System of Units

Over the years, physics has used several different systems of units, and it helps to know all of them because older instruments, textbooks and regional habits still sometimes use them:

1. CGS system — Centimetre, Gram, Second, used for length, mass and time respectively.
2. MKS system — Metre, Kilogram, Second.
3. FPS system — Foot, Pound, Second (an imperial system, still used in a few English-speaking countries for everyday measurement).
4. SI (Système International / International System of units) — the modern, globally standardised system. The first three systems (CGS, MKS, FPS) were all in extensive use until fairly recently, and each had its own internal consistency but did not talk to the others easily. In 1971, the 14th General Conference on Weights and Measures (an international body of metrologists) formally recommended universal adoption of the 'International system' of units, now called SI units. The great practical advantage of SI is that it is a fully decimal system — every unit relates to every other unit within the same quantity by a power of ten (kilometre, centimetre, millimetre, etc.), which makes conversions within the system simple and avoids the awkward non-decimal conversion factor …