Invar is a metal alloy invented by Charles Edouard Guillaume that exhibits nearly zero thermal expansion. That is, it grows/shrinks very little when subjected to moderate heating/cooling. This property is very useful in precision machinery. Invar is used frequently in very precise mechanical clocks, scientific instruments, color CRT shadow masks, some surveying tapes (now replaced by laser rangefinding), VHF/UHF/microwave duplexers and other resonant cavities. NASA used a rare alloy of Invar to make the camera on the spacecraft Cassini.

Invar should be treated nicely as dropping it will disturb the grain structure and possibly make it magnetic.

Invar is relativley simple alloy made of steel with about 36% nickel and tiny amounts of other elements. Invar 36 is a common grade and exhibits a very small coefficent of thermal expansion (1.6*10-6/K) but is a pain in the ass to machine. Another Invar alloy, FM (free machining) is much nicer to work with. It has more maganese and selenium as well as twice as much carbon as Invar 36 (which can lead to lower long-term stability). Super Invar has a much lower coefficent of thermal expansion (0.63*10^-6/K) but has some wack properties like spontaneously changing size and is still a son of a bitch to machine.