The Carbon Microphone

Author: N.H. Crowhurst

There is still another kind of microphone in common use; in fact, the carbon microphone forms the basis of millions of telephone instruments in use today. Any loose contact can be susceptible to vibrations around it that will alter its effectiveness in sympathy with the vibrations. A single loose contact, however, makes a poor microphone - all it can do is make noises that keep time with the speech or music. (It is, for example, suitable for relaying the ticking of a watch.)

The principle of operation of the contact microphone

The principle of operation of the carbon microphone

A carbon microphone extends this principle by using thousands of very small loose contacts. The space behind the diaphragm is loosely filled with tiny carbon granules. When the diaphragm vibrates due to sound waves reaching it, the granules are agitated. Because of the large number of contacts, the overall resistance of the microphone through the granules averages out in such a way that it follows the waveform of the sound striking the diaphragm. When a steady source of voltage is applied to the microphone, the current passed through it will fluctuate in sympathy with the sound waves.