42 THE DIFFUSION OF GASES THROUGH 



weighs about 35 grams, including disk and sinker. The essential part is 

 the tube// of copper about 0.5 cm. in diameter. This float is not intended 

 to rise and fall as in the case of fig. 1 1 a, but to move to a definite level 

 under the influence of the electrical forces of the condenser. 



Water has been referred to as the liquid charge of the apparatus. It has 

 an advantage, inasmuch as the whole of the lower half of the condenser may 

 be earthed and the guard ring and disk are necessarily at the same potential. 

 It has the very serious disadvantage, however, that large capillary forces 

 are involved, particularly in case of the wide stem of fig. 11 a. Hence a 

 charge of kerosene oil or even of the heavier clear paraffin oil is preferable. 



In cases where the disk e in its uncharged position is to be flush with the 

 surface dd, it is convenient to provide the tube gg with opposite glass win- 

 dows (not shown), through which a mark on the tube//" or the bottom of 

 the tube or the sinker may be distinctly seen. The adjustment is made 

 once for all, so that when e is flush with d, the mark seen at the window may 

 coincide with a definite line or two lines in the same horizontal plane on 

 the opposed windows. 



28. Equations for the Tubular Float. — Let V be the difference of poten- 

 tial of the plates of the condenser in the absolute electrometer and D their 

 distance apart. Let F be the electric field, so that F = V/D. Further- 

 more let f e be the electric pressure between the plates, i. e., the pull per 

 square centimeter. 



Suppose the disk e is raised a small distance / above the level just char- 

 acterized by D. Then we may write, since 87r(3oo) 2 = 2.262 Xio 6 



f e =VV2.262Xio 6 X(D-l) 2 (1) 



if V is given in volts. 



The total mechanical force evoked by the same rise / above the position 

 of equilibrium of the float is vpg, where v is the volume of the stem sub- 

 merged, p the density of the liquid, and g the acceleration of gravity. 

 Let r be the radius of the stem (ff, fig. 1 1 a), and R the radius of the disk 

 (e, fig. 11 a). Then the amount of force evoked per square centimeter of 

 the disk, i. e., the mechanical or restoring pressure f m , is 



r 2 

 fm=-gilpg (2) 



In the case of equilibrium these two pressures are equal,/ e =/ ra ,and there- 

 fore 



F 2 = 2.262Xio 6 (-^)pg(L>-/) 2 / (3) 



To measure V, therefore, both D, the original distance apart of the un- 

 charged plates of the condenser (disk flush with the level of the guard ring) 

 and the rise of the disk, /, on charging, must be known. It will also be 

 desirable to raise the guard ring to the level of the disk in the charged 

 apparatus. 



