The general principle of the telephone may be explained in the statement that it is an apparatus for the conveyance of the human voice, or indeed any sounds which are the direct result of vibration.
Sound is due to the vibrations of matter. A piano string produces sound because of its vibration when struck, or pulled to one side and then released. This vibration sets the air in rapid motion, and the result is the recording of the sound on our ear-drums, the latter corresponding to the film of sheepskin or bladder drawn over the hollow cup or cylinder of a toy telephone. When the head of a drum is struck with a small stick it vibrates. In this case the vibrations are set in motion by the blow, while in the telephone a similar phenomenon is the result of vibratory waves falling from the voice on the thin membrane, or disk of metal, in the transmitter. When these vibrations reach the ear-drum the nervous system, corresponding to electricity in the mechanical telephone, carries this sound to our brains, where it is recorded and understood. In the telephone the wire, charged with electricity, carries the sound from one place to another, through the agencies of magnetism and vibration.
Over short distances, however, magnetism and electricity need not be employed for the transmission of sound. A short-line telephone may be built on purely vibratory principles. Almost every boy has made a “phone” with two tomato-cans over which a membrane is drawn at one end and tied. The middle of the membrane is punctured, and a button, or other small, flat object, is arranged in connection with the wires that lead from can to can.
A really practical talking apparatus of this simple nature may be made from two fresh beef bladders obtained from a slaughter-house or from the butcher. You will also need two boards with holes cut in them, two buttons, some tacks, and a length of fine, hard, brass, copper, or tinned iron wire. The size should be No. 22 or No. 24. The boards should be ten by fourteen inches and half an inch in thickness. Cut holes in them eight inches in diameter, having first struck a circle with a compass. This may be done with a keyhole saw and the edges sand-papered to remove rough places. Prepare the bladders by blowing them up and tieing them. Leave them inflated for a day or two until they have stretched, but do not let them get hard or dry.