File:Branly coherer.png

English: Diagram of the first coherer, invented in 1890 by French scientist Édouard Branly, an early radio signal detector used in the first radio receivers. The coherer was the first practical radio wave receiver, used in the first radio communication systems by Oliver Lodge, Alexander Stepanovich Popov, and Guglielmo Marconi until about 1910.

It consists of a tube (1) containing metal powder (3), in contact with two pairs of electrodes (2,4 and 5,6) The top and bottom electrodes were connected to a dipole antenna to pick up the radio waves, and also to a DC circuit consisting of a battery and a galvanometer. When a radio signal from a spark gap radio transmitter was received by the antenna and applied across the electrodes, it caused the metal powder to become conductive. This allowed current from the battery to pass through the coherer and register on the galvanometer, indicating the presence of the radio wave. To restore the coherer to its high resistance receptive condition, the metal particles had to be disturbed by tapping it. The thumscrew (2) at top allows the pressure on the powder to be adjusted. Branly used the side electrodes (5,6) in experiments to measure the resistance of the powder at right angles to the radio current, establishing that the conductivity of the powder was increased in all directions.