English: A tripod coherer, an experimental radio wave detector invented in 1902 by French radio researcher Édouard Branly . Coherers were used in the first radio receivers around 1900. In this device the radio signal from an antenna was applied between the metal 'tripod' and the bottom plate. Although most coherers functioned as "switches", turning on a DC current from a battery to record the radio signal, the tripod coherer may have been one of the first rectifying| (diode) detectors, because Branly reported it could produce a DC signal without a battery. The imperfect contact between the metal points and the iron plate, mediated by a semiconducting coating of iron oxide on the points, functioned as a crude PN junction, rectifying the radio waves. The rectified signal was applied to an earphone connected across the plates, and the "dots" and "dashes" of the Morse code transmitter could be heard. This made it one of the first semiconductor devices, a predecessor to the crystal detector and the semiconductor diode