Centimetre–gram–second system of units
The centimetre-gram-second system (abbreviated CGS or cgs) is a variant of the metric system. The derived units are on the centimeter as the unit of length, the gram as a unit of mass and the second as a unit of time. The system evolved in the nineteenth century when it was used to define both electrical and mechanical units.
The cgs units of energy and force are the dyne and the erg.
- The dyne is defined as the force needed to accelerate a mass of one gram by one centimetre per second squared (cm s-2).
- The erg is defined as the work done when a mass of one gram is moved a distance of one centimetre against a force of one dyne.
The coherent cgs unit of pressure was the barye (symbol Ba). A bayre is one dyne per square centimeter. Many non-coherent units of pressure were used in the cgs system. They include:
- The atmosphere (atm). A standard atmosphere is 1.01325×106 Ba.
- Millimetres of mercury (mmHg). One millimeter of mercury is 133.32 Ba.
The dyne and the erg were used to define the units for electrical quantities. Two different forms of cgs units were developed. One form was developed for electrostatic units and another for electromagnetic units. Electrical engineering units were based on the MKS (metre-kilogram-second) system instead of the CGS system.
During the nineteenth and early twentieth centuries many other units were defined using cgs units. In 1960 the BIPM published the International System of Units (SI) which was based on the metre, the kilogram, the second and the ampere. SI has now replaced the CGS system of units in most applications.