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.



The Electrical Congress of Paris, 1884

The first commission virtually dealt with the length of a column of mercury of one square millimetre section which represented the ohm—it having been decided at the Congress of 1881 that this should be the unit of resistance. Many physicists had been working on this in different countries and on different methods. M. Mascart grouped the results in the following useful table :—

Methods Experimenters Column of Mercury
in Centimetres
1. B.A. British Association 104.83
Rayleigh-Schuster 106.00
Rayleigh (1882) 106.27
H. Weber 106.16
2. Weber (I.) Kohlrausch 105.81
Wiedemann 106.19
Mascart 106.33
3. Kirchoff F. Weber 105.02
Rowland 105.79
Glazebrook 106.29
Mascart 106.33
4. Ròiti 105.9
5. Fr. Weber 105.33
6. Lorenz Lorenz (first) 107.1
Rayleigh 106.24
Lenz 106.13
Lorenz (second) 106.19
7. Weber (II.) Dorn 105.46
Fr. Weber 105.26
Wild 105.68
Baille 105.37
8. Heat Joule 106.22

From this it appears that the figures obtained by the different methods were—

B.A. 106.21
Weber's I. 106.14
Kirchoff's 105.93
Lorenz 106.19
Weber's II. 105.47
Joule 106.22

The mean of which was 106.02, but 106 was taken as a round figure sufficiently near the truth for all practical and useful purposes. Hence the Congress decided that “the legal ohm should be the resistance of a column of mercury of one square centimetre section and of 106 cm. of length at the temperature of freezing,” and a resolution was passed desiring the French Government to transmit this resolution to the different Governments, with a view of making its adoption international.

We can now congratulate ourselves upon having a scientific system of electrical units independent of any particular instruments or of any particular process. It is not absolutely exact. That is, the new ohm is not 10⁹ C.G.S. units, but is the nearest approach to it that can be practically attained. It will probably be known as the Congress ohm, to distinguish it from the true ohm (10⁹ C.G.S.) or the B.A. ohm of 1864.

The Electrical Congress of Paris, 1884.
Nature, vol. 30, page 26 (May 8, 1884).

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