Standard (metrology)

Historical standards for length, volume, and mass were defined by many different authorities, which resulted in confusion and inaccuracy of measurements.

Modern measurements are defined in relationship to internationally standardized reference objects, which are used under carefully controlled laboratory conditions to define the units of length, mass, electrical potential, and other physical quantities.

The IPK is a one kilogram mass of a platinum-iridium alloy maintained by the International Bureau of Weights and Measures (BIPM) in Sèvres, France.

Formerly it was defined in terms of standard cell electrochemical batteries, which limited the stability and precision of the definition.

Currently the volt is defined in terms of the output of a Josephson junction,[3] which bears a direct relationship to fundamental physical constants.

[5] A machine shop will have physical working standards (gauge blocks for example) that are used for checking its measuring instruments.

The international prototype of the kilogram (IPK) is an artefact or prototype that was defined to have a mass of exactly one kilogram.
Standard units for length would be embedded in the cornerstones of churches or important public buildings, so that all people trading in an area could agree on the units.
A set of gauge blocks are used as a working standard to check the calibration of measurement tools such as micrometers .