ensure good contact. The back of the microphone is closed, except for a series of holes that permit' the
air and sound to reach the back of the diaphragm. The bridge, E, extends across the opening in the front
of the microphone and supports the front carbon granule cup, or button, F. A similar one is supported by
the back. These carbon cups, or buttons, do not touch the diaphragm and are partly filled with fine
carbon grains. The size of these grains determines the sensitivity of the instrument, and soft felt washers
prevent the carbon from getting out of the cup.
a. Figure 2-3B illustrates both the mechanical structure and the equivalent electrical circuit of
the simple single-button carbon microphone. The single-button carbon microphone is characterized by
high output level and ruggedness. It is practically unaffected by heat and humidity. When space and
weight are limited in an installation, its high output is advantageous because fewer amplifier circuits are
required. The output ranges from 0.1 to 0.3 volt across a normal transformer impedance of 50 to 100
ohms.
b. To secure a more uniform response from various frequencies, the double-button type of
carbon microphone, illustrated in Figure 2-3C, is generally used in place of the single-button type. As
you can see, the diaphragm is placed between the two cups which contain carbon grains. Vibration
causes the grains of carbon on one side of the diaphragm to be compressed; at the same time, it causes
the grains of carbon on the opposite side of the diaphragm to be loosened. This action permits more
current through the first carbon button than through the second. The output voltage of the double-button
carbon microphone ranges from 0.02 to 0.07 volts across a normal transformer impedance of 200 ohms.
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