The weak points in this first attempt at a direct determination of consisted in: (1) the assumption that the number of ions is the same as the number of drops; (2) the assumption of Stokes’s Law of Fall which had never been tested experimentally, and which from a theoretical standpoint might be expected to be in error when the droplets were small enough; (3) the assumption that the droplets were all alike and fell at a uniform rate wholly uninfluenced by evaporation or other causes of change; (4) the assumption of no convection currents in the gas when the rate of fall of the cloud was being measured. II. SIR JOSEPH THOMSON’S WORK ON This first attempt to measure was carried out in Professor J. J. Thomson’s laboratory. The second attempt was made by Professor Thomson himself[34] by a method which resembled Townsend’s very [Pg 50] closely in all its essential particulars. Indeed, we may set down for Professor Thomson’s experiment precisely the same five elements which are set down on p. 45 for Townsend’s. The differences lay wholly in step 2, that is, in the way in which the electrical charge per cubic centimeter carried by the gas was determined, and in step 3, that is, in the way in which the total weight of the cloud was obtained. Thomson produced ions in the space (Fig. 1) by an X-ray bulb which ran at a constant rate, and measured first the current which, under the influence of a very weak electromotive force , flows through between the surface of the water and the aluminum plate which closes the top of the vessel. Then if is the whole number of ions of fine sign per cubic centimeter, the velocity of the positive and that of the negative ion under unit electric force, i.e., if and are the mobilities of the positive and negative ions, respectively, then the current per unit area is evidently given by and 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. [Pg 51] Having thus obtained of equation (4), Thomson had only to find and then solve for . To obtain he proceeded exactly as Townsend had done in letting the ions condense droplets of water about them and weighing the cloud thus formed. Fig. 1