Modern cars use electric temperature gauges to keep track of radiator and engine temperature. They operate like a voltmeter, with a special hairpin-like assembly opening and closing to create an electrical circuit. Old cars used a Bourdon Tube, which is a mechanical temperature gauge.
The electrical temperature gauge actually reads voltage, which is translated into temperature readings on the gauge. It needs an electrical circuit and a sending unit to work. The former is provided by the vehicle battery, while the latter is made up of a temperature-sensitive material that is sealed inside a watertight unit that hangs in the engine's coolant stream.
The voltage from the battery enters the gauge on one side, runs through the bimetallic spring and goes to the sending unit, which acts as a ground. When an engine is cold, very little current passes and the gauge has a low reading. As the engine warms, the resistance lowers, allowing more current through and making the temperature on the gauge rise.
The Bourdon Tube is a thin brass or copper tube that is filled with alcohol or another vaporizing fluid and sealed at both ends. The end that attaches to the gauge is made into a spiral with the indicator needle attached. The other end is attached to a watertight connector and sits in the engine coolant. As the engine heats up, the vaporizing fluid in the tube expands, the spiral unwinds, and it moves the gauge needle. This setup is usually only found on antique cars.