Text 7. The Telephones

The telephones are connected in series with the detector.

A brief explanation of how the telephone works. Is here necessary. The sensation of sound is excited in the ear by the motion imparted to the air by vibrating bodies. If a flat steel spring be fixed in a vertical position in a vice, and the free end of it be displaced, on releasing it a vibratory motion will follow. The free end will pass backwards and forwards along a gradually decreasing arc. During its first movement to the right, it compresses the air on its right- hand side, and causes a state of rarefaction on its left-hand side. A revers movement has exactly the opposite effect. As long as the spring continues to vibrate, waves of rarefaction and compression are propagated, the frequency of these waves or the number of complete vibrations per second determining whether they are audible or not. It the frequency be anything between 30 and 20,000 per second, audible sounds are produced. The telephone is an instrument capable of producing waves in the air of such a frequency. A disc of thin soft iron, varnished to prevent rusting, takes the palace of the spring just described, and it is set in vibration by fluctuations in the intensity of a magnetic field. Fig. I shows an electro – magnet with its two poles in close proximity to a disc of soft iron D, which is firmly clamped in position by its edges. The core of the magnet is permanently magnetized and exercises a force of attraction on the disc. If a current be passed through the coils wound round its pole pieces, this force of attraction is increased or decreased according to the direction of the current. It the force be increased, the centre ofthe disc is pulled towards the magnet: and if the force be decreased, it is released to some extend. If, then, rapid alternations of current, or intermittent unidirectional currents, be passed through the windings, the disc or “ diaphragm ”, as it is called, is caused to vibrate; and if the frequency of the vibrations be within the limits stated above, they will produce the sensation of sound in the ear.

On account of its shape the telephone receiver used in a wireless installation is called a “watch” receiver.

Two complete watch receivers are connected in series at the ends of a steel or aluminium strip spring, to form the telephone head-gear. As the space available is very small, the wire used in the coils of the electro-magnets must of necessity by very thin, in order to obtain the necessary ampere-turns required for the high degree of sensitiveness of the telephone. In low resistance telephones the wire is insulated with silk, but where a much greater number of turns is required, as in the case of telephones of from two to eight thousand ohms resistance used with a valve or crystal receiver, the wire insulation usually consists of a coating of enamel, as space is thus economized. In the high resistance telephone a pair of protective spark points is often included, as a guard for the coil windings against excess voltage due either to direct application, inductive kick on suddenly breaking circuit, or high –frequency surge – all tending to damage the insulation. Again, where enamelled wire is used, the interior of case is filled with paraffin wax, further to ensure good insulation and prevent moisture from reaching the windings.