Centimetre–gram–second system of units

The centimetre–gram–second system of units (CGS or cgs) is a variant of the <a href="/facts/Metric_system/to24H0YK">metric system</a> based on the <a href="/facts/Centimetre/urPkBdGR">centimetre</a> as the unit of <a href="/facts/Length/EwFyAqGH">length</a>, the <a href="/facts/Gram/sRSW5RJb">gram</a> as the unit of <a href="/facts/Mass/HHdc8XmF">mass</a>, and the <a href="/facts/Second/jtFxnJXx">second</a> as the unit of <a href="/facts/Time/HceJKUGB">time</a>. All CGS <a href="/facts/Mechanics/A3yHLQA5">mechanical</a> units are unambiguously derived from these three base units, but there are several different ways in which the CGS system was extended to cover <a href="/facts/Electromagnetism/oAkmALHT">electromagnetism</a>.
The CGS system has been largely supplanted by the <a href="/facts/MKS_units/fMpV1vDw">MKS system</a> based on the <a href="/facts/Metre/hPhzc1CV">metre</a>, <a href="/facts/Kilogram/lU2zbPk8">kilogram</a>, and second, which was in turn extended and replaced by the <a href="/facts/International_System_of_Units/S7YLglNz">International System of Units</a> (SI). In many fields of science and engineering, SI is the only system of units in use, but CGS is still prevalent in certain subfields.
In measurements of purely mechanical systems (involving units of length, mass, <a href="/facts/Force/533Q1uzq">force</a>, <a href="/facts/Energy/wd8PrQM7">energy</a>, <a href="/facts/Pressure/eyc37GRG">pressure</a>, and so on), the differences between CGS and SI are straightforward: the <a href="/facts/Unit_conversion/BY9Az4t6">unit-conversion factors</a> are all <a href="/facts/Exponentiation/pac6QY2g">powers of 10</a> as 100 cm = 1 m and 1000 g = 1 kg. For example, the CGS unit of force is the <a href="/facts/Dyne/gHYd2NyK">dyne</a>, which is defined as 1 g⋅cm/s2, so the SI unit of force, the <a href="/facts/Newton_(unit)/Ral9XgNl">newton</a> (1 kg⋅m/s2), is equal to 100000 dynes.
On the other hand, in measurements of electromagnetic phenomena (involving units of <a href="/facts/Charge_(physics)/jnQEDBI3">charge</a>, electric and magnetic fields, <a href="/facts/Voltage/EN2po6N2">voltage</a>, and so on), converting between CGS and SI is less straightforward. Formulas for physical laws of electromagnetism (such as <a href="/facts/Maxwell%2527s_equations/LfJ1drHS">Maxwell's equations</a>) take a form that depends on which system of units is being used, because the electromagnetic quantities are defined differently in SI and in CGS. Furthermore, within CGS, there are several plausible ways to define electromagnetic quantities, leading to different "sub-systems", including <a href="/facts/Gaussian_units/1mtqI96c">Gaussian units</a>, "ESU", "EMU", and <a href="/facts/Heaviside%25E2%2580%2593Lorentz_units/2XTE2Tkt">Heaviside–Lorentz units</a>. Among these choices, Gaussian units are the most common today[according to whom?], and "CGS units" is often intended to refer to CGS-Gaussian units.

Centimetre–gram–second system of units open-in-new

Centimetre–gram–second system of units