Noun
an alloy of iron and nickel having a low coefficient of thermal expansion; used in tuning forks and measuring tapes and other instruments
Source: WordNetInvar pendulums were first used in 1898 in the Riefler regulator clock Milham 1945, p.331-332 which achieved accuracy of 15 milliseconds per day. Source: Internet
Invar and fused quartz Around 1900 low thermal expansion materials were developed which, when used as pendulum rods, made elaborate temperature compensation unnecessary. Source: Internet