International System
of Electrical and Magnetic Units

This system is NOT the current “metric system,” known as SI, the Systéme International, or International System of Units. (Click here for that system.)

A practical system of electrical units defined by the International Electrical Congress at Chicago in 1893. A “practical” system is one whose units have magnitudes convenient for the purposes for which they are used, which often leads to their being much bigger than the absolute units on which they are ultimately based.

The absolute units on which the International System was based are those of the cgs electromagnetic system. The international ohm is 1,000,000,000 times the size of the cgs unit of resistance, its volt is 100,000,000 times the cgs unit of potential, its farad is  ¹/1,000,000,000th of the cgs unit of capacitance, and so on. These multipliers were the same as those of the practical systems that preceded it, and the units were roughly the same size, all tracing their ancestry back to the British Assn. committee of 1861.

The International System was, nonetheless, a new system. Since 1884 the ohm (the legal ohm) was the resistance of a column of mercury with a cross sectional area of 1 square centimeter and a length of 106.0 cm, which was—not incidentally—about 1,000,000,000 cgs units of resistance. The 1893 Congress said the international ohm was 1,000,000,000 units of resistance of the cgs electromagnetic system and that it could be represented for practical purposes by a column of mercury 106.3 cm long. The change from 106.0 to 106.3, about a quarter of a per cent, was the result of another 9 years of increasingly more sophisticated measurements of the value of the cgs unit of resistance, which enabled the new standard to come within 0.1% of 1,000,000,000 cgs units of resistance. The change in the ohm alone was enough to alter the values of all the other electric units; hence the need for a new name for the system.

The International System was modified by the London Electrical Conference in 1908, and discarded by the CGPM in 1948.

international ampere

international coulomb

international ohm

Further Reading

J. H. Dellinger.
International System of Electric and Magnetic Units.
Scientific Paper 292.
Bulletin of the [U. S.] Bureau of Standards, volume 13, number 4 (March 6, 1917).

Frank A. Wolff.
The so-called international electrical units.
Scientific Paper 3.
Bulletin of the Bureau of Standards, volume 1, page 39. (1904-1905).

Frank A. Wolff.
The principles involved in the selection and definition of the fundamental electrical units to be proposed for international adoption.
Scientific Paper 102.
Bulletin of the Bureau of Standards, volume 5, page 39. (1908-1909)

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