Units of Measurement — The SI System

From Many Systems to One Global Standard

Historically, different regions used different systems of units for the same physical

quantities — the CGS system (centimetre, gram, second), the FPS or British system

(foot, pound, second), and the MKS system (metre, kilogram, second) were all in

widespread scientific and everyday use. A measurement of force in CGS (the dyne) and the same

physical force expressed in FPS (the poundal) are not directly comparable without an

error-prone conversion — a serious practical problem once science and trade became

international.

The solution adopted worldwide is the International System of Units, universally

abbreviated SI (from the French Système International d'Unités), maintained by the

Bureau International des Poids et Mesures (BIPM). SI is a coherent, purely decimal system —

converting between related SI units (e.g. metres to kilometres) only ever involves shifting a

decimal point, unlike the FPS system's irregular factors (12 inches to a foot, 3 feet to a

yard).

The Seven SI Base Units

SI is built on exactly seven base quantities, each considered independent of the others —

none of the seven can be expressed as a combination of the rest. Every other physical

quantity's unit (called a derived unit, such as the newton for force or the joule for

energy) is obtained by multiplying and dividing powers of these seven.

Defining Units by Constants of Nature, Not Physical Objects

Older definitions relied on physical artefacts — for a long time, the kilogram was

literally defined as the mass of a specific platinum-iridium cylinder kept in France, and the

metre was once a fraction of the Earth's meridian. The problem with an artefact-based

definition is that a physical object can be damaged, contaminated, or simply drift in mass

over decades of handling.

The 2018 redefinition of the SI (which took effect in 2019) fixed every base unit in

terms of an unchanging fundamental constant of nature instead: