Constantan

Constantan
Type	Copper-nickel alloy
Physical Properties
Density (ρ)	8 885.24941 kg/m^3
Mechanical Properties
Young's modulus (E)	24 x 106 PSI
Tensile strength (σt)	60,000 PSI
Elongation (ε) at break	25%
Thermal Properties
Melting temperature (Tm)	1210° C
Thermal conductivity (k)	21.2W/m· K
Specific heat capacity (c)	.094 gram· cal./°C
Electrical Properties
Surface resistivity	0.49 μΩ·m

Constantan is a copper–nickel alloy also known as Eureka, Advance, and Ferry. It usually consists of 55% copper and 45% nickel. Its main feature is its resistivity, which is constant over a wide range of temperatures. Other alloys with similarly low temperature coefficients are known, such as manganin (Cu₈₆Mn₁₂Ni₂).

In 1887, Edward Weston discovered that metals can have a negative temperature coefficient of resistance, inventing what he called his "Alloy No. 2." It was produced in Germany where it was renamed "constantan".

Of all modern strain gauge alloys, constantan is the oldest, and still the most widely used. This situation reflects the fact that constantan has the best overall combination of properties needed for many strain gauge applications. This alloy has, for example, an adequately high strain sensitivity, or gauge factor, which is relatively insensitive to strain level and temperature. Its resistivity (49.0 x 10⁻⁸ Ω·m) is high enough to achieve suitable resistance values in even very small grids, and its temperature coefficient of resistance is fairly low. In addition, constantan is characterized by good fatigue life and relatively high elongation capability. However, constantan tends to exhibit a continuous drift at temperatures above 65 °C (150 °F); and this characteristic should be taken into account when zero stability of the strain gauge is critical over a period of hours or days. Constantan is also used for electrical resistance heating and thermocouples.

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