Patent Drawing of Engine - 613809-teslaRotatingCoherer - Public domain portrait drawing Public domain image

Drawings of a rotating coherer invented by Serbian-American engineer Nikola Tesla, from his US patent no. 613,809, application filed July 1, 1898, granted November 8, 1898. Drawings from Sheet 4 of patent application, fig. 7 (right) and 8 (left). with part labels removed and explanatory caption added. The coherer, invented in 1890 by Edouard Branly, was an antique radio component, an early radio wave detector used in the first radio receivers around 1900. It consists of a tube containing metal filings between two electrodes. A radio wave applied between the electrodes reduces the device's resistance, causing it to "turn on" and conduct electric current. This triggers a DC circuit with a battery, also connected to the electrodes. However, after the coherer is "turned on" by the radio waves, it's resistance stays low. To prepare it to receive the next radio signal, the filings must be "decohered", mechanically disturbed in order to return the device to the high resistance "off" state. Tesla's innovation in his coherer was to continuously rotate the coherer tube, to disturb the filings. Tesla used this device in a radio-controlled boat, one of the first remotely controlled vehicles. However simpler methods of "decohering" coherers were found, and Tesla's coherer wasn't much used.