DISRUPTIVE DISCHARGE — DIFFERENCE OF POSITIVE AND NEGATIVE DISCHARGE. 129 



throwing out its brushes into the air, it is acting exactly as the small balls have acted 

 in the experiments already described, and by virtue of the same properties and rela- 

 tions. 



1300. It may very properly be said with respect to the experiments, that the large 

 negative ball is as essential to the discharge as the small positive ball, and also that 

 the large negative ball shows as much superiority over the large positive ball (which 

 is inefficient in causing a spark from its opposed small negative ball) as the small 

 positive ball does over the small negative ball ; and probably when we understand 

 the real cause of the difference, and refer it rather to the condition of the particles 

 of the dielectric than to the sizes of the conducting balls, we may find much im- 

 portance in such an observation. But for the present, and whilst engaged in inves- 

 tigating the point, we may admit, what is the fact, that the forces are of higher in- 

 tensity at the surfaces of the smaller balls than at those of the larger (1372. 1374.) ; 

 that the former, therefore, determine the discharge, by first rising up to that exalted 

 condition which is necessary for it ; and that, whether brought to this condition by 

 induction towards the walls of a room or the large balls I have used, these may fairly 

 be compared one with the other in their influence and actions. 



1501. The conclusions I arrive at are: first, that when two equal small conducting 

 surfaces equally placed in air are electrified, one positively and the other negatively, 

 that which is negative can discharge to the air at a tension a little lower than that 

 required for the positive ball : second, that when discharge does take place, much 

 more passes at each time from the positive than from the negative surface (1491.). 

 The last conclusion is very abundantly proved by the optical analysis of the positive 

 and negative brushes already described (1468.), the latter set of discharges being 

 found to recur five or six times oftener than the former*. 



1502. If, now, a small ball be made to give brushes or brushy sparks by a powerful 

 machine, we can, in some measure, understand and relate the difference perceived 

 when it is rendered positive or negative. It is known to give when positive a much 

 larger and more powerful spark than when negative, and with greater facility (1482.); 

 in fact, the spark, although it takes away so much more electricity at once, com- 

 mences at a tension higher only in a small degree, if at all. On the other hand, if 

 rendered negative, though discharge may commence at a lower degree, it continues 

 but for a very short period, very little electricity passing away each time. These 

 circumstances are directly related, for the extent to which the positive spark can 

 reach, and the size and extent of the positive brush, are consequences of the capa- 

 bility which exists of much electricity passing off at one discharge from the positive 

 surface (1468. 1501.). 



1503. But to relate these effects only to the form and size of the conductor, would, 

 according to my notion of induction, be a very imperfect mode of viewing the whole 



* A very excellent mode of examining the relation of small positive and negative surfaces would be by the 

 use of drops of gum water, solutions, or other liquids. See onwards (1581. 1593.). 

 MDCCCXXXVIII. 8 



