\^ ^5-^-)^,^^ Electricity in Equilibriuni^j,^ 1....<T 4© 



linear dimensions, so that tlie distribution of electricity on each 

 may be uninfluenced by the presence of the other. Also the 

 bodies should be spheres, so that the attraction may be the same 

 as if the whole electricity of each were collected at its centre ; 

 and the distance to be measured will then be the distance 

 between the centres. These conditions have been expressly 

 mentioned by Coulomb, and they have been fulfilled, as far as 

 possible,, in his researches, as we see by the descriptions of the 

 experiments made, which we find in his memoirs. He has thus 

 arrived by direct measurement at the law, which we know by a 

 mathematical demonstration*, founded upon independent experi- 

 ments, to be the rigorous law of nature, for electrical action. 

 None of these precautions however have been taken in the 

 experiments described in Mr. Harris's memoir, and the results 

 are accordingly unavailable for the accurate quantitative verifi- 

 cation of any law, on account of the numerous unknown disturb- 

 ing circumstances by which they are affected. The phaenomena 

 which he observes, however, afford qualitative illustrations of the 

 mathematical theory of a very interesting nature, as may be 

 seen from the following examples of his results. 



[a) When the distance between the bodies is great with 

 reference to their linear dimensions, the repulsion is inversely 

 as the square of the distance, and directly as the product of the 

 masses. 



{b) When the distance is small, the action becomes apparently 

 irregular. Thus if the quantities of electricity on the two 

 bodies be equal, the force, which is always of repulsion, does 

 not increase so rapidly when the bodies approach, as if it followed 

 the law of the inverse square of the distance. 

 V' (i^) If the charges be unequal, the repulsion ceases at a certain 



««>.; ' 



ciifj See Murphy's " Electricity," p. 41, or Pratt's " Mechanics," Art. 154. 

 ' tCavendish demonstrates mathematically that if the law of force be any 

 other than the inverse square of the distance, electricity could not rest in 

 equilibrium on the surface of a conductor. But experiment has shown, 

 that electricity does rest at the surface of a conductor. Hence the law of 

 force must be the inverse square of the distance. Cavendish considered 

 the second proposition as highly probable, but had not experimental evidence 

 to support this opinion, in his published work (An attempt to explain the 

 phrenomena of Electricity by means of an Elastic Fluid). Since his time, 

 the most perfect experimental evidence has been obtained that electricity 

 resides at the surface of a conductor ; in such facts, for instance, as the 

 perfect equivalence in all electro-statical relations of a hollow metallic con- 

 ductor of ever so thin substance, or of a gilt non-conductor (possessing a 

 conducting film of not more than ^^oWjt of ^"^ i^^ch thick) and a solid 

 conductor of the same external form and dimensions j the minor premise 

 of his syllogism is thus demonstrated, and the conclusion is therefore 

 established.] 



Phil Mag. S. 4. Vol. 8. No. 49. July 1854. E 



