A student is investigating how the resistance of a precision strain gauge (made of a very thin foil of constantan alloy) changes under mechanical stress. The strain gauge is connected in a circuit with a DC power supply, a microammeter, a digital voltmeter, and a variable resistor (rheostat) as shown in the diagram.
The variable resistor is adjusted to keep the current very low to prevent 'self-heating' of the strain gauge.
Explain how the variable resistor is used to prevent the strain gauge from experiencing significant self-heating.