CHAPTER XII. The Telephone

THE TELEPHONE

IN the electric telegraph, the electric current is made to turn a magnetic needle, or move an electric pen at a distance. When the distance becomes very great, as in the telegraph cables which connect us with America, a more delicate instrument is used for receiving the messages. It consists of a kind of small ink bottle, which is turned this way or that by the electric current. It squirts out a fine jet of ink, and the ink falls on paper which is moved past the instrument. In this way the signals are recorded on paper without any friction.

The electric telegraph, however, is a very simple and coarse instrument in comparison with the telephone, for the telephone enables us to send speech itself over several hundred miles, over distances which the most powerful light from the highest lighthouse could not cover.

That any one can speak into an instrument in London, and that at the same time a friend in Paris can hear his voice, and understand what he is saying, is a thing so wonderful that a person must have tried it for himself before he can quite believe it. Yet what is wonderful about it is not so much the fact of speech being communicated, as the distance across which the communication is made. Why should we not be able to cover such great distances by shouting?

The reason is, that when we shout we create a wave of sound in the air. Such a wave of sound is in many respects like a wave in water. When we throw a stone into a pond, the wave starts from the place where the stone fell into the water, and travels out in circles which become wider and wider. Not only do the circles become wider, but they also grow fainter and fainter the wider they get. If the pond is very large, the wave may get so faint that when it reaches the bank it is too faint to be perceived.

If, instead of a pond, we have a long channel or gutter filled with water and make a splash in it at one end, we find that the wave can travel a very great distance along the channel without growing much feebler. That is because it cannot spread out sideways. Similarly, if we speak into a long pipe or tube, our voice carries much farther than it does in the open air, again because the wave of sound cannot spread out sideways. This is the reason why it is so easy to talk through a speaking-tube.

If we could send the same waves along a wire, we might succeed in carrying the sound much farther than we do even through a speaking-tube. This fact can be proved by taking two small tin cans, and stretching a wire from a hole in the bottom of one to a hole in the bottom of the other. Then, if one person speaks into one of the cans, and the other person holds the other can to his ear, and the wire is stretched between them, the second person will hear distinctly what the first person is saying. In this case the wave of sound travels along the wire, stretching it and compressing it in turn, until it reaches the other can, and throws the air into similar vibrations. Such an instrument could be called a telephone, but nowadays the word is used only for instruments which are worked by electricity or magnetism.

To understand the difficulties which the inventors of the telephone had to overcome, let us consider for a moment the nature of the sound waves which represent speech. Spoken language consists of vibrations of the air. These vibrations are very small and very quick puffs of air, which we produce by means of our throat and mouth. Now if we were to try to give a very rapid succession of puffs, we could not give more than about ten per second. But if we had a machine which could puff much more quickly than that, we should find that after the puffs had become more than twenty per second, there would be a kind of hum. This result can be illustrated by means of a toothed wheel such as we find in clocks. When such a toothed wheel is turned quickly, and we hold the edge of a piece of paper against it, then, as it is made to turn more and more quickly, the little ticks of the paper merge more and more into a hum, and finally become a musical note. The faster the wheel, the higher the pitch of the note.

Every note has its own pitch, that is to say, it has a certain number of vibrations per second. The notes in the middle of a piano have from two hundred to five hundred vibrations per second. The same note may be either loud or soft, just as a wave of water may be either high or low. The high wave corresponds to a loud sound. Very small ripples on the surface of water correspond to a very high, but faint, note of music, like the singing of an insect.

Each note has its own pitch and its own loudness. In addition to that, it also has a certain “quality” of its own. We may, for instance, play the same note on a violin, a trumpet, or an organ. They may have the same pitch and the same loudness, and yet we can distinguish them from each other easily. What is this “quality” which enables us to distinguish the note of one instrument from the same note played on another instrument? It must be something in the position of the waves which travel through the air and reach our ear.

This question long remained in doubt, until people found out that no musical sound was quite simple. Even in water one usually sees little ripples on the top of big waves. A wave with a perfectly smooth surface is very rare. In the same way, a perfectly simple and pure sound wave is hardly ever produced. Each wave has little ripples on it. The wave from a piano has one kind of ripples, the wave from an organ has another kind, and the wave from a trumpet--which is particularly shrill and penetrating--has a great number of very tiny ripples, which mean very high notes. These facts can be proved by playing a soft note on an organ, and playing a number of very high notes at the same time. If the high notes are properly chosen, the sound becomes shrill and penetrating, like the sound of a trumpet.

Musical sounds are therefore quite fixed as soon as we know their loudness, pitch, and quality. But not all sounds are musical. There are such things as noises, and many forms of human speech are anything but musical. As a matter of fact, speech consists of a very rapid succession of all kinds of notes, which last only a very short time, and rapidly change from one to the other.

Of these notes, the highest in pitch are the hissing sounds, such as s and sh, and these are most difficult to send through the telephone. Shortly after the electric telegraph was invented, the idea occurred to many electricians that it ought to be possible to send speech along the wire conducting the electric current. They knew that sound consisted of waves, and they thought that if an electric current could be interrupted a great many times per second, a musical note would be produced at the other end of the wire.

An observation made in America in the year 1837 confirmed this idea. It was found that when a current is sent suddenly through a long coil of wire wound round a piece of soft iron, a little sound is heard. Such a coil is called an electromagnet, because the soft iron is a powerful magnet so long as a current is passing through the coil. Another sound is heard when the current is broken suddenly. This observation suggested that if the electromagnet were at the other end of a long wire, and the current at one end were made and broken rapidly, the electromagnet would give a musical note at the other end.

In the year 1854, a French telegraphist proposed to send speech by electricity on that principle. He proposed to stretch a skin over a drum, and fix one end of a fine wire to the middle of the skin. The end of the wire should dip into a drop of quicksilver connected with another wire, and this contact should establish an electric circuit. On speaking into the drum the skin would vibrate, the wire would move in and out of the drop, and the electric circuit would be made and broken in turn. It would be made and broken very rapidly when a high note was sung into it, and less rapidly in the case of a low note. It would follow all the various notes of ordinary speech, and would produce the necessary vibrations at the receiving end.

The Frenchman never carried out his proposal. In the year 1861 it was again taken up by a German professor of the name of Reis. His instrument was able to transmit musical notes, but not spoken language. It was a very wonderful instrument at the time, but it could not give the correct quality to notes. The sudden induction of the electric current was too jerky to give the very fine ripples on the sound waves, and especially to render the delicate shades of human speech. The inventor died poor and neglected in 1874, and it was not till 1876 that the telephone problem was solved really.

The credit of the successful solution of this great problem is due to an American of the name of Graham Bell. As is very often the case, the successful instrument was extremely simple, so simple indeed that people were disappointed on seeing it. It could be put together by a schoolboy, and yet it was able to talk. From the very first, Bell had seen the correct principle which must govern the construction of a good telephone. The fault of Reis’s telephone was that it could not follow the delicate wavelets of sound which constitute human speech. It was necessary to make an instrument which should be extremely sensitive and delicate, as much so as the human ear, which is saying a great deal. In order to do this Bell used Faraday’s induction currents.

We have seen already that when Faraday pulled off the cap of the electromagnet a momentary current was induced in the coil of wire. Small currents could be induced by simply shaking the cap to and fro. Now, Bell had the idea of mounting a small piece of iron in front of the bar of an electromagnet. The ends of the coil of wire of the electromagnet were pulled out to a very long distance and passed round another electromagnet. The small piece of iron was stuck upon a piece of parchment, which was stretched on a drum in front of the first electromagnet. On speaking into this drum the parchment was set into vibration, and with it the small piece of iron. This produced slight and very rapid currents in the coil of the electromagnet, and these were transmitted along the wire to the other electromagnet.

Another parchment with a similar small piece of iron was mounted on a drum in front of the second electromagnet, and it was found that the second drum reproduced exactly all that was spoken into the first drum. For the small currents arriving at the second electromagnet made its magnetism stronger and weaker in rapid succession, and attracted the iron sometimes more and sometimes less. Every motion of the first parchment, therefore, was rendered faithfully by the second parchment, and the vibrations of the air inside the first drum were reproduced accurately in the second drum. That was all that was necessary for the complete transmission of human speech.

But this apparatus was still capable of considerable improvement. In the first place, we know that an electromagnet loses all its magnetism when the current in its coil is stopped. But Graham Bell found that it was not at all necessary to use an electromagnet.

Instead of a piece of soft iron, he found he could use a similar piece of steel which had been magnetised strongly. Such a piece of steel is called a permanent magnet. The little horseshoe magnets with the red paint on them that are sold in shops are such permanent magnets. This discovery simplified matters greatly, as it was no longer necessary to use a battery at all.

Another improvement was to do away with the parchment. Graham Bell found that a sheet of tin would do very well instead of the parchment. What is called “tin” is usually in reality a sheet of iron covered on both sides with a very thin layer of metallic tin; and as the magnetism acts quite easily through tin, the tinned iron acts like a sheet of pure iron. When a sound wave reaches it, it is set vibrating; and its vibrations slightly alter the magnetism of the permanent magnet, and produce the small and rapid currents mentioned above. The whole apparatus, therefore, consists simply of two steel magnets, two discs of tin, sufficient wire to make coils round the magnets and cover the distance between the two stations, and the necessary boxes to keep the discs and magnets in the proper position. It is surely the simplest instrument ever invented to perform such a wonderful task. What is almost quite as wonderful is that this telephone is still in general use for receiving telephone messages.

It says a great deal for the excellence of the instrument that thirty-three years of active search for improvements have not been able to suggest any real improvement in Graham Bell’s instrument. Bell’s receiver is still used at the present day.

But his instrument is not as good for sending or transmitting speech as some other instruments which have since been invented.

It was found that Bell’s transmitter was rather feeble as regards loudness, and an improved instrument has been invented in America since his time.

It was Edison, the inventor of the phonograph, who first proposed to use carbon for sending the message. The new instrument was in some ways a revival of the idea of Reis, but instead of breaking and making the circuit, it was thought better to make the contact between the ends of the circuit weaker and stronger in succession. This is now done in an instrument called the Blake transmitter.

An electric circuit containing a battery is interrupted at one point, and joined again through a little ball of platinum and a ball of carbon, such as is used in electric batteries. This contact is connected with a vibrating plate which transmits the speech. The circuit is never really broken, but is weakened and strengthened alternately by a more or less perfect contact between the two balls. The result is that human speech is conveyed with great clearness.

This principle of loose contact has since been employed for another instrument which is perhaps even more wonderful than the telephone. It is the “microphone,” which was first constructed by Edwin Hughes in 1878. A current was sent through a Bell telephone and through two pieces of carbon joined by a rod of carbon which was laid loosely across them. The slightest sound produced on the board which held the carbons was converted into a loud sound in the telephone. When a fly walked across the board, the patter of its feet could be heard in the telephone.