. The principles underlying radio communication . is the magnetic effect in the receiver for a current ofgiven strength. The use of telephone receivers in radio com-munication is discussed in Section 180. Above the pole pieces and very close to them is a thin cir-cular soft-iron disk D, called the diaphragm. The diaphragmof a -receiver can be seen through the hole in the center of theearpiece. The distance between the pole pieces and the dia- WIRE TELEPHONY. 149 phragm is important in determining the sensitivity of tliereceiver; in standard instruments this distance is about The per
. The principles underlying radio communication . is the magnetic effect in the receiver for a current ofgiven strength. The use of telephone receivers in radio com-munication is discussed in Section 180. Above the pole pieces and very close to them is a thin cir-cular soft-iron disk D, called the diaphragm. The diaphragmof a -receiver can be seen through the hole in the center of theearpiece. The distance between the pole pieces and the dia- WIRE TELEPHONY. 149 phragm is important in determining the sensitivity of tliereceiver; in standard instruments this distance is about The permanent magnet pulls the diaphragm toward thepole pieces a certain distance, which depends upon the flexi-bility of the diaphragm. The variations in the current in thereceiver windings, corresponding to the sound vibrations ofthe voice spoken into the transmitter, produce correspondingvariations in the magnetic field of the pole pieces, and thediaphragm moves in accordance with these variations andreproduces the voice spoken into the Fig. 93b.—Watch case telephone receiver. It is possible to use a telephone receiver as a a circuit containing only two identical sensitive telephonereceivers and no battery, the same instrument can be usedalternately as receiver and transmitter by the person at eachend of the line, and speech thus transmitted. This was, in fact,done in the early days of telephony, but the currents so gen-erated by using the receiver as a transmitter are so feeble thatother devices are now used for practical purposes. Operation.—Words spoken into the transmitter vary the pres-sure on the carbon granules, and hence the resistance betweenthe transmitter terminals and corresponding variations in theoutput current of the transmitter are thus produced. Thenature of the electric current transmitted by the wires leadingto the receiving station depends upon the auxiliary apparatusused with the transmitter. The electric current passing be- 1
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