What are Carbon Microphones?
Carbon mics worked because the sound energy vibrated the carbon particles pushing them together and making the capsule a better or worse conductor of electricity. By forcing a small current through the carbon, the electric flow could be broken up by the effect of sound on the carbon. Carbon microphones were cheap and durable.
A carbon microphone, formerly used in telephone handsets, is a capsule containing carbon granules pressed between two metal plates. Their problem was that they did not have a very good frequency response. The carbon particles did not respond well to higher frequencies, and the low notes did not have enough energy to compress the particles to make a good connection. Carbon microphones have extremely low-quality sound reproduction and a very limited frequency response range, but are very robust devices. Carbon mics aren’t good for recording music.
In a carbon microphone a voltage is applied across the metal plates, causing a small current to flow through the carbon. One of the plates, the diaphragm, vibrates in sympathy with incident sound waves, applying a varying pressure to the carbon. The changing pressure deforms the granules, causing the contact area between each pair of adjacent granules to change, and this causes the electrical resistance of the mass of granules to change. The changes in resistance cause a corresponding change in the voltage across the two plates, and hence in the current flowing through the microphone, producing the electrical signal.
Unlike other microphone types, the carbon microphone can also be used as a type of amplifier, using a small amount of sound energy to produce a larger amount of electrical energy. Carbon microphones found use as early telephone repeaters, making long distance phone calls possible in the era before vacuum tubes. These repeaters worked by mechanically coupling a magnetic telephone receiver to a carbon microphone: the faint signal from the receiver was transferred to the microphone, with a resulting stronger electrical signal to send down the line.