File:Branly copper rod coherer.png

English: A simple radio wave detector or coherer invented by French scientist Édouard Branly around 1890 and used in some of the first experiments at receiving radio communication. It consists of two copper bars which are roughened and oxidized and laid one over the other, creating a high resistance junction. A dipole radio antenna consisting of two wires were attached to the two bars. The bars were also connected 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 to the bars, the resistance of the contact between them dropped to a low level. Electric current from the battery passed through the coherer and was detected by the galvanometer, registering the presence of the radio wave. Before receiving another signal, the device had to be restored to its receptive condition by tapping it, disturbing the junction between the bars to reset it to its previous high resistance state. Branly wrote that the device could be made more sensitive by grinding the top bar into a knife edge and resting it very lightly on the other.