systems of units

The principal systems of units used in scientific work, late 19ᵗʰ – early 20ᵗʰ centuries. In the United States, it was abbreviated cgs (no periods); in the rest of the world, C.G.S.. The form cm-gm-sec is sometimes encountered.

A cgs system had been recommended by the great German physicist Wilhelm Weber. Its use was firmly established by the endorsement of a committee of the British Association for the Advancement of Science in 1872, influenced by the equally great J. Clerk Maxwell.

Cgs systems were gradually superseded by meter-kilogram-second-ampere (MKSA) systems, and finally rendered obsolete in 1960 by the CGPM's adoption of SI, which is a MKSA system. Many cgs units, however, are still in daily use; for example, in the controversy concerning the effect of electromagnetic fields on health, the field strengths have almost always been reported in milligauss.

The mechanical units are the same in all cgs systems. Length, mass, and time, however, are not sufficient to define electric and magnetic quantities; a fourth dimension must be included. The different cgs systems arose from differing choices of the additional dimension. See:

The cgs electrostatic system of units

The cgs electromagnetic system of units

The cgs electric units were much too small for practical use in engineering, which lead to the creation of the International System of units (not be confused with SI!), which is not a cgs system. All of these systems share certain names, such as volt and ampere, but with different meanings. To avoid confusion, the prefix “ab-” was often added to cgs electromagnetic units (especially by Americans) and “stat-” to cgs electrostatic system units. Many workers did not use the special names, but simply referred to, for example, the “esu unit of charge” or the “emu unit of resistance.”

Generalized cgs systems

During the transition from cgs to MKSA it was often necessary to convert data expressed in units in a cgs system to units in MKSA. However, there is no easy way to do this, as the cgs systems are based on three fundamental quantities and the MKSA on four. If a cgs system could be created which had four dimensions and in which each unit in the cgs system corresponded to a unit in the four-dimensional cgs system, and physical quantities had the same numerical values, then values in this “generalized cgs system” could be converted to values in MKSA units by simple conversion factors.

At a meeting in Copenhagen in 1951, the International Union of Pure and Applied Physics approved the introduction of two such generalized cgs systems (Resolution 5):

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