© 



386 RECENT PROGRESS IN PHYSICS. 



If electricity be induced upon an insulated conductor, we find, as a 

 general rule^ that the electricity dissimilar to that of the inducino- 

 body approaches as near as possible to it, and the similar removes as 

 far as possible from it. This rule, however, even if we regard it only 

 as a general guide, leaves much that is undetermined. 



i-'ifr- 26. Let an insulated disk a h (fig. 26) be exposed 



fm , ^ . if to the inductive action of an electrified sphere 



"^ c. How will the two electricities be distributed 



on ahf Are the edges a &, or the middle of 

 the back d, to be regarded as the most remote 

 parts of the disk ? If c be positive, is positive 

 electricity to be expected at d f 

 Fechner has made a series of experiments in answer to these ques- 

 tions. The attracted — E is collected in the greatest quantity at the 

 middle of the front surface, and decreases towards the edges; the 

 repelled -|-B is found on the back, and its intensity, which is but 

 slight in the middle, at d increases towards the edges. The repelled 

 -\- E embraces the edge, so that it is found at the edge even on the 

 front face; the line of indiiference between the -|-E and — E is on the 

 front, and approaches the middle the closer the sphere is brought to 

 the disk. 



Fechner has made similar experiments with rods and strips of metal. 



[§ 18 is omitted because it is occupied with the refutation of the views of Knochenhauer, 

 which liave never been generally adopted, and which are sufliciently disproved in other 

 parts of this report.] 



§ 19. Experimental proof that the quantity of latent electricity 

 IS in the inverse proportion to the square of the distance from the 

 inditing body. In order to set aside definitely the objections of 

 Knochenhauer, I have myself made a series of experiments on the law, 

 according to which the strength of the induction decreases when the 

 distance between the bodies acting on each other increases. 



The method of observation was essentially the same as that em- 

 ployed by Knoclienhauer , except that I substituted a straw electrome- 

 ter, with a graduated arc, for the torsion balance. I had first to find 

 the proportion of the increase of charge to the increase of divergence 

 of the leaves of the electrometer, in order to determine subsequently 

 from tlie divergences the magnitude of the electrical force which pro- 

 duced them. This was accomplished in the following manner: 



A large Leyden jar, having about two square feet of interior coat- 

 ing, was charged with positive electricity: the knob of the jar might 

 be considered as a tolerably constant source of electricity, from which 

 the KumG small quantity could always be taken and conveyed to the 

 electrometer. This transfer was made by means of a brass knob of 

 about three lines in diameter, insulated by a sealing wax handle of 

 sufiicient length. This small knob was brought into contact with the 

 knob of tlie Leyden jar, and thus charged with a certain quantity of 

 electiicity, which we will designate by 1. This quantity 1 was then 

 transferred to the electrometer by touching its jjlate with the charged 

 knob; the pendulum diverged, and the amount of the divergence was 

 noted. 



Tlie small knob was again brought into contact with the knob of 



