What kind of instrument uses two dissimilar metals to measure temperature changes due to thermal expansion?

A bimetal thermometer is designed specifically to measure temperature changes through the principle of thermal expansion using two dissimilar metals. When these metals are bonded together and exposed to temperature variations, they expand at different rates based on their unique coefficients of thermal expansion. This differential expansion causes the bimetallic strip to bend in response to temperature increases or decreases. The degree of bending is then translated into temperature readings via a calibrated dial or scale.

This type of thermometer is particularly useful in various applications where direct temperature measurement is needed. Unlike a thermocouple, which generates a voltage based on the temperature difference between its two junctions, the bimetal thermometer relies on the physical movement of the thermally responsive metal. Infrared thermometers, on the other hand, measure temperature from a distance based on the infrared radiation emitted by an object, which is a different principle altogether. Liquid-in-glass thermometers use thermal expansion of liquids, typically mercury or alcohol, and do not involve dissimilar metals. Hence, the bimetal thermometer stands out for its method of incorporating two metals to directly gauge temperature changes through mechanical movement.