Coherer - Imperfect Junction Coherer

Imperfect Junction Coherer

There are several variations of what is known as the imperfect junction coherer. The principle of operation (microwelding) suggested above for the filings coherer may be less likely to apply to this type because there is no need for decohering. The iron and mercury variation is important because the Thomasina form was used by Marconi for the first transatlantic radio message. An earlier form was invented by Jagdish Chandra Bose in 1899. The device consisted of a small metallic cup containing a pool of mercury covered by a very thin insulating film of oil; above the surface a small iron disc is suspended. By means of an adjusting screw the lower edge of the disc is made to touch the oil-covered mercury with a pressure small enough not to puncture the film of oil. Its principle of operation is not well understood. The action of detection occurs when the radio frequency signal somehow breaks down the insulating film of oil, allowing the device to conduct, operating the receiving sounder wired in series. This form of coherer is self-restoring and needs no decohering.

In 1899, Bose announced the development of an "iron-mercury-iron coherer with telephone detector" in a paper presented at the Royal Society, London. He also later received U.S. Patent 755,840, "Detector for electrical disturbances" (1904), for a specific electromagnetic receiver.