Josephson junction, which bears a direct relationship to fundamental physical constants. In contrast, the reference standard for the metre is no longer defined by a physical object (as the former international prototype of the metre (IPM) or originally the mètre des Archives). In 1983, the standard metre was redefined as the distance light travels in a vacuum during 1/ of a second. Secondary reference standards Secondary reference standards are very close approximations of primary reference standards. For example, major national measuring laboratories such as the US's National Institute of Standards and Technology (NIST) will hold several "national standard" kilograms, which are periodically calibrated against the IPK and each other. Working standards A machine shop will have physical working standards (gauge blocks for example) that are used for checking its measuring instruments. Working standards and certified reference materials used in commerce and industry have a traceable relationship to the secondary and primary standards. Working standards are expected to deteriorate, and are no longer considered traceable to a national standard after a time period or use count expires. Laboratory standards National organizations provide calibration and private industrial laboratories with items, processes and/or certification so they can provide certified traceability to national standards. (In the United States, NIST operates the NVLAP program.) These laboratory standards are kept in controlled conditions to maintain their precision, and used as a reference for calibration and creating working standards. Sometimes they are (incorrectly) called "secondary standards" because of their high quality and reference suitability. See also History of measurement International System of Units Measurement Measurement uncertainty Measuring instrument Metre Standardization Technical standard Units of measurement Kibble balance 2019 redefinition of the SI base units References Metrology Standards by type