50 Prof. Thomson on the Mathematical Theory of 



distance, and at all smaller distances there is attraction between 

 the bodies. 



These results are, with all their peculiarities, in full accordance 

 with the theory of Coulomb, which indicates that, if the quan- 

 tities of electricity be equal, and the bodies equal and similar, 

 there will be repulsion in every position : but if there be any 

 difference, however small, between the charges, the repulsion 

 will necessarily cease, and attraction commence, before contact 

 takes place, when one body is made to approach the other. Un- 

 less, however, the difference of the charges be sufficiently consi- 

 derable, a spark may pass between the bodies, and render the 

 charges equal, before attraction commences. In Mr. Harris's 

 experiments, in which the bodies seem to have been nearly oblate 

 spheroids, the attraction is generally sensible before the distance 

 is small enough to allow a spark to pass, if the charge on one 

 be double of that on the other. 



Mr. Harris next proceeds to investigate the theory of the 

 proof plane, and to examine whether it can be considered as 

 indicating with certainty the intensity of electricity at any part 

 of a charged body, and, principally from an experiment made on 

 a charged non-conductor (a hollow sphere of glass), comes to a 

 negative conclusion. It should be remembered, however, that, 

 the proof plane having never been applied to determine the in- 

 tensity at points of the surface of a charged non-conductor, such 

 conclusions in no way interfere with adopted ideas. Since there 

 can be no manner of doubt as to the theory of this valuable 

 instrument, as we find it explained by M. Pouillet*, nor as to 

 the experimental use of it made by Coulomb, it is unnecessary 

 to enter more at length on the subject here. 



7. Mr. Faraday's researches on electro-statical induction, 

 which are published in a memoir forming the eleventh series of 

 his " Experimental Researches in Electricity,'' were undertaken 

 with a view to test an idea which he had long posstessed, that 

 the forces of attraction and repulsion exercised by free electri- 

 city are not the resultant of actions exercised at a distance, but 

 are propagated by means of molecular action among the conti- 

 guous particles of the insulating medium surrounding the 

 electrified bodies, which he therefore calls the dielectric. By 

 this idea he has been led to some very remarkable views upon 

 induction, or in fact upon electrical action in general. As it is 

 impossible that the phsenomena observed by Faraday can be 

 incompatible with the results of experiment which constitute 

 Coulomb's theory, it is to be expected that the difference of his 

 ideas from those of Coulomb must arise solely from a different 

 method of stating, and interpreting physically, the same laws : 

 • See Note IV. 



