Wikipedia
Fernico describe a family of metal alloys made primarily of iron, nickel and cobalt. The family includes Kovar, FerNiCo I, FerNiCo II, and Dumet. The name is made up of the chemical symbols of its constituent three elements. "Dumet" is a portmanteau of "dual" and "metal," because it is a heterogeneous alloy, usually fabricated in the form of a wire with an alloy core and a copper cladding. These alloys possess the properties of electrical conductivity, minimal oxidation and formation of porous surfaces at working temperatures of glass and thermal coefficients of expansion which match glass closely. These requirements allow the alloys to be used in glass seals, such that the seal does not crack, fracture or leak with changes in temperature.
Dumet is most commonly used in seals where lead-in wires pass through the glass bulb wall of standard household electric lamps (light bulbs) among other things.
The two Fernico alloys both consist of iron (Fe), nickel (Ni), and cobalt (Co). Fernico is used at high temperatures and is identical to Kovar. Fernico II is used at cryogenic temperatures in the range. Both are used to create electrically conductive paths through the walls of sealed borosilicate glass containers. Dumet is used for a similar purpose, but is tailored for seals through soda lime and lead alkali silicate glasses.
These alloys adhere to lead-tin, tin, and silver solders. Other metals, including copper, molybdenum, nickel, and steel can be spot-welded to the FerNiCo alloys forming low resistance electrical connections.