碳粒式麦克风

碳粒式麦克风，有时称作纽扣式麦克风或碳粒式传声器，是一种能将声音转化为电子信号的传感器，由两片分开的金属薄片及其之间的碳粒组成。朝外的那块金属薄片作为传导震动片。当声音使薄片振动时，会使其改变对碳粒的压力，会进一步改变两薄片之间的电阻 (挤压碳粒使其更加紧密，因此压力越大薄片间电阻越小)。当在两金属薄片之间施加直流电压，变化的电压会带来电流的变化，这些变化可以通过电话系统传输，或在电子系统中将声音转化成电信号。
在20世纪20年代，真空管电子放大器开始广泛应用之前，碳粒式麦克风是获得高品质音频信号的唯一实际的方法，被广泛用于电话系统中。它的制造成本低，输出信号强，以及良好的高频输出特性使其非常适用于电话系统，直到20世纪80年代，新安装的电话仍然在使用碳粒式麦克风。在此之前，它们在其他应用领域早已被其他类型的麦克风取代（在绝大多数的西方铜线电话网络中，旧式基于碳粒式麦克风的电话仍在使用未被淘汰）。
碳粒式麦克风曾被广泛应用于早期的调幅无线广播系统中(一般为改进过的电话麦克风)，但由于它们有限的响应频率，以及有较高的噪音，遂于19世纪20年代在该领域中被淘汰。接下来的数十年，它们继续被广泛应用于低端播音、军事以及业余无线电应用。 
Carbon microphones used as amplifiers.
One of the surprising attributes of carbon microphones is that they can actually be used as amplifiers. This capability was used in early telephone repeaters, making long distance phone calls possible in the era before vacuum tube amplifiers. In these repeaters, a magnetic telephone receiver (an electrical-to-mechanical transducer) was mechanically coupled to a carbon microphone. Because a carbon microphone works by varying a current passed through it, instead of generating a signal voltage as with most other microphone types, this arrangement could be used to boost weak signals and send them down the line. These amplifiers were mostly abandoned with the development of vacuum tubes, which offered higher gain and better sound quality. Even after vacuum tubes were in common use, carbon amplifiers continued to be used during the 1930s in portable audio equipment such as hearing aids. The Western Electric 65A carbon amplifier was 1.2" in diameter and 0.4" high and weighed less than 1.4 ounces. Such carbon amplifiers did not require the heavy bulky batteries and power supplies used by vacuum tube amplifiers. By the 1950s, carbon amplifiers for hearing aids had been replaced by miniature vacuum tubes (only to be shortly replaced by transistors). However, carbon amplifiers are still being produced and sold.
One illustration of the amplification provided by carbon microphones was the oscillation caused by feedback, that resulted in an audible squeal from the old "candlestick telephone" if its earphone was placed near the carbon microphone.
Early radio.
Early AM radio transmitters relied on carbon microphones for voice modulation of the radio signal. 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. Later systems using vacuum tube oscillators often used the output from a carbon microphone to modulate the grid bias of the oscillator or output tube to achieve modulation.
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" the all 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.