The basic information about the carbon microphone

Carbon microphone is also known as carbon button microphone or carbon transmitter. It consists of two metal plates separated by carbon granules. One of the plates faces outwards and acts as a diaphragm. When sound waves hit these metal plates, the granules change, which in turn alters the electrical resistance between the plates.

DC current passes through the two plates and the changing resistance results in a change in current. The current is then passed through a telephone system to convert the sound into an electrical signal.

Carbon microphones were once known for their high-output, low-impedance performance and reasonably low cost. One of the main drawbacks of these microphones was poor sound reproduction quality and limited frequency response, as well as a high level of hissing noise. Due to these reasons they became less popular.

One of the merits of carbon microphones is that they can be used as amplifiers. This capability was used in early telephone repeaters, making long-distance telephone calls possible in the pre-vacuum tube period. These carbon microphones were mechanically coupled with a magnetic telephone receiver.

Another advantage of the carbon microphone is that it could be used to boost weak signals and send them down the line. This is possible since it works by changing the current that passes through it, rather than generating a signal voltage as is the case with most types of microphones. These amps were soon abandoned with the development of vacuum tubes as they offered higher gain and better sound quality.

An illustration of the amplification of these carbon microphones was the oscillation caused by feedback. This results in an audible screeching sound from the old candlelight telephone, if your earpiece is placed close to the microphones.

The main advantage of carbon microphones is that they have the ability to produce high level audio signals from very low DC voltages. And you don’t need any additional amplification or battery.

They are widely used in safety-critical applications such as mining and chemical manufacturing where higher line voltages cannot be used due to the risk of sparks and resulting explosions. Installing a large base of communications infrastructure around carbon microphones is already a costly affair; it is considerably cheaper to maintain the existing structure than to replace it with new technology.

They are also famous for being able to withstand damage from high voltage transients, such as those caused by lightning strikes and electromagnetic pulses of the type generated by nuclear explosions. They are still maintained as a backup communication system at critical military installations.

In the early days, AM radio transmitters were connected to carbon microphones for voice modulation of the radio signal. In Reginald Fessenden’s early audio transmissions, a continuous wave from an Alexanderson alternator was fed through a carbon microphone.

Carbon microphones are still used in certain niche applications in the developed world. This is due to its wide compatibility with existing equipment.

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