LIQUIDS AND ALLIED EXPERIMENTS. 



51 



throughout, the suddenness with which the charged diver breaks off being 

 specially marked. Care must be taken to decrease pressure slowly in order 

 to avoid thermal discrepancies. Table 13, part IV, is an example of the 

 data obtained. 



Table 13. — Measurement of potential. 



I. — M=35 grams. Disk, 3.5 cm. in di- 

 ameter. Kerosene oil,p=o.799 at 24°. 

 Barometer, 76.14 at 17°. Diameter 

 of diver tube, 3.85 cm.;length, 8.8cm. 



III. — il/=3i.98grams. Diameter, 2i? = 

 5.0 cm.; length, 6 cm.,* slightly top- 

 heavy. 



II. — M= 35.795 grams. Barometer, 75.70 

 at 19°. Diameter of diver tube, 3.85 

 cm.; length, 8.8 cm. hpu,/pm=o.^. 



IV. — M=37. 89 grams. Diameter, 2i?: 

 5.0 cm.; length 6 cm. 



These values of V for so large variation of D {H — H' increasing about 

 5 times) are to be regarded as a satisfactory test, the fluctuation of values 

 being again attributable to the electrical machine. In these absolute values, 

 however, the potentials V found were somewhat larger than would follow 

 from the results of an interposed spark-gap with balls about 2 cm. in diam- 

 eter. The spark would have been equivalent to about 16,000 volts. 

 Sparks, however, here occur too rapidly for measurement. 



A number of experiments tried by interposing solid disks between the 

 condenser plates led throughout to inadmissible results. In case of a mica 

 plate lying on the guard ring, the whole cylinder of oil between the metal 

 disk at the top and the mica plate must eventually have reached constant 

 potential, the charge being carried by convection to the top face of the mica. 

 The diver, therefore, clings to the mica plate, which in an insulating medium 

 can not be again discharged. All measurement is thus out of the question. 

 In case of thick glass plates completely filling the condenser space, other 



