Enheter

Enheter (units) are standard quantities used to express and compare physical properties such as length, time, and mass. They provide a reference for communicating measurements, ensuring reproducibility, and enabling calculations across contexts.

The International System of Units (SI) is the dominant framework for scientific measurement. It defines seven base units: metre for length, kilogram for mass, second for time, ampere for electric current, kelvin for thermodynamic temperature, mole for amount of substance, and candela for luminous intensity. Derived units are formed from these base units, such as the newton (kilogram metre per second squared) and the joule (kilogram metre squared per second squared). SI prefixes, such as kilo-, milli-, micro-, and nano-, express multiples and submultiples of units, allowing convenient scaling without altering relationships between quantities.

Many non-SI units remain in common use. Examples include the litre, hour, and tonne, and some fields use specialized systems like CGS or Imperial units. In scientific work, units are typically converted to a consistent system to avoid arithmetic errors and to enable comparison.

Metrology is the science of measurement. National and international bodies, led by the International Bureau of Weights and Measures (BIPM) and its committees, maintain definitions, standards, and the framework for traceability. Measurements are reported with uncertainty estimates and calibrated against SI standards.

In 2019, the SI base units were redefined in terms of fundamental constants rather than artifact standards. The metre is defined by the speed of light, the kilogram by the Planck constant, the ampere by the elementary charge, the kelvin by the Boltzmann constant, the mole by Avogadro’s number, and the candela by a fixed luminous efficacy. Time remains defined by the cesium frequency. These changes ensure long-term stability and universality of enheter.