STRIKING DISTANCE. 195 



spherical ; but its radius of curvature is very great, and the dis- 

 tribution of potentials is sensibly the same as between the planes. 

 The following table sums up the results of the experiments.* The 

 numbers in the third column were obtained by multiplying those in 



3*io 10 

 the preceding column by = 300 (510). 



Distance Difference of Potential^ Electrical 



Force 



V 



~d 



267*1 



257-0 



224*2 

 200'6 



iS^S 



144-1 

 139-6 



140*8 



134-9 

 132-1 

 131*0 



This table demonstrates the remarkable fact that the electrical force 

 is greater for small distances than for larger ones. It results from 

 this that the electrical density and the electrostatic pressure go on 

 diminishing as the striking distance increases. 



In the experiments of Sir W. Thomson the striking distance did 

 not exceed 0*15 cm. It is difficult to measure directly the difference 

 of potential for greater differences. The electrometer is connected 

 with an insulated conductor B under the influence of a conductor A 

 in connection with one of the electrodes between which the spark 

 strikes, the other being in connection with the earth. So long as 

 the system B does not take its own charge, its potential is in a 

 constant ratio with that of the system A. This ratio, which can be 

 varied with the distance of the two conductors, is determined in 

 each case by a special experiment. 



This method was used by Gaugainf to investigate the law of the 

 striking distance between two balls or between two plates, and thus 

 demonstrate the law of the variation of the electrical density 



* THOMSON. Reprint of Papers, p. 258. 



t GAUGAIN. Ann. de Chim. et de Phys. [4], Vol. viu., p. 75. 1866. 



O 2 



