This is one of two micrometers made by Joseph Finney of Liverpool between 1760 and 1772. The micrometer acts as a form of pyrometer, measuring the elongation of a bras bar when heated and comparing it with the elongation of another material similarly heated. A mahogany bar was originally with the instrument, probably to compare the expansion of metal with that of wood. The bars would have been fixed securely to the mahogany frame at one end, and presed against levers geared to the pointers which move over the dials at the other end. The dials enabled any expansion to be measured very precisely.
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