Carbon microphone

Sound waves striking the diaphragm cause it to vibrate, exerting a varying pressure on the granules, which in turn changes the electrical resistance between the plates.

The frequency response of most carbon microphones, however, is limited to a narrow range, and the device produces significant electrical noise.

Before the proliferation of vacuum tube amplifiers in the 1920s, carbon microphones were the only practical means of obtaining high-level audio signals.

Carbon microphones, usually modified telephone transmitters, were widely used in early AM radio broadcasting systems, but their limited frequency response, as well as a fairly high noise level, led to their abandonment in those applications by the late 1920s.

They continued to be widely used for low-end public address, and military and amateur radio applications for some decades afterward.

Although Edison was awarded the first patent in mid-1877, Hughes had demonstrated his working device in front of many witnesses some years earlier, and most historians credit him with its invention.

[9] This capability was used in early telephone repeaters, making long-distance phone calls possible in the era before vacuum tube amplifiers.

In the first long-distance audio transmissions by Reginald Fessenden in 1906, a continuous wave from an Alexanderson alternator was fed directly to the transmitting antenna through a water-cooled carbon microphone.

Apart from legacy telephone installations in various conditions according to areas and countries, carbon microphones may be still used today in certain niche applications although manufacturers are discontinuing distribution.

The microphone's low-voltage performance is particularly useful in remote locations served by very long telephone lines, where the electrical resistance of the wires can lead to severe DC voltage drop.

Carbon microphones are also widely used in safety-critical applications such as mining and chemical manufacturing, where higher line voltages cannot be used, due to the risk of sparking and consequent explosions.