were easily measured in any experiment;
was already known from Rutherford’s previous work, so that
, the charge of one sign per cubic centimeter of gas under the ionizing action of a constant source of X-rays, could be obtained at once from (4). This then simply replaces Townsend’s method of obtaining the charge per cubic centimeter on the gas, and in principle the two methods are quite the same, the difference in experimental arrangements being due to the fact that Townsend’s ions are of but one sign while Thomson’s are of both signs.
Having thus obtained
of equation (4), Thomson had only to find
and then solve for