155. Opisometer

 An opisometer, also called a meilograph or map measurer, is an instrument for measuring the lengths of arbitrary curved lines. A simple opisometer consists of a toothed wheel of known circumference on a handle. The wheel is placed in contact with the curved line to be measured and run along its length. By counting the number of teeth passing a mark on the handle while this is done, the length of the line can be ascertained: line length = wheel circumference × teeth counted/teeth on wheel.

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