A bolometer element with a positive temperature coefficient of resistance, used to detect and measure power at radio, microwave, infrared, and optical frequencies. The temperature of the barretter increases when electromagnetic energy is absorbed. Barretters are made of metal; therefore, the electrical resistance increases when the temperature increases. The resulting resistance change of the barretter is measured by using direct-current or low-frequency instruments. Early radio demodulators and current regulator tubes using fine iron wires also have been called barretters.