Definition
A coiled spring made by bonding two different metals together, used in temperature-sensitive instruments. Because the two metals expand and contract at different rates with temperature change, the spring tightens or loosens as temperature varies, which compensates for temperature-induced errors in the instrument's reading.
Plain English
A small coiled spring built from two metals stuck together. The two metals react differently to heat, so the spring automatically adjusts itself when the temperature changes. This keeps the instrument it's part of accurate even when it gets hotter or colder.
Context Anchor
Seen in aircraft maintenance discussions of engine instruments, especially gauges that must stay accurate as cockpit or engine-area temperature changes.
Derivation
Bi- (Latin, 'two') + metallic ('made of metal'), combined with hairspring — a very fine, hair-thin coiled spring. So it literally means a hair-thin coiled spring made of two metals. The two-metal construction is the whole point: it's what gives the spring its self-adjusting behavior.
Why Pilots Care
Instruments that use bimetallic hairsprings stay accurate across the wide temperature range an aircraft sees from ground to altitude. A maintenance technician needs to recognize the part and understand that damage to it can cause subtle, temperature-dependent instrument errors that may not show up during a warm hangar check.
Analogy
It is like a tiny metal coil that bends when warmed, similar to the curled metal strip used in some simple thermostats.
Intuition Check
Do not read “hairspring” as anything soft or flexible like a hair. Here it means a very small metal spring used inside a precision instrument.
Example Sentence 1
The airspeed indicator uses a bimetallic hairspring to compensate for temperature changes inside the instrument case.
Example Sentence 2
Replacement of the bimetallic hairspring restored the instrument's accuracy after it had been giving erroneous readings on hot days.