Thermocouple iron is used for the positive leg of thermocouples type J and L. The alloy is also used as the positive leg of compensating cables type KCA (WX).

The alloy is transformed magnetically at 770°C (1420°F) and a crystallographic transformation occurs at 900°C (1650°F). Both transformations affect the thermoelectric properties of the alloy.

Iron can not be exposed to sulphur-containing atmospheres and to temperatures below zero. Thermocouple iron wire is supplied copper-coated to prevent oxidation.

Thermocouple type J and L are frequently used because of their high emf output and their low cost. Compensating cables of type KCA are used together with thermocouples of type K.