50 THE DIFFUSION OF GASES THROUGH 



With the disks of the condenser i mm. apart, it should therefore be 

 possible to measure 50 volts, the difference of manometer pressure resulting 

 being about 0.1 mm. of mercury. From this limit the manometer head 

 rises rapidly as the square of V or inversely as the square of D. Thus for 

 50,000 volts and D = 1 centimeter, IV — 11= 100 centimeters. Hence if meas- 

 urements are to be made with the air pump, i. e., with H' — H less than 

 the equivalent of one atmosphere, D will have to be increased to D = 2 cm., 

 where II' — H = 25 cm., etc. 



Practically the determination of V will depend upon the measurement 

 H' — H or of D. The latter seems to be the more convenient datum, 

 though it requires a screw with graduated head and a stuffing box in place 

 of the rod aa in fig. 15, i. e., a screw micrometer for D. In such a case let 

 H' — H= 36 cm., a convenient mean value. In other words, the uncharged 

 diver is to just float when the artificial atmosphere is about 36 cm. below 

 the normal barometer, whereas the charged rider floats for different values 

 of D. Hence by equation (7) for this typical case, 



F = 3Xio 4 Z> volts, roughly 

 or numerically 



D = o.oi 0.1 1.0 2.0 cm. 



7 = 300 3,000 30,000 60,000 volts. 



In other words, the electrometer, beginning with one electrostatic unit, will 

 be suitable for measuring the ordinary sparking potentials in air of electro- 

 static and similar machines; for the distance apart of the plates of the 

 submerged condenser would probably suffice to prevent sparking within. 

 Moreover, H' — H is large enough for accurate measurement. 



35. Measurements. — For convenience the following experiments are made 

 in terms of H — H' the difference of manometer pressures. The poten- 

 tials used were produced by a small Wimshurst machine, eventually kept 

 in rotation by a small motor. The data are given in table 13, part I. 

 Different values of H were used, as the apparatus was not quite tight below 

 the rider, so that small accessions of air entered. The variations of V are 

 probably due to the electrical machine, which was here turned by hand. 

 The table shows a consistent series of relative values for V, notwithstanding 

 the large variation given to D. 



The diver was now modified by soldering it to a small sinker below in 

 order to secure greater stability of vertical notation. The results in table 

 13, part II, show considerable improvement and there was no leakage. 

 Changes of V are again probably due to the electrical machine. 



Another diver was now inserted having a breadth of tube somewhat 

 larger than the above, being 5 cm. in diameter and 6 cm. long. Its mass 

 was 31.98 grams, but unfortunately it proved to be slightly top-heavy in the 

 lighter oil, so that only a few measurements were taken (table 13, part III). 



The addition of a sinker increased the weight of the diver to 37.89 grams, 

 the other constants being the same. The experiment was satisfactory 



