Text 12. 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.

Text 13. Radio waves

Electrical energy that has escaped into free space exists in the form of electromagnetic waves. These waves, which are commonly called radio waves, travel witch the velocity of light and consist of magnetic and electrostatic fields at right angles to each other and also at right angles to the direction of travel.

One half of the electrical energy contained in the wave exists in the form of electrostatic energy, while the remaining half is in the form of magnetic energy.

The essential properties of a radio wave are the frequency, intensity, direction of travel, and plane of polarization. The radio waves produced by an alternating current will vary in intensity witch the frequency of the current and will therefore be alternately positive and negative.

The distance occupied by one complete cycle of such an alternating wave is equal to the velocity of the wave divided by the number of cycles that are sent out each second and is called the wave length.

The relation between wave length in meters and frequency in cycles per second is therefore.

The quantity 300 000 000 is the velocity of light in meters per second. The frequency is ordinaliry expressed in kilocycles, abbreviated KC; or in megacycles, abbreviated MC. A low-frequency wave has a long wave length while a high frequency corresponds to a short wave length.

The strength of a radio wave is measured in terms of the voltage stress produced in space by the electrostatic field of the wave and is usually expressed in microvolts stress per meter.

Since the actual stress produced at any point by an alternating wave varies sinusoidally from instant to instant, it is customary to consider the intensity of such a wave to be the effective value of the stress, which is 0.707 times the maximum stress in the atmosphere during the cycle. The strength of the wave measured in terms of microvolts per meter of stress in space is exactly the same voltage that the magnetic flux of the wave induces in a conductor I meter long when sweeping across this conductor with the velocity of light.

Thus the strength of a wave is not only the dielectric stress produced in space by the electrostatic field, but it also represents the voltage that the magnetic field of the wave will induce in cutting across a conductor.

In fact, the voltage stress produced by the wave can be considered as resulting from the movement of the magnetic flux of the same wave.

The minimum field strength required to give satisfactory reception of a wave depends upon a number of factors, such as frequency, type of signal involved, and amount of interference present. Under some conditions radio waves having signal strengths as low as 5.000 to 30.000 per meter are required to ensure entirely satisfactory reception at all times.

In most cases the weakest useful signal strength lies somewhere between these extremes.

A plane parallel to the mutually perpendicular lines of electrostatic and electromagnetic flux is called the wave front.

The wave always travels in a direction at right angles to the wave front, but whether it goes forward or electromagnetic and electrostatic flux.

If the direction of either the magnetic or electrostatic flux is reversed, the direction of travel is reversed; but reversing both sets of flux has effect.

The direction of the electrostatic lines of flux is called the direction of polarization of the wave. If the electrostatic flux lines are vertical the wave is vertically polarized; when the electrostatic flux lines are horizontal and the electromagnetic flux lines are vertical. The wave is horizontally polarized.

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