Bimetallic Coil Basics A bimetallic coil is made of two separate metal strips bonded to each other, then rolled into a spiral in order to make the strip more compact. One end of the coil is anchored, and the other is attached to a pointer. When the temperature changes, one of the metals expands more than the other, causing the coil to tighten or unroll slightly. This motion, in turn, causes the pointer to move.
Although not quite as accurate as a spirit gauge, these mechanisms are very durable while allowing for a large pointer and numerals for ease of reading. They are accurate to within +2/-2 degrees, compared to +1/-1 degree for spirit gauges.
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