Abstract
AN article by A. Herckmans (Bell Lab. Rec., 21, No. 2; October 1942) describes a new amplifying set which recently has been developed for use with the regular telephone set to assist the hard of hearing, and which permits all the added equipment except a small 4½-volt battery to be incorporated in the base of the telephone. The gain of the amplifier is adjusted by turning one of the switchhook plungers, and the amplifier can be disconnected by a second switch controlled by this same plunger. The switch-hook plunger at the right of the set gives gain control and it may be turned to any one of three positions to give low, medium or high gain. Vertical motion of the plunger removes the amplifier from the circuit entirely. The very small amplifier unit consists of a granular-carbon microphone the diaphragm of which is actuated by a bipolar receiver element using permanent magnets. A soft rubber mounting prevents it from picking up vibrations from the surface on which the set is resting. When the amplifier is in use its receiver element is connected in place of the regular handset receiver, while its transmitter element is connected to the handset receiver in series with a 4½ -volt battery which may be placed in any convenient location and connected to the set by two wires. With this arrangement the handset receiver is operated by the amplified speech from the transmitter element of the amplifier.
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