international ohm

The unit of electric resistance in the international system of electric and magnetic units established by the International Electrical Congress in Chicago in 1893. At first it was known as the “reproducible” ohm. The international ohm was defined as the resistance of a column of mercury of constant cross section at the temperature of melting ice, 106.3 centimeters long and with a mass of 14.4521 grams.

Public Bill 105, passed by Congress on July 12, 1894, made this the legal definition of the ohm in the United States. The definition was confirmed by International Conference of London in 1908.

In German-speaking areas, the international ohm was also called the Reichanstalt ohm, from its having been adopted by the Physikalisch-Technische Reichanstalt in Charlottenburg (Berlin).

A laboratory definition of a unit, such as this one, was unsatisfactory to scientists, who wanted an absolute unit. The international ohm became obsolete when Resolution 2 of the 1946 CIPM, approved by 9th CGPM (1948), introduced a new, absolute, definition of the ohm.