Bimetal strips consists of two different metals welded together to form a cantilever as shown in Figure (a). When heated, both metals expand, but the bottom strip (assuming that the lower strip has the higher expansion coefficient), expands more than the top metal. The result is that the bottom of the strip becomes longer than the top, and the cantilever curls upwards as shown in Figure (b).
A bi-metal thermometer uses a bi-metal strip, shaped in a helix or spiral form, as shown in Figure a. The one end is fixed and the other end is free to rotate as the helix curls in or out with changing temperature. A shaft and pointer is linked to the rotating helix, to indicate the temperature. The stem is filled with silicone fluid, to provide damping and thermal conductivity between the stem and bi-metal strip.