History Carbon microphone

the first microphone enabled proper voice telephony (loose-contact) carbon microphone (then called transmitter). independently developed around 1878 david edward hughes in england , emile berliner , thomas edison in us. although edison awarded first patent in mid-1877, hughes had demonstrated working device in front of many witnesses years earlier, , historians credit him invention.

hughes device used loosely packed carbon granules - varying pressure exerted on granules diaphragm acoustic waves caused resistance of carbon vary proportionally, allowing relatively accurate electrical reproduction of sound signal. hughes coined word microphone. demonstrated apparatus royal society magnifying sound of insects scratching through sound box. contrary edison, hughes decided not take out patent; instead, made invention gift world.

in america, edison , berliner fought long legal battle on patent rights. federal court awarded edison full rights invention, stating edison preceded berliner in transmission of speech...the use of carbon in transmitter is, beyond controversy, invention of edison , berliner patent ruled invalid.

the carbon microphone direct prototype of today s microphones , critical in development of telephony, broadcasting , recording industries. later, carbon granules used between carbon buttons. carbon microphones used in telephones 1890 until 1980s.

use amplifier

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.

early radio applications

early radio transmitters relied on carbon microphones voice modulation of radio signal. in first long-distance audio transmissions reginald fessenden in 1906, continuous wave alexanderson alternator fed directly transmitting antenna through water-cooled carbon microphone. later systems using vacuum tube oscillators used output carbon microphone modulate grid bias of oscillator or output tube achieve modulation.