A Little About Strain Gauges

Strain gauges are often made of constantan wire, which consists of 60 percent copper and 40 percent nickel. The electrical resistance of constantan is relatively independent of changes in temperature. On the other hand, the resistance of constantan does change somewhat when the wire is stretched. The wire is said to have a 'gauge factor' of two. The phrase means that the percent change in the wire's electrical resistance corresponds to twice the percent change in length.

Another popular material for strain gauges is isoelastic wire, an alloy of iron, nickel, chromium and other metals. It has a large temperature coefficient and a gauge factor of 3.5. I recommend that amateurs buy the Type AR-1 strain gauge for this experiment, because the substance to be studied must be cooled far below room temperature. The resistance of this gauge at room temperature is about 120 ohms; the gauge factor is two. The unit is available from distributors of scientificsupplies.