226 MR. HARRIS ON SOME ELEMENTARY LAWS OF ELECTRICITY. 



the balls c & of the discharging electrometer. Similar results ensue in accumulating 

 different quantities on simple conductors, the distances through which a discharge 

 occurs in air of the same density being directly as the quantity accumulated. 



33. In order to conduct these and other experiments on electrical attraction, by 

 means of simple conductors, with greater accuracy, I employed the mechanical 

 arrangement represented in fig. 15. It consists of an oblong base, a b, a portion of 

 which, m n, may be drawn out to a certain length by means of an easy groove in 

 which it slides. There is a micrometer-screw and frame, f, fixed on this sliding 

 portion, which moves the insulating glass rod q between the guides m o, either back- 

 ward or forward, and by very small quantities. On the distant extremity a of the 

 base h a, is fixed a second insulating glass rod, r, which passing with friction through 

 some compressed cork in the ball r, may be either elevated or depressed for an 

 inch or more. By this machine two conductors, h h\ placed on the glass rods r q, 

 may be exactly opposed to each other in the same right line, and may be also set to 

 any given distance within the '01 of an inch, measured between their nearest points, a 

 graduated circle and index being affixed to the micrometer-screw at S for this pur- 

 pose. We may also charge either of the conductors h K with a given quantity of elec- 

 tricity, without the influence of the other, by withdrawing the sliding portion of the 

 base m n. 



(h.) Two conductors, h K', fig. 15, being separated by a given distance, measured 

 between the nearest points, one of them A.^ was withdrawn, so as not to influence the 

 quantity which the opposed conductor h could receive. When this last h had been 

 charged, then the conductor h^ was again restored, in an uninsulated state, to its pre- 

 vious position, and the precise distance at which the discharge took place observed 

 by a final approximation with the micrometer-screw s. This distance being found, the 

 same was repeated when the conductor ^ was charged with only one half the previous 

 quantity, and so on. In these experiments the distances of discharge varied directly 

 with the respective quantities accumulated*. 



34. Comparing these results with those before arrived at (17. 20.), it may be seen, 

 that whilst the distances of discharge between two points increase in the simple ratio 

 of the quantity, the attractive forces increase as its square. 



35. This is not only applicable to discharges produced by different quantities dis- 

 posed on the same conductor, but it is also true in disposing the same quantity on 

 many conductors precisely similar, so as to double, treble, &c., the extent of similar 

 surface : we have in all cases the distance of discharge, in a simple ratio of the quan- 

 tity contained on a unit of similar surface. 



36. The distance, therefore, through which an electrical accumulation can discharge 

 in air of a given density, is an accurate measure of the comparative quantity contained 



* I do not advert to these experiments as containing any very new or unexpected results in electricity, but 

 in explanation of the application of particular methods of research, in demonstrating more completely than has 

 been hitherto done, a class of facts essentially involved in the subject of these inquiries. 



