Electric units, called “international units,” for current and resistance, were introduced by the International Electrical Congress held in Chicago in 1893, and definitions of the “international ampere” and “international ohm” were confirmed by the International Conference in London in 1908.

Although it was already obvious on the occasion of the 8th CGPM (1933) that there was a unanimous desire to replace those “international units” by so-called “absolute units,” the official decision to abolish them was only taken by the 9th CGPM (1948), which adopted the ampere for the unit of electric current, following a definition proposed by the CIPM (1946):

The

ampere(A) is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to \(2\cdot 10^{−7}\) newton per meter of length.

It follows that the magnetic constant \(\mu_0\), also known as the permeability of vacuum, is exactly \(4\pi \cdot 10^{−7}\) henries per meter, \(\mu_0 = 4\pi \cdot 10^{−7}\) H/m.

The expression “MKS unit of force” which occurs in the original text of 1946 has been replaced here by “newton,” a name adopted for this unit by the 9th CGPM (1948).