Definition
An iron-nickel alloy, approximately 36% nickel and 64% iron, that exhibits an extremely low coefficient of thermal expansion. Because its dimensions change very little with temperature variation, it is used in components where dimensional stability is critical, such as the balance weights and bimetallic compensating elements in precision instruments and certain piston engine valve mechanisms.
Plain English
A special metal alloy that barely changes size when it gets hot or cold. This makes it useful in parts that must keep their exact shape across a wide range of temperatures.
Context Anchor
Seen in aircraft instrument, maintenance, and materials discussions, especially where temperature changes could affect measurement accuracy.
Derivation
The name 'Invar' is short for 'invariable,' meaning 'not changing.' It was coined to describe the alloy's near-invariable length across temperature changes.
Why Pilots Care
Maintains the accuracy of instruments such as altimeters and gyros when outside air temperatures fluctuate during flight.
Intuition Check
Invar does not mean the metal never changes at all. It means it changes very little compared with many other metals when temperature changes.
Example Sentence 1
The balance wheel in the precision aircraft clock was made of Invar to keep its timing accurate from the cold of high altitude to the heat of the cockpit on the ramp.
Example Sentence 2
During instrument overhaul, technicians verify that Invar components are present to preserve calibration stability.