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 55
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).

Iron-nickel alloy

Type: NiFe55

Speicification: φ0.5mm～φ6.0mm

Standard: GB5253-85

Applicaion: Mainly used in the production of electrode for cast iron.

Technical index: