Fil:Pyrometer example.png

English: Early pyrometer measuring the coefficient of expansion of a metal bar, from a 19th century chemistry textbook The metal bar (a) expands when heated by the lamps (f) and presses against a lever (b), which moves a pointer (c) along a scale that serves as a measuring index. (e) is an immovable prop which holds the bar in place. A spring on (c) pushes against (b), causing the index to fall back once the bar cools.