SOUND POWERED TELEPHONE

THEORY OF OPERATION

The RCA Sound Powered Telephone consists of atransmitter ami one or more receiver units. Thesystem generates its own transmitting power andshould never he used in conjunction with battery­operated telephone systems. The motors used in thetransmitters and receivers are essentially the samebut differ in a few poims. such as length of contacts,shape oi armature, and number of turns and size ofwire in the operating' coil. In the Sound PoweredTelephone system, all motor units, hoth transmittingand receiving, afe connected in parallel. All opencircllit lille switch is inserted in serles with eachtransmitter, thus eliminating unnecessary noises onthe line fr0111 transmitters not in tlse. The switchIlllht be depressed while a transmitter is in use.

Sound Powered Telephones are moisture proofedand will witlhtand immersion without damage to themechanism. These telephones are designed to oper­ate efficiently under most conditions encountered inmarine practice, even under pressures varying­widely frOlll normal atmospheric pressure.

Figures (A) (ll)"and (C) illustrate. in a sim­plified manner. the functioning of a Sound PoweredTelephone lIlotor unit. Notice that the unit cOll~ists

of a coil located within the field of a permanent mag~

net. An armature passing through the coil. shownin (A). i~ centered within the air gap between thet\\'o poles. The armature is connected by means ofa drive rod to a diaphragm. Notice that the mag­netic lines of force, represented by the arrows, passfrom the "N" to the "5" pole of the magnet. \Viththe armature in this position, 110 magnetic lines offorce are passing vertically through the coi\.

\Vhen the motor unit is used as a transmitter. thesound waves compress the air before the diaphragm.forcing the diaphragm in. thus moving the armatureto the right. as illustrated in (B). Tn this positionthe armature nearly bridges the air gap and forms a

llluch better path for the magnetic lines of force thanthe air did. i\fagnetic lines of force are, therefore,condncted along the armature and up through thecoil in a vertical direction, inducing an electric cur­rent within the coiL

Since sound waves are vibrations, causing com­pression and rareficatioll of the atmosphere in whichthey travel, the diaphragm will vibrate with thewan~s and remain in anyone position for only amoment. After reaching the position shown in (B)the diaphragm moves outward to the position shownin (C), forming a path for the magnetic lines offorce through the coil in the opposite direction, andinducing an electric current within the coil flowingin the opposite direction. Thus it can be seen thatwhen the diaphragm is actuated by sound waves, themotor unit generates corresponding electrical im­pulses which are fed to a second motor unit em~

ployed as a receiver.

\Vhen a motor unit is used as a receiver. the actiolltaking place is the reverse of that when used as atransmitter. -\Vhen used as a receiver, electricalimpulses entering Ihe coil magnetize the armature.:'.fagnetic poles of the same polarity repel each otherand those of opposite polarity attract each other.The current flowing through the coil in (B) mag­netizes the armature in such a direction that the topof the armature is "N": therefore. it would be re~

pelled by the "N" pole and attracted by the "5" poleof the permanent magnet, thus drawing the dia­phragm inward. As the direction of current re­verses. the polarity of the armature reverses. and itsposition in the air gap will be reversed, forcing thediaphragm outward. thus compressing the atmos­phere before it. It can be seen that the diaphragmof the receiver will vibrate in unison with thai of thetransmitter and excite corresponding sound wavesin the atmosphere before it.

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