"Some people change their ways when they see the light, others when they feel the heat."
Caroline Schroeder, pianist
The bimetallic bar was invented by the Yorkshire clockmaker John Harrison (1693-1776), and was used in his third and fifth chronometers to cancel out thermally induced variations in the balance springs.
Imagine two straight strips of metal bar—steel and brass, say—riveted, brazed, or welded together along their length. Brass expands by nineteen parts in a million for every increase in temperature of 1.8°F (1°C), and steel by thirteen parts. Heating the bar will make one metal expand more than the other and cause the bar to bend. In the above example, the brass will be on the outer side of the curve. Cooling the bar will cause it to curve the other way, with the steel on the outside.
A spiral bimetallic strip unwinds or tightens as a function of temperature; a cheap, robust thermometer may be made by attaching a pointer. Strips are also used in clocks, forming the circular rim of the balance wheel, where the size of the wheel changes in a way that compensates for temperature variations in the strength of the controlling spring. But by far the most common usage is as a temperature-sensitive contact breaker in thermostatically controlled devices such as refrigerators, ovens, and irons. The fact that the metals conduct electricity means that the moving end of the strip can be used to open and close an electrical switch that is connected to a heater or cooler.
