Iron–hydrogen resistor

This characteristic made it useful for stabilizing circuits against fluctuations in power-supply voltages.

A modern successor to the iron–hydrogen resistor is the semiconductor PTC thermistor.

The hydrogen gas protects the iron against oxidation and also enhances the effect, since the solubility of hydrogen in iron increases as temperature increases, resulting in higher resistance.

Iron–hydrogen resistors were used in the early vacuum tube systems in series with the tube heaters, to stabilize the heater circuit current against fluctuating supply voltage.

In 1930s Europe it was popular to combine them in the same glass envelope with an NTC-type thermistor made 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.