Other units

In SI, the ampere was defined as the constant current that, flowing in two parallel conductors one meter apart in a vacuum, will produce a force between the conductors of 2 if 10-7newtons per meter of length.

In 1971 the mole was defined as the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12.

The international unit of light intensity, the candela, was originally defined as 1/60 of the light radiated from a square centimeter of a blackbody, a perfect radiator that absorbs no light, held at the temperature of freezing platinum. It is now more precisely defined as the intensity of a light source, in a given direction, with a frequency of 540 x 1012hertz and a radiant intensity of 1/683 watts per steradian in that direction.

The radian is the plane angle between two radii of a circle that cut off on the circumference an arc equal in length to the radius.

The steradian is defined as the solid angle that, having its vertex in the center of a sphere, cuts off an area of the surface of the sphere equal to that of a square with sides of length equal to the radius of the sphere.

The SI units for all other quantities are derived from the seven base units and the two supplementary units. Some derived units are used so often that they have been assigned special names-usually those of scientists.

One feature of SI is that it is a coherent system-that is, derived units are expressed as products and ratios of the base, supplementary, and other derived units without numerical factors. This results in some units being too large for ordinary use and others too small. To compensate, the prefixes developed for the metric system have been borrowed and expanded. These prefixes are used with all three types of units: base, supplementary, and derived. Examples are millimeter (mm), kilometer/hour (km/h), megawatt (MW), and picofarad (pF).

Because double prefixes are not used, and because the base unit kilogram already contains a prefix, prefixes are not used with kilogram, although they are used with gram. The prefixes hecto, deka, deci, and centi are used only rarely, and then usually with meter to express areas and volumes. Because of established usage, the centimeter is retained for body measurements and clothing.

Certain units that are not part of SI are used so widely that it is impractical to abandon them.

In cases where their usage is already well established, certain other units are allowed for a limited time, subject to future review. They are the nautical mile, knot, angstrom, standard atmosphere, hectare, and bar.