THE TELEPHONE

You probably all know that the telephone is an electrical instrument by which one person may talk to another who is at a distance. Not only can we talk to a person who is in a different part of the city, but such great improvements have been made in these instruments that we can talk through the telephone to a person in another city, even though it be hundreds of miles away.

The main principle of the telephone is electromagnetism, as in the telegraph, but there are other important points in addition to those we mentioned in describing the latter.

Let us take first the

INDUCTION-COIL

You will remember that an electromagnet is made by winding many turns of wire around a piece of iron and sending a current of electricity through this wire.

Now, suppose this current of electricity was being supplied by two cells of a battery. If you took in your hands the wires coming from these two cells, giving, say, four volts, you could not feel any shock; but if you were to take hold of the ends of the wires on the electromagnet and separate them while this same current was going through, you would get a decided shock.

This separation would "break" the circuit, and the reason you would get a shock is that, while the electricity is acting on the wire, the iron itself is magnetized, and on breaking the circuit reacts upon the wire, producing for a moment more volts of pressure in every turn of it. Thus, you see, this weak pressure of electricity as it travels through the wire can yet produce, through its magnetism, strong momentary effects, but you cannot feel it unless you break the circuit.

HOW THE INDUCTION-COIL IS MADE

The object of the induction-coil is to produce high intensity, or pressure, from a comparatively weak pressure and large current of electricity; so, if we add still more wire, the magnet has a larger number of turns to act upon and thus makes a very strong pressure, or large number of volts, but a lesser number of ampÈres.

Instead of taking one piece of iron, as we would for an ordinary electromagnet, we take a bundle of iron wires in making an induction-coil, as these give a stronger effect. Around this bundle of wires we wrap many turns of insulated copper wire. This is called the primary coil, and the ends of this wire are to be attached to the battery.

On top of, or over, this primary coil we wrap a great many turns of very fine wire, of which, as it is so fine, a great length can be used. This is called the secondary coil, and it is in this coil that the volts, or pressure, of electricity become strongest.

Above we show you a sketch of an induction-coil. (Fig. 9.)

At the left-hand side of the cut is a "circuit-breaker," which is simply a piece of iron (armature) on a spring placed opposite the iron core. This armature is made a part of the wire leading to the primary coil. When the current from the battery is sent through the wires, the core becomes magnetized and draws this armature away from a fixed contact point, thus breaking the circuit, but the spring pulls it back, again completing the circuit, and so it keeps going back and forth very rapidly with a br-r-r-ing sound.

If you were now to take hold of the ends of the secondary coil you would get a continuous series of quick shocks which would feel like pins and needles running into you.

Perhaps most of you have taken hold of the handles of a medical battery and have had shocks therefrom. In so doing, you have simply had the current from the secondary of an induction-coil. The current may be made weaker by sliding a metallic cover over part of the iron core and so shutting off part of the magnetic effect.

SPARKING COILS

While on this subject we may add that these coils will produce sparks from the two ends of the wire of the secondary coil. These sparks vary in length according to the amount of wire in the coil. Small ones are made which give a spark a quarter of an inch in length, while others are made which will give sparks 10, 12, and 16 inches in length. In the latter, however, there are many miles of wire in the secondary coil.

The largest induction-coil known is one which was made for an English scientist. There are 341,850 turns, or 280 miles, of wire in the secondary coil. With 30 cells of Grove battery this coil will give a spark 42 inches in length. You may form some idea of the effect of this induction-coil when we state that if we desired to produce the same length of spark direct from batteries, without using an induction-coil, we should require the combined volts of pressure of 60,000 to 100,000 cells of battery.

Having explained to you briefly the induction-coil—how it is made and its action—we must ask you to bear these principles in mind, and presently we will tell you how it is used in the telephone.

The next thing we shall try to explain will be

THE VIBRATING DIAPHRAGM

Did you ever take the end of a cane in your hand, raise it up over your head, and then bring it down suddenly and sharply, so that it nearly touched the ground, as though you were about to strike something? If not, try it now with a thin walking-cane or with a pine stick about three feet long and one-half inch thick, and you will find that there is a peculiar sound given out. It is not the stick that makes this sound, but it is owing to the fact that you have caused the air to vibrate, or tremble, and thus give out a sound.

If you strike a tuning-fork sharply you will see the ends vibrate and a sound will be given. If you put your fingers on top of a silk hat and speak near it you will feel vibrations of your voice.

Every time you speak you cause vibrations of the air; and the louder and higher you speak the greater the number of vibrations.

Suppose you take a thin piece of wood in your hands (say, for instance, the lid of a cigar-box cut in the shape shown in the picture, Fig. 10) and hold it about two inches from your mouth and then speak. You will feel the wood tremble in your hand. This is because the vibrations of the air cause the wood to vibrate in the same manner. These vibrations are very minute and cannot be seen with the naked eye, but they actually take place, and could be measured with a delicately balanced instrument.

Now let us try another experiment in further illustration of this principle. We will take a tube about three inches long and one and one-half or two inches in diameter. This tube may be made of cardboard. Now cut out a piece of thin cardboard which will just fit over one end of the tube. This piece we will call the "diaphragm." Fasten the diaphragm by pasting it with two strips of thin paper to the tube. These strips of paper should be fastened only on the ends, and the middle of the paper allowed to be slack, as shown in the picture, so that the diaphragm may work backward and forward easily. Take a small shot about the size seen in the sketch and tie it to a single thread of fine silk, then let it hang as shown in the sketch (Fig. 11), so that it will only just touch the diaphragm. Now, if you speak into the open end of the tube the diaphragm will vibrate and the shot will be seen to move to and from it according to the strength of the vibrations. If we could by any means make a diaphragm in another tube reproduce these same vibrations, we should hear the same words respoken, if the tube were held to the ear.

While the vibrations caused by the human voice are too minute to be seen, it may seem surprising that they can be made to produce power. This is done by an ingenious mechanism called a Phonomotor, perfected by the great inventor Thomas A. Edison, of whom every one has probably heard. This mechanism, when spoken or sung at (or into) immediately responds by causing a wheel to revolve. No amount of blowing will start the wheel, but it can instantly be set in motion by the vibrations caused by sound.

The Phonomotor (which is shown in the engraving Fig. 12) has a diaphragm and mouthpiece. A spring, which is secured to the bedpiece, rests on a piece of rubber tubing placed against the diaphragm. This spring carries a pawl that acts on a ratchet or roughened wheel on the fly-wheel shaft. A sound made in the mouthpiece creates vibrations in the diaphragm; the vibrations of the diaphragm move the spring and pawl with the same impulses, and as the pawl thus moves back and forth on the ratchet-wheel it is made to revolve.

The instrument, therefore, is of great value for measuring the mechanical force of sound waves, or vibrations, produced by the human voice.

THE TRANSMITTER

That part of the telephone into which we speak is called the transmitter. This is usually a piece of hard rubber having a round mouthpiece cut through it. At the other side of this mouthpiece is placed a diaphragm made of a thin piece of metal, which is held m place by a light spring. Behind this diaphragm, and very close to it, is placed a carbon button. Between this carbon button and the diaphragm is a small piece of platinum, which is placed so as to touch both the button and diaphragm very lightly. This platinum contact piece is connected with one of the wires running to the primary of the induction-coil, and the spring attached to the carbon button is connected with the battery to which the other wire of the primary is connected. This is all shown in the sketch of a transmitter. (Fig. 13.)

A is the mouthpiece; B, the diaphragm; C, the carbon button; D, the wire at the end of which is the platinum contact; E, the battery; and F, the induction-coil; P, P are the wires to the primary, and S, S to the secondary wires.

We will now say a few words about the receiver, and then describe the manner in which the telephone works.

THE RECEIVER

This is that part of the telephone which is held to the ear, and by which we can hear the words spoken into the transmitter of the telephone at the other end of the line.

The receiver is made of hard rubber, and contains a permanent bar magnet, which is wound with wire so as to make it also an electromagnet when desired. In front of this magnet is placed loosely a diaphragm of thin sheet iron. This diaphragm is placed so as to be within the influence of the magnet, but just so that neither one can touch the other.

Fig. 14 is a sketch of the receiver. A and B are the wires leading to the magnet, C, and D is the diaphragm. E and F are where the wires connect, one from the secondary of the induction-coil in the other telephone, and the other connected with the earth.

THE CARBON BUTTON

The little carbon button plays an important part in the telephone. You will see from the sketch of the transmitter that the current of electricity will flow through the carbon button to the contact point and through the wire to the primary of the induction-coil.

Now, carbon has a peculiarity, which is this, that if we press this carbon button, ever so slightly, against the platinum contact, there would be less resistance to the flow of the electricity through the wire to the primary, and the more we press it the less the resistance becomes. The consequence of this would be that more current would go to the primary, and the secondary would become correspondingly stronger. If the carbon button were left untouched, and nothing pressed against it, the flow of current through it would be perfectly even.

Having examined the inside of the transmitter and receiver, and understanding the effect of pressure on the carbon button, let us now see

HOW THE TELEPHONE WORKS

When we speak into the mouthpiece of the transmitter, the vibrations of the air cause the diaphragm to vibrate very rapidly, and, of course, every movement of the diaphragm presses more or less against the carbon button, in consequence of which the currents passing through the primary of the induction-coil are constantly increased or diminished and thus produce similar effects, but magnified, in the secondary.

The effect of this is that the magnet in the receiver of the other telephone is receiving a rapidly changing current, which, producing corresponding magnetic changes, makes the magnet alternately weaker or stronger. This influences, by magnetism, the iron diaphragm accordingly, and makes it reproduce the same vibrations that were caused by the speech at the transmitter of the sending telephone. Thus, the same vibrations being reproduced, the original sounds are given out, and we can hear what the person at the sending telephone is saying.

The action of the telephone illustrates well the wonderfully quick action of the electric current by the reproduction of these sound waves, or air vibrations, for they number many thousands in one minute's speech.

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