Overview
The centimetre–gram–second system, commonly abbreviated as CGS or cgs, is a coherent set of physical units that uses the centimetre for length, the gram for mass and the second for time. It is a variant of the metric tradition and was developed in the 19th century to provide simple, consistent units for mechanics and later for electromagnetism. The CGS scheme served as an important intermediate step between older customary units and the internationally adopted SI system.
Base and derived units
In CGS, base units are chosen so that many physical formulae do not require extraneous constants when expressed in those units. Common derived units include:
- Dyne — the unit of force. One dyne is the force required to accelerate a mass of one gram at one centimetre per second squared (1 dyne = 1 g·cm·s−2).
- Erg — the unit of energy or work. One erg equals the work done by a force of one dyne acting through a distance of one centimetre (1 erg = 1 dyne·cm).
- Barye (Ba) — the coherent unit of pressure in CGS, defined as one dyne per square centimetre.
Other mechanical quantities, such as momentum, power and specific energy, are expressed in combinations of centimetres, grams and seconds. This coherence made CGS practical for many laboratory calculations in physics during its period of prominence.
Electrical variants: ESU and EMU
A distinctive feature of CGS is that it produced multiple, internally consistent systems for electrical quantities. Two major variants arose: the electrostatic units (ESU) system and the electromagnetic units (EMU) system. These differ in the placement of factors of the speed of light in Maxwell's equations and in the numerical values of unit magnitudes. Engineers and applied technologists tended to prefer metre–kilogram–second (MKS) or later SI units because those align more naturally with practical current and circuit quantities defined using the ampere.
History and adoption
The CGS framework evolved in the 19th century as scientists sought a rational and reproducible set of units. It was widely used in theoretical and experimental physics into the early 20th century. Over time, the need for larger practical units and the development of electrical technologies encouraged adoption of systems based on the metre, the kilogram and the second (MKS). In 1960 the General Conference on Weights and Measures adopted the International System of Units (SI), which replaced CGS as the international standard for most applications.
Uses, conversions and examples
Although SI is dominant, CGS persists in some subfields and historical literature. Astrophysics, theoretical papers in electromagnetism and older textbooks may still use CGS expressions. Common conversions include:
- 1 dyne = 10−5 newton (N) in SI.
- 1 erg = 10−7 joule (J).
- 1 barye = 0.1 pascal (Pa).
Practical pressure measures historically expressed in CGS-derived terms included atmospheres and millimetres of mercury; one standard atmosphere equals about 1.01325×106 barye, while 1 mmHg is close to 133.32 barye. Such numbers illustrate why larger SI units (pascal, newton, joule) are often more convenient for everyday engineering.
Notable distinctions and legacy
Key distinctions between CGS and SI include the choice of base units and the treatment of electromagnetic quantities. CGS systems can make some theoretical expressions more compact, but the multiplicity of CGS electrical variants can cause confusion. Modern practice favors SI for its universality and because electrical units are defined directly from measurable base quantities like the ampere. Nevertheless, understanding CGS remains useful for reading older scientific literature and for appreciating the historical development of unit systems within physics and engineering. For further background on the metric tradition, see general material on the metric system and the relationship between mechanical concepts such as mass, time and mechanical units; consult introductory sources on the erg and the history of how work has been quantified.