International System of Units

Here we go. Far superior to the British Gravitational or English Engineering systems of units.

Basic units are length, force, and mass, which are interrelated by Newton's Second Law of Motion.

Length--meters (m).

Force--newtons (N). This is the derived one.

Mass--kilograms (kg). Actually, the gram is the base of the kilogram, but kilograms are used as the base of this system

So by the Law, one newton equals one kilogram*m/s^2, i.e. the force needed to accelerate a one kilogram mass at a rate of one meter/second/second.

Many people are familiar with the kilogram, even if they don't use it much, but erroneously think it is a measure of weight--likely because metric scales read in kilograms. As noted above, one kilogram will weigh 9.81 newtons on the earth, as the acceleration due to gravity here is 9.81 m/s^2. Take those scales to the moon, and what they say is one kilogram is really quite a bit more massive.

The name Système International d'Unités (International System of Units) was adopted in 1960. The abbreviation and/or symbol for this is SI (no full stops). This renamed the MKSA, the then-current scientific form of the metric system.

The history of the SI in brief:

1799 Platinum standard mètre and kilogramme deposited in Paris. Metric system official in France and soon spreads to most European countries.

1832 Gauss uses a system of measurement using the second (defined by astronomy) and the two metric units the millimetre and the gram as its basis. He and Weber then extend its use to electrical measurements.

1874 The British Association for the Advancement of Science (BAAS), under the guidance of Maxwell and Kelvin, adopts the CGS system, a coherent system of units with the centimetre, gram, and second as their basis, and derived units in terms of these. They adopt the prefixes mega and micro to supplement the original French prefixes (from milli to myria = 10000).

1875 The Treaty of the Metre signed in Sèvres, in France, establishes the world body that controls the metric system, the BIPM (Bureau Internationale des Poids et Mesures).

1889 The BIPM adopts the MKS, a coherent system of units using a new platinum prototype metre and kilogram together with the astronomical second as its basis.

1939 MKSA system proposed, which brings electrical and magnetic units into conformity with the MKS by rewriting equations so that the ampere can be used as a fourth base unit.

1954 The BIPM officially extends the MKS system to include three new base units, the ampere, the kelvin, and the candela.

1960 The name Système International d'Unités, symbolized SI, was adopted.

1971 A seventh base unit, the mole, was introduced to the SI.

This node with its long title can serve as a history. For details of all the SI units now used, see SI units.

For details of the prefixes for multiples and submultiples, see Standard SI prefixes.

For a history of the metric system generally, and uses and conversions of all the everyday (non-scientific) units such as the litre, centimetre, degree Celsius, and nautical mile, see metric system.