Iron–hydrogen_resistor
An iron–hydrogen resistor consists of a hydrogen-filled glass bulb (similar to a light bulb), in which an iron wire is located. This resistor has a positive temperature coefficient of resistance. This characteristic made it useful for stabilizing circuits against fluctuations in power-supply voltages.[1] This device is often called a "barretter" because of its similarity to the barretter used for detection of radio signals. The modern successor to the iron–hydrogen resistor is the current source.