Answers for the Teacher Only

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What Do You Think?

The telephone mouthpiece converts sound into an electrical signal. Older telephones contained a carbon transducer, but more recent models have a variable capacitor, with one of its plates fastened to a membrane vibrated by the sound of the user's voice. The capacitor is part of an electronic circuit that produces an electrical signal proportional to the pressure vibrations on the membrane. The ear piece resembles a headphone transducer, although the coil is wrapped around a fixed magnet, and the magnetic field produced by the current in this coil vibrates another permanent magnet attached to the diaphragm.