25 Gauge Constantan Wire

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. Constantan is also used to form thermocouples with wires made of iron, copper, or chromel.

This alloy has, for example, an adequately high strain sensitivity, or gauge factor, which is relatively insensitive to strain level and temperature. Its resistivity is high enough to achieve suitable resistance values in even very small grids, and its temperature coefficient of resistance is not excessive.

In addition, constantan is characterized by good fatigue life and relatively high elongation capability. It must be noted, however, that 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.

Electrical resistivity at room temperature 3.2 Ω per meter for 25 gauge wire.

Temperature coefficient at 20 °C 8 ppm/K-1

Temperature coefficient -55 to 105 °C[1] ±40 ppm/K-1

Density 8.9 × 103 kg/m³

Melting point 1221 to 1300 °C

Specific heat capacity 0.39 J/(g·K)

Thermal conductivity at 23°C 19.5 W/(m.K)

Linear coefficient of thermal expansion at 25-105°C  14.9 × 10-6 K-1

Tensile strength 455-860 MPa

Elongation at fracture <45%

Elastic modulus 162 GPa