An early name for a device that "detected" radio signals. A detector is actually a rectifier that only passes the (intermittent) DC component of an AC signal - thereby allowing the signal to be used as a representative voltage to pass to an amplifier. Early detectors were made from carbon powder or crystals of Galena touched by a thin wire of copper, brass or sometimes silver (the Cat's Whisker) or carbon

A device that measures the amount of energy radiated by an object. Can be a thermal detector or a photodetector. Thermal detectors respond to radiation by changing their volume, capacitance, or generation of millivoltages; they can be thermocouples, thermopiles, pneumatic detectors, or bolometers. Their common feature is their relatively slow response. Photodetectors are Semiconductors which produce a signal in proportion to the photon flux which strikes them.