13 21.26 30.27 155.6 .1114 67.14 61.26 14 13.20 36.80 206.4 .1477 68.90 61.11 15 15.92 31.25 200.7 .1437 68.97 61.39 16 21.11 20.58 227.8 .1630 69.88 61.27 17 13.12 29.10 262.4 .1878 70.85 60.94 18 17.32 20.54 281.4 .2014 71.60 60.98 19 23.00 13.24 321.4 .2297 73.34 61.20 20 14.00 20.72 345.4 .2472 74.27 61.22 21 20.47 13.62 359.1 .2570 74.54 60.97 22 13.17 20.47 371.5 .2659 75.00 60.97 23 12.69 20.74 380.6 .2724 75.62 61.24 24 19.65 13.12 388.5 .2781 75.92 61.24 25 16.57 13.80 438.3 .3137 77.94 61.18 Mean = 61.126. [Pg 123] Fig. 10 [Pg 124] Let us now review, with Figs. 5 and 10 before us, the essential elements in the measurement of . We discover, first, that electricity is atomic, and we measure the electron in terms of a characteristic speed for each droplet. To reduce these speed units to electrical terms, and thus obtain an absolute value of , it is necessary to know how in a given medium and in a given field the speed due to a given charge on a drop is related to the size of the drop. This we know accurately from Stokes’s theory and Arnold’s experiments when the holes in the medium, that is, when the values of are negligibly small, but when is large we know nothing about it. Consequently there is but one possible way to evaluate e, namely, to find experimentally how the apparent value of , namely, , varies with or , and from the graph of this relation to find what value , approaches as or approaches zero. So as to get a linear relation we find by analysis that we must plot instead of against or . We then get from the intercept of an experimentally determined straight line on the -axis of our diagram. This whole procedure amounts simply to reducing our drop-velocities to what they would be if the pressure were so large or so small that the holes in the medium were all closed up. For this case and for this case alone we know both from Stokes’s and Arnold’s work exactly the law of motion of the droplet. [Pg 125] CHAPTER VI THE MECHANISM OF IONIZATION OF GASES BY X-RAYS AND RADIUM RAYS I. EARLY EVIDENCE