Centimetre–gram–second system of units
coherent system of units using centimeter gram and second as base units
The centimetre–gram–second system of units (CGS or cgs) is a variant of the metric system based on the centimetre as the unit of length, the gram as the unit of mass, and the second as the unit of time. All CGS mechanical units are unambiguously derived from these three base units, but there are several different ways in which the CGS system was extended to cover electromagnetism.
The CGS system has mainly been supplanted by the MKS system based on the metre, kilogram, and second, which was in turn extended and replaced by the International System of Units (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, force, energy, pressure, and so on), the differences between CGS and SI are straightforward: the unit-conversion factors are all powers of 10 as 100 cm = 1 m and 1000 g = 1 kg. For example, the CGS unit of force is the dyne, which is defined as 1 g⋅cm/s2, so the SI unit of force, the newton (1 kg⋅m/s2), is equal to 100000 dynes.
In contrast, converting measurements of electromagnetic quantities—such as electric charge, electric and magnetic fields, and voltage—between CGS and SI systems is considerably more complex. This is because the form of the equations governing electromagnetic phenomena, including Maxwell's equations, depends on the system of units employed; electromagnetic quantities are defined differently in SI and in CGS. Moreover, several distinct versions of the CGS system exist, each defining electromagnetic units differently. These include the electrostatic (ESU), electromagnetic (EMU), Gaussian units, and Heaviside–Lorentz units. Gaussian units are the most widely used in modern scientific literature, and the term "CGS units" is often understood to refer specifically to the CGS–Gaussian system.
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