A Chinese bronze caliper from 9 AD was already marked in tenths of an inch
During the brief Xin dynasty, Chinese craftsmen made a bronze caliper with a slot and pin, graduated in inches and tenths, and dated by inscription to the first day of the first month of the dynasty's first year. The oldest known caliper is older still: a wooden one from a sixth-century BC Greek shipwreck.
Calipers measure length, width, thickness, depth or the diameter of a hole. The name is a corrupted form of caliber; Americans spell it with one L, the British with two, and like scissors it is often treated as plural. The Giglio wreck, a Greek ship sunk off the Italian coast, yielded a wooden caliper that already had one fixed and one sliding jaw. Such tools stayed in use among Greeks and Romans, though finds are rare.
The familiar vernier caliper is named after Pierre Vernier, who improved on the nonius devised by the Portuguese mathematician Pedro Nunes. A vernier scale lets the user interpolate precisely between the main markings. Reading one well means accounting for zero-point error: if closed jaws show 0.013 centimetres, that figure must be subtracted from every result, so a raw reading of 2.475 becomes 2.462. Dial and digital calipers do the same job with a needle or screen.
The simplest calipers have no scale at all. Two legs are set to span an object, then carried to a ruler or directly to the workpiece. Outside calipers of high-carbon steel can straddle a pipe too wide for a vernier's jaws. Oddleg calipers, also called hermaphrodite calipers or oddleg jennys, run one bent leg along an edge while the other scribes a parallel line.
Dividers, a pointed relative, have a clever trick: by walking the points end to end across a board and adjusting until the last step lands exactly on the edge, a worker can mark equal sections without measuring anything. Navigators use them on charts, where one minute of arc along a meridian equals roughly one nautical mile, or 1,852 metres. Cardiologists use a version to measure the spacing of heartbeats on an ECG trace, and Robert A. Mackin invented a pocket model.
Source: Calipers
