How a dial thermometer works: This is the mechanism that powers a typical dial thermometer, illustrated in a patent by Charles W. Putnam from 1905. At the top, we have the usual pointer and dial arrangement. The bottom artwork shows what's happening round the back. A bimetallic strip (yellow) is tightly coiled and attached both to the frame of the thermometer and the pointer. It's made up of two different metals bonded together, which expand by different amounts as they heat up. As the temperature changes, the bimetallic strip curves more or less tightly (contracts or expands) and the pointer, attached to it, moves up or down the scale.
It's easy to see how coiled bimetallic strip from an actual dial thermometer works: if you turn the pointer with your hand toward colder temperatures, the coiled strip tightens up; turn the pointer the other way and the strip loosens.
One problem with dial thermometers is that they take a while to react to temperature changes. Electronic thermometers don't have that problem: you simply touch the thermometer probe onto the object whose temperature you want to measure and the digital display gives you an instant temperature reading.
