Invar is a nickel-iron alloy that is notable for its low coefficient of thermal expansion (CTE) . It is a single-phase alloy, consisting of approximately 36% nickel and 64% iron. Invar is used in applications where high dimensional stability is required, such as precision instruments, clocks, seismic creep gauges, television shadow-mask frames, valves in engines, and large aerostructure molds. Invar was first used in watch balance wheels and pendulum rods for precision regulator clocks, where its low CTE helped to reduce thermal variations in the length of clock pendulums. In land surveying, Invar is used to make the level staff (leveling rod) instead of wood, fiberglass, or other metals, when high-precision elevation leveling is to be performed. In the astronomical field, Invar is used as the substrate for some precision mirrors. Invar 36 is a specific type of Invar that exhibits controlled expansion qualities and is effective in extreme temperatures ranging from -150°C (-238°F) all the way up to 260°C (500°F) . Invar 36 and Invar 42 are the most effective alloys for the manufacturing of precision instrumentation due to their near-zero CTE and improved dimensional stability.