Application
The coherer used in practical receivers was a glass tube, sometimes evacuated, which was about half filled with sharply cut metal filings, often part silver and part nickel. Silver electrodes made contact with the metal particles on both ends. The electrodes were slanted so the width of the gap occupied by the filings could be varied by rotating the tube about its long axis, thus adjusting its sensitivity to the prevailing conditions.
In operation, the coherer is included in two separate electrical circuits. One is the antenna-ground circuit shown in the untuned receiver circuit diagram below. The other is the battery-sounder relay circuit including battery B1 and relay R in the diagram. A radio signal from the antenna-ground circuit "turns on" the coherer, enabling current flow in the battery-sounder circuit, activating the sounder, S. The coils, L, act as RF chokes to prevent the RF signal power from leaking away through the relay circuit.
One electrode, A, of the coherer, (C, in the left diagram) is connected to the antenna and the other electrode, B, to ground. A series combination of a battery, B1, and a relay, R, is also attached to the two electrodes. When the signal from a spark gap transmitter is received, the filings tend to cling to each other, reducing the resistance of the coherer. When the coherer conducts better, battery B1 supplies enough current through the coherer to activate relay R, which connects battery B2 to the telegraph sounder S, giving an audible click. In some applications, a pair of headphones replaced the telegraph sounder, being much more sensitive to weak signals.
The problem of the filings continuing to cling together after the removal of the signal was solved by tapping the coherer with a small arm attached to the sounder after the arrival of each signal, shaking the filings and raising the resistance of the coherer to the original value. This apparatus was called a decoherer. In practical implementations, the decoherer was the clapper of a door bell that was powered by the coherer current itself. This is referred to as 'decohering' the device and was subject to much innovation during the life of the popular use of this component. Tesla, for example, had the tube rotating continuously along its axis, following each successive activation.
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