.) A body is called malleable when it can be hammered out into thin sheets. Copper, for instance, is very malleable.

Crystals.--Buy three ounces of alum at the druggist's and pound it into a fine powder and put the powder into a tumbler full of very hot water, stirring the alum in with a glass rod until all is dissolved. They lay a bit


Fig. 139. How to make Alum Crystals

of stick across the mouth of the tumbler with a short string hanging down into the water. (See Fig. 139.) Put the tumbler in a cool place and look at it the next day and see the beautiful crystals of alum that have formed. The hot water kept all the alum dissolved. As the water cooled,

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some alum was freed, and it formed into its own kind of crystal. Everything has its particular way of crystallizing. Alum makes one kind of crystal, quartz another.

You can guy some rock salt, some bichromate of potash, and some blue vitriol at the druggist's also, and make crystals out of these substances.


Fig. 140. Different forms of Snow Crystals.

just as you made the alum crystals. Each substance will crystallize in its own way. You can save some of the best crystals in wide-mouthed glass bottles, tightly corked, and begin to collect a cabinet of crystals for yourself.

Freshly falled snow (that is, frozen water) makes cyrstals, as you can see on a window pane in the winter time.




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