Text 3.

Detection

The process by which the signal being transmitted is reproduced from the radio-frequency currents present at the receiver is called de­tection, or sometimes demodulation. Where the intelligence is trans­mitted by varying the amplitude of the radiated wave, detection is accomplished by rectifying the radio-frequency currents.

The rectified current thus produced varies in accordance with the signal originally modulated on the wave radiated at the transmitter and so reproduces the desired signal.

Thus, when the modulated wave is rectified, the resulting current has an average value that varies in accordance with the amplitude of the original signal.

In the transmission of code signals by radio, the rectified current reproduces the dots and dashes of the telegraph code and could be used to operate a telegraph sounder. When it is desired to receive the telegraph signals directly on a telephone receiver, it is necessary to break up the dots and- dashes at an audible rate in order to give a note that can be heard, since otherwise the telephone receiver would give forth a succession of unintelligible clicks.

The detection of a frequency-modulated wave involves two steps.

First, the wave is transmitted through a circuit in which the rela­tive output obtained from the circuit depends upon the frequency.

The circuit output is then an amplitude-modulated wave since, as the frequency of the constant-amplitude input wave varies, the output will vary correspondingly.

The resulting amplitude-modulated wave is then rectified.

Text 4.

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 reverse .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 is 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 place of the spring just described, and it is set in vibration by fluctuations in the intensity of a magnetic field. Fig. 1 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. If the force be increased, the centre of the disc is pulled towards the magnet; and if the force be decreased, it is released to some extent. 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 be 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.