Middle English gauge, from Anglo-French
- 1a : a measurement (as of linear dimension) according to some standard or system: as (1) : the distance between the rails of a railroad (2) : the size of a shotgun barrel's inner diameter nominally expressed as the number of lead balls each just fitting that diameter required to make a pound <a 12-gauge shotgun> (3) : the thickness of a thin material (as sheet metal or plastic film) (4) : the diameter of a slender object (as wire or a hypodermic needle) (5) : the fineness of a knitted fabric expressed by the number of loops per unit width
- b : dimensions, size
- c : measure 1 <surveys are a gauge of public sentiment>
- 2: an instrument for or a means of measuring or testing: as
- a : an instrument for measuring a dimension or for testing mechanical accuracy
- b : an instrument with a graduated scale or dial for measuring or indicating quantity
- 3: relative position of a ship with reference to another ship and the wind
- 4: a function introduced into a field equation to produce a convenient form of the equation but having no observable physical consequences
A caliper (British spelling also calliper, or in plurale tantum sense a pair of callipers) is a device used to measure the distance between two opposite sides of an object. A caliper can be as simple as a compass with inward or outward-facing points. The tips of the caliper are adjusted to fit across the points to be measured, the caliper is then removed and the distance read by measuring between the tips with a measuring tool, such as a ruler.
The earliest caliper has been found in the Greek Giglio wreck near the Italian coast. The ship find dates to the 6th century BC. The wooden piece already featured a fixed and a movable jaw. Although rare finds, caliper remained in use by the Greeks and Romans.
A bronze caliper, dating from 9 AD, was used for minute measurements during the Chinese Xin dynasty. The caliper had an inscription stating that it was "made on a gui-you day at new moon of the first month of the first year of the Shijian guo period." The calipers included a "slot and pin" and "graduated in inches and tenths of an inch."
The modern vernier caliper, reading to thousandths of an inch, was invented by American Joseph R. Brown in 1851. It was the first practical tool for exact measurements that could be sold at a price within the reach of ordinary machinists.