X. On the determination of the number of electrostatic units in the electromagnetic unit of electricity

This paper contains an account of some experiments which have been made during the last two years in the Cavendish Laboratory, Cambridge. These experiments were made to determine “v ” by comparing the electrostatic and electromagnetic measures of the capacity of a condenser. The condenser consisted of two cylinders fitted with guard-ring pieces. The electrostatic measure of the capacity was calculated from the dimensions of this condenser. The electromagnetic measure of the capacity was determined by a very slight modification of the method given in § 775 of Maxwell’s “Electricity and Magnetism.” In this method the condenser has to be repeatedly charged and discharged by a commutator, and a very elaborate commutator would be required to work the guard-ring part of the condenser; for this reason the capacity of the guard-ring condenser was experimentally compared with the capacity of another condenser without a guard-ring, the capacity of the latter being altered until the capacities of the two condensers were equal. The electromagnetic measure of the capacity of the condenser without a guardring was then determined by Maxwell’s method. The ratio of the electrostatic to the electromagnetic measure of the capacity isv 2 . The result of the experiments, using Lord Rayleigh’s value of the ohm, was that “v ” = 2·963 × 1010 in C. G. S. units.