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primary of a transformer. The changing resistance of the carbon pile produces a corresponding change in
the current of the circuit. The varying current in the transformer primary produces an alternating voltage
in the secondary. The transformer steps up the voltage and matches the low impedance of the microphone
to the high impedance of the first amplifier. The voltage across the secondary may be as high as 25 volts
peak. The impedance of this type of microphone varies from 50 to 200 ohms. This effect is caused by the
pressure of compression and rarefaction of sound waves, discussed in chapter 1 of NEETS, Module 10,
Introduction to Wave Propagation, Transmission Lines, and Antennas.
Figure 1-33A.—Carbon microphones. SINGLE-BUTTON CARBON MICROPHONE.
The DOUBLE-BUTTON CARBON MICROPHONE is shown in figure 1-33, view (B). Here, one
button is positioned on each side of the diaphragm so that an increase in resistance on one side is
accompanied by a simultaneous decrease in resistance on the other. Each button is in series with the
battery and one-half of the transformer primary. The decreasing current in one-half of the primary and the
increasing current in the other half produces an output voltage in the secondary winding. The output
voltage is proportional to the sum of the primary winding signal components. This action is similar to that
of push-pull amplifiers.
Figure 1-33B.—Carbon microphones. DOUBLE-BUTTON CARBON MICROPHONE.
