UCUM base unit
The Unified Code for Units of Measure defines seven units of measure as a basic set from which all other UCUM units can be derived. The UCUM base units and their physical quantities are the meter for measurement of length, the second for time, the gram for mass, the coulomb for charge, the kelvin for temperature, the candela for luminous intensity, and the radian for plane angle.
The UCUM base units form a set of mutually independent dimensions as required by dimensional analysis commonly employed in science and technology.
The names and symbols of UCUM base units are written in lowercase, except the symbols of those named after a person, which are written with an initial capital letter. For example, the metre (US English: meter) has the symbol m, but the kelvin has symbol K, because it is named after Lord Kelvin and the coulomb with symbol C is named after Charles-Augustin de Coulomb.
Several other units, such as the litre (US English: liter), are formally not part of the UCUM, but are accepted for use with UCUM. Some of the base units in the UCUM are different from the SI base units. The UCUM is compatible, but not isomorphic with the SI. There are four changes in the UCUM from the SI. Instead of the kilogram, the gram is the unit of mass since a base unit cannot have a prefix and a meaningful unit is required before it is modified by a prefix. This first change has no effect on the system. Instead of electric current, charge is a base quantity for electreomagnetic phenomena since the elementary charge of electrons defines electric phenomena including electric current. Since the mole can be defined in terms of Avogadro's number, the mole is dimensionsless in the UCUM. The radian is a distinct base unit for the plane angle to distinguish angular velocity from rotational frequency and to distinguish the radian from the steradian for a solid angle. Because of the last two changes, the UCUM is not isomorphic with the SI.
Seven UCUM base units
- International vocabulary of metrology
- Metric prefix
- Physical constant
- Electric constant
- Magnetic constant