Those are iron and nickel based alloys which are characterised by their high temperature coefficient which confers them a very important variation of resistance.

Alloys in question are:

- FeNi 70
– Invar® 
 
They are also characterized by their dilatation at some temperatures.

Their resistivity is relatively high (from 0.20 to 0.83 ohm mm²/m).
They often intervene in the applications where an autoregulation of the power (thermostats) is necessary, but also in the manufacture of leading-in for lamps (capping with the glass).

The alloy Invar® is used for its property to have a linear expansion coefficient null.