Historical Metric System
The metric system was developed at the time of the French Revolution, with standards set for the meter and kilogram on June 22, 1799.
The metric system was an elegant decimal system, where units of like type were defined by power of ten. The degree of separation was relatively straightforward, as the various units were named with prefaces indicating the order of magnitude of the separation.
Thus, 1 kilogram was 1,000 grams, because kilo- stands for 1,000.
In contrast to the English System, wherein 1 mile is 5,280 feet and 1 gallon is 16 cups (or 1,229 drams or 102.48 jiggers), the metric system had obvious appeal to scientists. In 1832, the physicist Karl Friedrich Gauss promoted the metric system heavily and used it in his definitive work in electromagnetics.
Formalizing Measurement
The British Association for the Advancement of Science (BAAS) began in the 1860s codifying the need for a coherent system of measurement within the scientific community. In 1874, the BAAS introduced the cgs (centimeter-gram-second) system of measurements. The cgs system used the centimeter, gram, and second as base units, with other values derived from those three base units. The cgs measurement for magnetic field was the gauss, due to Gauss' earlier work on the subject.
In 1875, a uniform meter convention was introduced.
There was a general trend during this time to make sure that units were practical for their use in the relevant scientific disciplines. The cgs system had some flaws of scale, especially in the field of electromagnetics, so new units such as the ampere (for electrical current), ohm (for electrical resistance), and volt (for electromotive force) were introduced in the 1880s.
In 1889, the system transitioned, under the General Convention of Weights and Measures (CGPM - the abbreviation of the French name), to having new base units of meter, kilogram, and second. It was suggested starting in 1901 that introducing new base units, such as for electrical charge, could complete the system. In 1954, the ampere, the Kelvin (for temperature), and the candela (for luminous intensity) were added as base units.
The CGPM renamed it to the International System of Measurement (SI - from the French Systeme International) in 1960. Since then, the mole was added as the base amount for substance in 1974, thus bringing the total base units to seven and completing the modern SI unit system.
SI Base Units
The SI unit system consists of seven base units, with a number of other units derived from those foundations. Below are the base SI units, along with their precise definitions, showing why it took so long to define some of them.
- meter (m) - The base unit of length; determined by the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 of a second.
- kilogram (kg) - The base unit of mass; equal to the mass of the international prototype of the kilogram (commissioned by the CGPM in 1889).
- second (s) - The base unit of time; duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state in the cesium 133 atom.
- ampere (A) - The base unit of electrical current; constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circuit cross-section, and placed 1 meter apart in vacuum, would produce between those conductors a force equal to 2 x 10^{-7} newtons per meter of length.
- Kelvin(degrees K) - The base unit of thermodynamic temperature; the fraction 1/273.16 of the thermodynamic temperature of the triple point of water (the triple point is the point in a phase diagram where three phases coexist in equilibrium).
- mole (mol) - The base unit of substance; the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilograms of carbon 12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.
- candela (cd) - The base unit of luminous intensity; the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10^{12} hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
SI Derived Units
From these base units, many other units are derived. For example, the SI unit for velocity is m / s (meter per second), using the base unit of length and the base unit of time to determine the length traveled over a given period of time.
Listing all of the derived units here would be unrealistic, but in general when a term is defined, the relevant SI units will be introduced along with them. If looking for a unit that isn't defined, check out the National Institute of Standards & Technology's SI Units page.
Edited by Anne Marie Helmenstine, Ph.D.