A mechanical temperature gauge uses a Bourdon tube filled with temperature-sensitive liquid to indicate temperature, while an electrical model uses a variable-resistance unit that varies current to the dial as temperatures change. Most modern vehicles use electrical gauges as the mechanical versions are sensitive and expensive to replace.
An electrical temperature gauge works like a voltmeter. A sending unit containing a temperature-sensitive material is installed as part of the cooling system of an engine and is affected by coolant temperatures. As an engine warms up, so does the coolant. This lowers the resistance of the sending unit, meaning more current flows through the circuit. A bimetallic spring sits behind the face of the gauge and expands as more current flows through it. This expansion moves the needle on the gauge higher as engine temperatures rise.