An iron-hydrogen resistor consists of a hydrogen-filled glass bulb like 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. The modern successor to this device is the current source.
When the current rises the temperature will increase. The higher temperature leads to a higher electrical resistance, opposing the increase in current. The hydrogen filling protects the iron not only against oxidation, but enhances the effect since the solubility of hydrogen in iron will increase when the temperature increases, resulting in the higher resistance.
Iron-hydrogen resistors were used in the early vacuum tube systems in series with the heaters of electron tubes to stabilize the heater circuit current against fluctuating supply voltage.
This device is often called a "barretter" because of its similarity to the Barretter used for detection of radio signals.
In 1930's Europe it was popular to combine them in the same glass envelope with an NTC-type thermistor made out of UO2 until 1936, known as Urdox resistor and acting as an inrush current limiter for the series heater strings of domestic ac/dc tube radios.
- Praktikum der Physik von Wilhelm Walcher Page 241
- Regulator, Type 4A1, Museum of Victoria exhibit No: ST 029230
- Paleoelectronics RDH4 Ch 33, Ch 35