meter-kilogram-second-ampere system

(physics) A system of electrical and mechanical units in which length, mass, time, and electric current are the fundamental quantities, and the units of these quantities are the meter, the kilogram, the second, and the ampere respectively. Abbreviated mksa system. Also known as Giorgi system; practical system.

(Metric) The formal recognition of the m.k.s. system augmented for electric quantities by use of the ampere rather than the volt (or other) as the relevant base unit. (The original c.g.s. system had all electrical units defined in mechanical terms, while subsequently, for a time, the ampere and the volt had independent definitions. With the SI, the ampere alone is defined, making our contemporary form of metric an m.k.s.A. system.)

The use of lower- versus upper-case letters in the system titles is very varied. If one regards the title in itself as deserving of capitalization, then upper-case letters are appropriate. Otherwise, if the title is composed from the initial letters of the names of the units, lower-case letters are appropriate. However, since the symbol for ampere is distinctively the capital A, many people find the use of small a unacceptable. But, while lower-case m for metre and s for second are proper symbols, k was not always the symbol for kilo-. This system first came into use when K rather than k stood for kilo-, and neither alone stands for kilogram. So m.k.s.A. is perhaps best today, but m.k.s.A., or m.Kg.s.A., was correct yesterday and M.K.s.A. and m.k.s.A. are defensible; m.k.s. and m.k.s.A. are used in this publication, the latter, as Metric-m.k.s.A., to mean the international standard system adopted in 1946, effective for 1948, which relegated the volt to derived status. (Interestingly, it is MKS in the official 1946 definition of newton.)