Marconi coherer tube (Rankin Kennedy, Electrical Installations, Vol V, 1903)

Drawing of a coherer, a radio wave detector used in the first radio receivers from about 1890 to 1910. The coherer was invented by Edouard Branly in 1890, but this widely used design was perfected by Guglielmo Marconi around 1900. It consists of an evacuated glass tube with two electrodes (P) spaced a few millimeters apart, with loose metal filings in the space between. When a radio signal from an antenna is applied across the electrodes, it causes the filings to clump together (cohere), reducing their resistance. This allowed a current from a battery to flow in a DC circuit attached to the electrodes, making a click sound in an earphone or a mark on a paper tape, to record the code symbol received. The filings used in this Marconi coherer were made of 90% nickel - 10% silver. The slanting ends of the electrodes allowed the sensitivity of the device to be adjusted somewhat by rotating the tube, changing the space occupied by the filings.