Microphone.
An apparatus which includes a contact of variable resistance; such
resistance can be varied in amount by slight vibrations, such as those
produced by sound waves. The apparatus in use forms part of a circuit
including a telephone and current generator. As the contact is varied
the resistance of the circuit and consequently the current intensity
changes and sounds are emitted by the telephone corresponding to such
changes. If the microphone is spoken to, the telephone will emit
corresponding sounds, reproducing the voice.
It has been found in practice that carbon gives the best microphone
contact. One of the simplest and earliest forms is shown in the cut. A
short rod or pencil of carbon, A, such as used in batteries, is
sharpened at the ends and rests loosely in a vertical position between
two blocks of carbon, C C, in each of which a hole is drilled to receive
one of the points. The blocks are carried on a standard and base D. The
blocks are connected with two terminals x, y, of a circuit, including a
telephone and battery. There are two contacts to be disturbed.
If delicately adjusted a fly walking over the base-board will disturb
the contacts enough to produce sounds in the telephone. These sounds are
possibly not due only to sound waves, but in part to absolute mechanical
disturbances.
The various forms of telephone transmitter are generally microphones.
Fig. 242. MICROPHONE.
Microphone Relay.
A combined microphone and telephone. A microphone is placed close to the
diaphragm of a telephone. The slight sound waves emitted by the
telephone affect the microphone and are repeated in its circuit. The
microphone circuit includes a local battery and telephone.
Microtasimeter.
An apparatus for indicating minute changes in temperature or atmospheric
moisture.