English: A 'ball' coherer, an antique radio wave detector used in the first radio receivers around 1900. This unusual type, invented by Edouard Branly in 1899, used a row of lightly touching steel balls in a tube between two electrodes. A radio wave from an antenna is applied to the end electrodes, directly across the balls. When a radio signal arrives, it causes the DC resistance of the contacts between the balls to decrease. This triggers a battery-powered circuit, also connected across the electrodes, to make a "click" sound or a mark on a paper tape to record the radio message. The balls must be "decohered", reset to the high resistance state, by tapping them mechanically after every signal. Branly found that radio waves from a nearby electric spark reduced the resistance of the device from 2000 to 100 ohms. The sensitivity can be varied by tilting the tube to different angles.