Detectors 094
Detectors 094 : Particle Detectors: (4.5) Coherer: (1) Operation: Unlike modern radio stations that transmit a radio signal modulated by an audio signal, the first radio transmitters transmitted unmodulated carrier wave signals used to send Morse code, consisting of "dots" and "dashes", by on-off keying, so early radio receiving apparatus merely had to detect the presence or absence of the radio signal, not convert it to audio. The device that did this was called a detector. The coherer was the most successful of many detector devices that were tried in the early days of radio. The basis for the operation of the coherer is that metal particles cohere (cling together) and conduct electricity much better after being subjected to radio frequency electricity. The radio signal from the antenna was applied directly across the coherer's electrodes. When the radio signal from a "dot" or "dash" came, the coherer would become conductive. The coherer's electrodes were also attached to a DCcircuit powered by a battery that created a "click" sound in earphones or a telegraph sounder, or a mark on a paper tape, to record the signal. Unfortunately, the reduction in the coherer's electrical resistancepersisted after the radio signal was removed. This was a problem because the coherer had to be ready immediately to receive the next "dot" or "dash". Therefore a decoherer mechanism was added, to tap the coherer, mechanically disturbing the particles to reset it to the high resistance state. Coherence of particles by radio waves is an obscure phenomenon that is not well understood even today. Recent experiments with particle coherers seem to have confirmed the hypothesis that the particles cohere by a micro-weld phenomenon caused by radio frequency electricity flowing across the small contact area between particles. The underlying principle of so-called "imperfect contact" coherers is also not well understood, but may involve a kind of tunneling of charge carriers across an imperfect junction between conductors
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