The experimental arrangements are shown in Fig. 7. The brass vessel was built for work at all pressures up to 15 atmospheres, but since the present observations have to do only with pressures from 76 cm. down, these were measured with a very carefully made mercury manometer , which at atmospheric pressure gave precisely the [Pg 112] same reading as a standard barometer. Complete stagnancy of the air between the condenser plates and was attained, first, by absorbing all of the heat rays from the arc by means of a water cell , 80 cm. long, and a cupric chloride cell , and, secondly, by immersing the whole vessel in a constant temperature bath of gas-engine oil (40 liters), which permitted, in general, fluctuations of not more than .02° C. during an observation. This constant-temperature bath was found essential if such consistency of measurement as is shown here was to be obtained. A long search for causes of slight irregularity revealed nothing so important as this, and after the bath was installed all of the irregularities vanished. The atomizer was blown by means of a puff of carefully dried and dust-free air introduced through cock . The air about the drop was ionized when desired, or electrons discharged directly from the drop, by means of Röntgen rays from X, which readily passed through the glass window . To the three windows (two only are shown) in the brass vessel correspond, of course, three windows in the ebonite strip , which encircles the condenser plates and . Through the third of these windows, set at an angle of about 28° from the line and in the same horizontal plane, the oil drop is observed through a short-focus telescope having a scale in the eyepiece to make possible the exact measurement of the speeds of the droplet-star. In plotting the actual observations I have used the reciprocal of the pressure in place of , for the reason that is a theoretical quantity which is necessarily proportional to , while is the quantity actually measured. [Pg 113] This amounts to writing the correction-term to Stokes’s Law in the form instead of in the form and considering the undetermined constant which is to be evaluated, as was before, by dividing the slope of our line by its -intercept.