Use as amplifier Carbon microphone

operation of carbon microphone. when sound wave presses on conducting diaphragm, granules of carbon pressed , decrease electrical resistance.

carbon microphones can used amplifiers. capability used in telephone repeaters, making long distance phone calls possible in era before vacuum tube amplifiers. in these repeaters, magnetic telephone receiver (an electrical-to-mechanical transducer) mechanically coupled carbon microphone. because carbon microphone works varying current passed through it, instead of generating signal voltage other microphone types, arrangement used boost weak signals , send them down line. these amplifiers abandoned development of vacuum tubes, offered higher gain , better sound quality. after vacuum tubes in common use, carbon amplifiers continued used during 1930s in portable audio equipment such hearing aids. western electric 65a carbon amplifier 1.2 in diameter , 0.4 high , weighed less 1.4 ounces. such carbon amplifiers did not require heavy bulky batteries , power supplies used vacuum tube amplifiers. 1950s, carbon amplifiers hearing aids had been replaced miniature vacuum tubes (only shortly replaced transistors). however, carbon amplifiers still being produced , sold.

an illustration of amplification provided carbon microphones oscillation caused feedback, resulted in audible squeal old candlestick telephone if earphone placed near carbon microphone.