systems of units

Systems of units that take the meter, kilogram, and second as their units of length, mass, and time. SI is such a system. Units for these three properties are enough to do Newtonian mechanics, a branch (some would say the trunk) of physics. In the United States, abbreviated as mks (no periods); in most of the world M.K.S.

The rise, rationalization, and replacement of M.K.S.

In the last half of the 19th century, when the “metric system” was beginning to gain worldwide acceptance, most of the scientific community decided to use a system of units based on the centimeter and gram (see centimeter-gram-second systems). During the same period, because the cgs electric units were much too small for everyday use, a separate system of much larger practical electric units arose. (See International System of Electric and Magnetic Units.)

Shortcomings of the cgs systems

The cgs systems’ electric and magnetic units were their undoing. Agreement was never reached on a satisfactory way of reconciling the electric and magnetic units with each other or with the practical units, which made life difficult for anyone who needed to use both electric and magnetic units in the same problem. Further, because some of the units had been defined more than once, by different parties at different times (unit electric flux, for example, or the gauss), there was some confusion about what certain terms meant. Many wanted units that would be numerically equivalent to the practical units, and many objected to the fact that, in their usual form, the cgs systems were not rationalized.

The Giorgi System

One of the critics of the cgs systems was an Italian physicist, Giovanni Giorgi (1871—1950), who as early as 1895 condemned cgs in a letter to the English magazine Electrician (April 1896). In October 1901, in a talk given before the Italian Electrical Engineering Assn.¹ Giorgi proposed a system that resolved most of the failings of the cgs systems. It used the meter, kilogram, and second, and in a slightly later revised form, set the permeability of free space at 10⁻⁷ henry per meter (for the significance of permeability, see centimeter-gram-second systems of units). This choice gave the ampere the same value that it had as a practical unit, and in fact the value for the permeability of free space is realized through the definition of the ampere. The Giorgi, or MKSA, system gradually caught on over the following decades.

A plenary session of the IEC at Scheveningen-Brussels in 1935 formally and unanimously adopted the Giorgi system, without rationalization, “for fear of producing dissension detrimental to the general acceptance of this system if a vote were taken upon the matter at that time” (from the minutes). It also left to the IPU (what is now the International Union of Pure and Applied Physics) and CIPM the question of what the fourth unit should be.

An insider writing in 1935 described the debate:

“In the E.M.M.U. committee [the committee on electrical and magnetic magnitudes and units of the International Electrotechnical Commission], a small majority has been in favor of rationalization; in the S.U.N. committee [the committee on symbols, units and nomenclature of the International Union for Pure and Applied Physics], a majority has been more definitely against it. It is clear that any attempt to force a decision one way or the other at the present time would divide the mks adherents into two opposing camps, the rationalists and the nonrationalists. It seems desirable, therefore, to avoid the issue and leave each writer free to follow his own choice, until experience may have crystallized opinion in the different countries.”²

A meeting of Advisory Committee #24 in Torquay in June 1938 again deferred the rationalization question for future consideration, and voted to name the system “the Giorgi (MKS) system.”

In 1950 the IEC decided to increase the value for the permeability of free space by a factor of 4π (the surface area of a sphere of radius 1), thus rationalizing the system. Expressions concerning spheres would contain “4π”; those concerning coils, “2π”; and those dealing with straight wires would not contain π at all. The resulting “rationalized MKSA system,” was the direct ancestor of SI. The fourth unit was permeability of free space, originally set at 10⁻⁷ henry per meter (compare cgs electromagnetic system of units).

1. “Razionali di elettromagnetismo”; published as
G. Georgi.
Unit� razionali di electtromagnetismo.
Atti dell' A.E. I, 1901.

2. A[rthur] E. Kennelly.
I.E.C. adopts MKS system of units.
Electrical Engineering, volume 54, page 1378, December 1935.

Kennelly was a member of the ICPM and professor emeritus of Harvard and MIT.

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