There are four general systems of mechanical units used in engineering namely F P S (food-pound-second) system, the English system, C.G.s. (centimeter-gram-second) system quantities i.e. of length, mass and time are foot, pound and second respectively. In English system the corresponding units are foot, slug and second. In metric or C.G.S. system the unit of length, mass and time are centimeter, gram and second respectively. In M.K.S. kilogram and second respectively.

Whereas, three basic concepts and three fundamental units are sufficient for description and measurement in mechanical science, experience shown that, in electrical science, four concepts or dimensions and four arbitrarily defined fundamental units are necessary to obtain a complete system or dimensions and units. At least one of these four units must be electrical in character.

The system employing meter, kilogram and second as fundamental units of mechanical quantities and ampere defined form its magnetic effect as fourth fundamental unit of electrical quantity is called the R.M.K.S.A system. The great advantage of this system is that its units are identical with those of practical system. Upto now this system of unit was very popular.

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