The MKSA Unit System

The Italian physicist Giovanni L.T Giorgi, in 1901, proposed a system which is based on five fundamental units. This system was in 1935 adopted by the International Electrotechnical commission (IEC) as the M.K. S. System of giorgi without specifying which electromagnetic unit would be the fourth base unit. In 1946 the International Committee for Weights and Measures (CIPM) approved a proposal to use the ampere as that unit in a four-dimnesional system, the MKSA system. The units of length was to be the standard metre, the unit of mass the standard kilogram, the nunit of time the second, and two new base units, the ampere for electric current intesity, and vacuum magnetic permeability which was defined as \(\mu_0 = 4\pi\times 10^{-7}\) [H\(\cdot\)m\(^{-1}\)]. This means that all units in the system could be used in electromagnetic or electrostatic context. The introduction of the factor \(4\pi\) in the expression for vacuum magnetic permeabiltiy meant that all units could be rationalized. For example the fractor of \(2\pi\) could be applied when a system had cylindrical symmetry, and of \(4\pi\) if it had spherical symmetry.

This system made a clear distinction between magnetic field strength \(H\) and magnetic flux density \(B\), and similarrly between electric field \(E\) and electric flux density \(D\). This distinction results from the expression for vacuum permittivity and magnetic permeability which is not equal to unity as in the cgs system. The vector equations relating these four quantities: $$ \mathbf{B} = \mu \mathbf{H} = \mu_0\mu_r \mathbf{H}$$ $$ \mathbf{D} = \varepsilon \mathbf{E} = \varepsilon_0 \varepsilon_r \mathbf{E} $$ The MKSA or Giorgi system became common in electrical engieerning from 1948. At the time, the 9th CGPM adopted the modern definition of ampere. The MKSA system can be described as precursor of the Interational System of Units (SI) adopted in 1960.