Constantan, also known in various contexts as Eureka, Advance, and Ferry, refers to a copper-nickel alloy commonly used for its stable electrical resistance across a wide range of temperatures.

In 1887, Edward Weston discovered that metals can have a negative temperature coefficient of resistance, inventing what he called his "Alloy No.

However, constantan tends to exhibit a continuous drift at temperatures above 65 °C (149 °F);[5] and this characteristic should be taken into account when zero stability of the strain gauge is critical over a period of hours or days.

[6] Very importantly, constantan can be processed for self-temperature compensation to match a wide range of test material coefficients of thermal expansion.

A-alloy is supplied in self-temperature-compensation (S-T-C) numbers 00, 03, 05, 06, 09, 13, 15, 18, 30, 40, and 50, for use on test materials with corresponding thermal expansion coefficients, expressed in parts per million by length (or μm/m) per degrees Fahrenheit.