The telephone

We use the electric telegraph to send written messages to people far away from us. We use the telephone to talk to people far away. In many ways the telephone is better than the telegraph as a means of communication. The cost of sending a telegram depends on the number of words in it.

We have to make our telegraph message as short as possible, but, even in a telephone call, we can say a lot of words. A telegram can only be sent from one post office to anot­her. There is a delay before it can reach the person it is addressed to. The telephone connects you to a person directly. You may have to wait several hours for an answer to a telegram. You can ask a person questions and get the answer immediately on the telephone.

Sound travels through the air in waves. When you play a violin, for example, the violin string vibrates. The vibrations from the vio­lin string pass through the air in little waves. When these waves reach the ear, the eardrum vibrates, and so you hear the violin. Different notes have different distances between the tops of the wa­ves. We call these different «wavelengths».

It was found that a thin sheet of metal, called a «diaphragm», would vibrate in the same way as the eardrum when sounds reac­hed it. In 1875 an inventor called Alexander Graham Bell got a U-shaped iron magnet and wound coils of wire around it. Then he placed the diaphragm very close to the poles of the magnet. Bell made sound waves reach the diaphragm, which vibrated, moving in­wards towards the magnet and outwards from it. This made small currents of electricity pass through the coils and these currents we­re sent along a wire.

At the other end of the wire Bell placed a similar instrument, with a diaphragm and coils round a U-shaped piece of iron, which we call the «receiver». The impulses of electric current flowed thro­ugh the coils of the receiver and magnetized the U-shaped piece of iron. The strength of the magnet was large or small according to the strength of the current. It made the diaphragm vibrate and the vibrations made waves of sound in the air exactly like the sound waves which originally reached the instrument at the other end of the wire. The sound waves had been turned into electricity, trans­mitted along a wire, and turned into sound again. The telephone had been invented.

But the sound from that telephone could only be transmitted over short distances because the microphone was not very sensiti­ve. A modern telephone has a carbon microphone in the part we call the mouthpiece. The diaphragm is still there, and when you speak into it, the waves of sound push it in and out. But there is a cur­rent of electricity, supplied by a battery, which is already flowing through the microphone. Behind the diaphragm there are small pieces, or granules, of carbon. When you speak, you make louder and softer sounds. The louder or softer the noise you make, the more or less the diaphragm is pushed in or out.

Pushing in the diaphragm packs the carbon granules closer to­gether. A soft sound does not push the diaphragm forward so far and the carbon granules are allowed to space out. Then it is more difficult for the current to flow through them and not so much gets through to the telephone line. The waves of electric current, vary­ing like this, pass along the line and finally reach the receiver of the telephone held by the person you are speaking to. This receiver has an electro-magnet and a diaphragm and works just like the one first invented.

The line from your telephone is connected to the line of the te­lephone of the person you want to speak to through the telephone exchange. In the early days of the telephone operators working in the exchange made all the connections between callers by hand. No­wadays, more and more exchanges are operated automatically.