That part of the apparatus which takes up the sounds and changes them into electric currents composes the transmitter. When words are spoken into the mouthpiece they strike a diaphragm, on the back of which is fastened a small cup-shaped piece of carbon. A second cup is mounted in a rigid position directly back of the first. The space between them is filled with small polished granules of carbon. When these granules are in a perfectly loose state and are undisturbed, their resistance to an electric current is very great and they allow almost none to flow.[2] When slightly compressed their resistance is greatly lowered and they permit the current to pass. The vibrations of the diaphragm cause the carbon cup mounted on its back to move and exert a varying pressure upon the granules with a corresponding variation in their resistance and the amount of current which will pass through.
Fig. 145.—A Telephone System, consisting of a Receiver, Transmitter, and a Battery connected in Series. Words spoken into the Transmitter are reproduced by the Receiver.
The receiver, or that part of the apparatus which transforms the pulsating current back into sound waves consists of a thin iron disk, placed very near but not quite touching the end of a small steel bar, permanently magnetized, and about which is wound a coil of fine insulated wire.
The transmitter and the receiver are connected together in series with a battery as in Figure 145. When words are spoken into the transmitter the little carbon granules are immediately thrown into motion, and being alternately compressed and released cause corresponding changes in the current flowing through the receiver from the battery. The magnetism of the receiver changes with each change in the electric current, and thus by alternately attracting and repelling the diaphragm causes it to vibrate and emit sounds. Such is the principle of the telephone. The telephones in actual service to-day are complicated with bells, magnetos, induction coils, condensers, relays, and various other apparatus, which fact renders them more efficient.
The bells and magnetos are for the purpose of calling the central operator or the person at the other end of the line and drawing attention to the fact that some one wishes to get into communication with him. The older styles of telephones used what is known as a polarized bell and a hand magneto for this purpose. A polarized bell is a very sensitive piece of apparatus which will operate with very little current. A magneto is a small hand dynamo which when turned with a crank will generate a current causing the bell at the other end of the line to ring. When the telephone receiver is raised off its hook in order to place it to the ear the bell and magneto are automatically disconnected from the line and the receiver and the transmitter are connected in their place. The current necessary to supply the telephone and receiver is supplied by two or three dry cells placed inside of each telephone.
The latest types of instruments employ what is known as the central energy system, wherein the current is supplied by a large storage battery located at the central office and serving as a current supply to all the telephones connected to that system.
It would be impossible to enter into the history of the telephone far enough to explain the details of some of the various systems in every-day use in such a book as this because of the immense amount of material it would be necessary to present. Such a work would occupy a volume of its own. Additional information may be readily found in any reference library. However, the "boy electrician" who wishes to make a telephone for communicating between the house and barn, or with his chum down the street, will find the necessary information in the following pages. If this work is carried out carefully and a home-made telephone system built and installed it will not only prove a very interesting undertaking but will also serve to dispel all mystery which may surround this device in the mind of the young experimenter.
How to Build a Telephone
Telephone receivers are useful for many purposes in electrical work other than to receive speech. They are used in connection with wireless instruments, in place of a galvanometer in measuring electrical circuits, and for testing in various ways.