An iron-hydrogen resistor consists of a hydrogen-filled glass bulb like a light bulb, in which an iron wire is located.

Early technologies included iron-hydrogen resistors, resonant transformers, nonlinear resistors, loading resistors, neon stabiliser tubes, vibrating contact regulators etc.

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.