The link between electromagnetic units and the more familiar units of length, mass and time was first demonstrated by Carl Friedrich Gauss in 1832 with his measurement of the Earth's magnetic field,[2] and the principle was extended to electrical measurements by Franz Ernst Neumann in 1845.
The chosen method was based on the resistivity of mercury, by measuring the resistance of a column of mercury of specified dimensions (106 cm × 1 mm2): however, the chosen length of column was almost 3 millimetres too short, leading to a difference of 0.28% between the new practical units and the CGS units which were supposedly their basis.
The international units did not have the same formal legal status as the metre and the kilogram through the Metre Convention (1875), although several countries adopted the definition within their national laws (e.g., the United States, through Public Law 105 of July 12, 1894[15]).
The 1893 system of units was overdefined, as can be seen from an examination of Ohm's law: By Ohm's law, knowing any two of the physical quantities V, I or R (potential difference, current or resistance) will define the third, and yet the 1893 system defines the units for all three quantities.
With improvements in measurement techniques, it was soon recognised that The solution came at an international conference in London in 1908.
An additional problem with the CGS system of electrical units, pointed out as early as 1882 by Oliver Heaviside,[19] was that they were not "rationalized", that is they failed to properly take account of permittivity and permeability as properties of a medium.
[17] The International Committee for Weights and Measures (CIPM) approved a new set of definitions for electrical units, based on the rationalized MKSA system, in 1946, and these were internationally adopted under the Metre Convention by the 9th General Conference on Weights and Measures in 1948.
The conversion factor for the "electrolytic" ampere (Aelec) can be calculated from modern values of the atomic weight of silver and the Faraday constant: