Force. — In Ohm's law we have two factors of great importance, namely, the resistance and the electromotive force or potential difference. Both of these are measured in units, which are determined by the magnetic effect of the electric current. As, however, these measurements are difi&cult to carry out, conventional values for the units have been adopted in a system in which they are easy to reproduce. As normal resistance we take the resistance offered by a column of mercury, at 0° and 760 mm. barometric pressure, 1 metre long and of 1 sq. mm. cross section. This choice has been made because it is comparatively easy to obtain pure mercury. This standard is called a Siemens' unit. The ratio adopted at the Paris Congress in 1881 between the ohm (legal ohm) and the Siemens' unit was 1 : 1'06. On account of more exact measurements, a meeting of deputies from Germany, Great Britain, and the United States in 1891 adopted the ratio : 1 ohm = 1*0630 Siemens' units (S.U.).
This new ohm ( = 10630 S.TJ.) is called an international ohm, and will be used in the sequel.
The value of a volt is so determined that it is the potential difference produced by a current intensity of 1 ampere at the ends of a resistance of 1 ohm, because according to Ohm's law : 1 volt = 1 ohm x 1 ampere. In the course of time the volt has undergone the same changes as the oHm. In recent times no change has been made in the ampere (compare p. 4).
For the comparison of potential differences use is made of the electromotive force of a " constant " galvanic element. In order to construct such a constant element, i.e. one whose electromotive force does not change with time, polarisation of the poles must be rigorously avoided ; the poles must be uon-polarisable, and this is accomplished by making the electrodes of the same metal as the cation of the salt solution in contact with them. The first element of this type was constructed by Daniell; it consists of a copper pole in a solution of copper sulphate and, separated from
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