A bimetallic strip is a piece of metal made by laminating two different types of metal together. The metals that make up the strip expand and contract when they're heated or cooled. Each type of metal has its own particular rate of expansion, and the two metals that make up the strip are chosen so that the rates of expansion and contraction are different. When this coiled strip is heated, the metal on the inside of the coil expands more and the strip tends to unwind.
The center of the coil is connected to the temperature-adjustment lever, and the mercury switch is mounted to the end of the coil so that when the coil winds or unwinds, it tips the mercury switch one way or the other.
In non-digital thermostats there are two switches. These switches move small metal balls that make contact between different traces on the circuit card inside the thermostat. One of the switches controls the mode (heat or cool), while the other switch controls the circulation fan. On the next page, we'll see how these parts work together to make the thermostat work.