The unit of electrical resistance; past history and impending change

In 1923 an amendment to the International Convention of 1875 relating to weights and measures gave the International Committee of Weights and Measures authority to deal with the electrical units. This committee was thereby confronted with two major problems concerning the magnitudes of the units and the best procedure for their legalization and maintenance. It set up an Advisory Committee of experts to advise it on all such questions. At its second meeting, in 1930, the Advisory Committee adopted a resolution to the effect that further comparisons of the manganin wire standards of resistance with mercury ohms are unnecessary. The mercury ohm has so long been the legalized material standard of resistance that it is regarded by many workers as a permanent institution, subject only to such slight revision of the length and mass of the mercury column as might be shown to be necessary as the technique of absolute measurements becomes more accurate. The present paper first gives a background by outlining the entire history of the unit of electrical resistance.