Harper's Electricity Book for Boys

by Joseph H. Adams

For direct communication over short or moderately long distances, nothing has been invented as yet that will take the place of the telephone. A few years ago, when this instrument was first brought out, it was the wonder of the times, just as wireless telegraphy is to-day. Starting with the simple form of the two cups with membranes across the ends, and a string or a wire connecting them, we have to-day the complex and wonderful electric telephone, giving perfect service up to a distance of two thousand miles. Some day inventors in the science of telephony will make it possible to communicate across or under the oceans, and when the boys of to-day grow to manhood they should be able to transact business by ’phone from San Francisco to the Far East, or from the cities near the Atlantic coast to London, Paris, or Berlin.

It is hardly necessary to enter into the history of telephones, as this information may be readily found in any modern encyclopædia or reference work. But the boy who is interested in electricity wants to know how to make a telephone, and how to do it in the up-to-date way, with the wire and ground lines, switches, cut-outs, bell connections, and other vital parts, properly constructed and assembled. In this laudable ambition we will endeavor to help him.

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 Bladder Telephone

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.

When the bladders are ready, cut off the necks, and also remove about one-third of the material, measuring from end to end. Soak the bladders in warm water until they become soft and white. Stretch them, loosely but evenly, over the opening in the boards, letting the inside of the bladder be on top, and tack them temporarily all around, one inch from the edge of the opening. Test for evenness by pushing down the bladder at the middle. If it stretches smoothly and without wrinkles it will do; otherwise the position and tacks must be changed until it sets perfectly smooth.