Definition
A copper-nickel alloy, typically about 55% copper and 45% nickel, valued for its nearly constant electrical resistance across a wide temperature range. In aviation maintenance, constantan is most commonly used as one of the two dissimilar metals in thermocouples, particularly the iron-constantan thermocouple used to measure cylinder head temperature on reciprocating engines.
Plain English
A special metal mix of copper and nickel that barely changes its electrical resistance when it gets hot or cold. That stable behavior makes it useful for building temperature-sensing wires in engine instruments.
Context Anchor
Seen in aircraft maintenance discussions of temperature indicators, thermocouple wiring, and electrical measuring circuits.
Derivation
The name comes from the Latin 'constans,' meaning 'standing firm' or 'unchanging.' It was named for the alloy's defining property: its electrical resistance stays nearly constant as temperature changes, which is exactly what you want in a precision sensing wire.
Why Pilots Care
Thermocouples made with constantan are how engine temperature gauges actually work. Knowing what the sensing element is helps a technician understand why thermocouple leads must be the correct alloy and correct length -- substituting ordinary copper wire will produce false readings.
Intuition Check
Constantan does not mean a wire that makes temperature constant. It means a wire material whose electrical behavior stays relatively steady while temperature changes.
Example Sentence 1
The cylinder head temperature probe uses an iron-constantan thermocouple, so the leads must be replaced with matching alloy wire, not standard copper.
Example Sentence 2
Constantan is paired with iron or chromel in thermocouples to produce a predictable voltage as temperature rises.