Current Usage
Apart from legacy telephone installations in Third World countries, carbon microphones are still used today in certain niche applications in the developed world. An example is the Shure 104c, which is still in demand because of its wide compatibility with existing equipment.
The principal advantage carbon microphones have over other microphone types is that they can produce high-level audio signals from very low DC voltages, without needing any form of additional amplification or batteries. This 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. Most all-electronic telephones need at least 3 volts DC to work, and so will often become useless in such situations, whereas carbon transmitter telephones will continue to work down to a fraction of a volt. Even where they do work, electronic telephones also suffer from the so-called "cliff effect", whereby they abruptly stop working when the line voltage falls below the critical level. In particular, this means that one telephone on a "party line" may tend to "hog" all the line current, cutting the others off. With carbon microphones, all receivers on the same line will still operate, albeit with reduced output.
Carbon microphones are also widely used in safety-critical applications such as mining and chemical manufacture, where higher line voltages cannot be used, due to the risk of sparking and consequent explosions. Carbon-based telephone systems are also resistant to damage from high-voltage transients such as those produced by lightning strikes and electromagnetic pulses of the type generated by nuclear explosions, and so are still maintained as backup communication systems in critical military installations.
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