Calipers

Caliper(s) or calliper(s) are an instrument used to measure the dimensions of an object, generally by placing two movable points of the instrument across the object or span to be measured. Many types of calipers permit reading out a measurement on a ruled scale, a dial, or an electronic digital display. A common association is to calipers using a sliding vernier scale. Some calipers can be as simple as a compass with inward or outward-facing points, but with no scale (measurement indication). The tips of the caliper are adjusted to fit across the points to be measured, and then kept at that span while moved to separate measuring device, such as a ruler. Calipers are used in many fields such as mechanical engineering, metalworking, forestry, woodworking, science and medicine. Caliper is the American spelling, while calliper (double "L") is the British spelling. A single tool might be referred to as a caliper or as calipers — a plural only (plurale tantum) form, like scissors or glasses. Colloquially, the phrase "pair of verniers" or just "vernier" might refer to a vernier caliper. In loose colloquial usage, these phrases may also refer to other kinds of calipers, although they involve no vernier scale. In machine-shop usage, the term "caliper" is often used in contradistinction to micrometer, even though outside micrometers are technically a form of caliper. In this usage, caliper implies only the form factor of the instrument. The earliest caliper has been found in the Greek Giglio wreck near the Italian coast. The ship's find dates to the 6th century BC. The wooden piece already featured a fixed and a movable jaw. Although rare finds, calipers 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 the gui-you day, the first day of the first month of the first year of Shijianguo ." The calipers included a "slot and pin" and "graduated in inches and tenths of an inch.