LIQUIDS AND AI^UE;d EXPERIMENTS. 53 



whence it appears that the lightest possible diver, observed at nearly atmos- 

 pheric pressure, for as large an area of the movable disk and as small a 

 distance apart of the condenser plates as possible, would here be conducive 

 to the best results. An apparatus constructed so that 



£^' = 75 cm. i?=i5cm. D = 0.05 cm. 1/ = 60 grams 



should show A.^= i cm. about for 100 volts, which appears to be the lower 

 limit of measurement. The higher limit, if the disks are made small, M 

 and D large, seems to be indefinite, providing all sharp edges can be obvi- 

 ated. So far as present experiments went the general behavior of the 

 apparatus was quite satisfactory. The drop is rapid and definite, facili- 

 tating accurate pressure measurement. The apparatus need not be quite air- 

 tight, since 11' — H is the variable in terms of which potential is to be found. 

 The large values of D (several centimeters) v/hich seem to be admissible 

 without vitiating the simple form of equation is an additional advantage, 

 so that sparks may in a measure be avoided. A heavier oil than kerosene 

 would perhaps be advisable, though sparks may occur without danger in 

 any case. 



