Wikipedia
Nisil
Nisil is an alloy of nickel (95.5% wt.) and silicon (4.4% wt.) with traces of Mg (0.1% wt.).
Nisil melts at 1341 - 1420 °C and has a density of 8.58 g/cm³, a Temperature resistance of 0.365 Ω⋅mm/m at 20 °C.
It is often used in conjunction with Nicrosil in type N thermocouples. In this use, it serves as the negative leg of the thermocouple. It can not be exposed to sulphur-containing gases.