422 



Mr. J. Buchanan. 



[May 27, 



The conclusion given above follows at once. 



To sum np the discussion, the result is that the equations (5), (6), 

 and (6') express the effect of heterogeneity in the constitution of the 

 dielectric medium. 



Note. — It may be well to notice here an objection that might be 

 raised against the validity of the above theorem. It could be urged, 

 that since Dr. Hopkinson has found by experiment* that no change 

 of specific inductive capacity could be detected when glass was 

 subjected to electric stress varied through a very wide range of 

 magnitudes, the quantity ir in the theorem has no existence. The 

 experiments just referred to, however, only prove that the quantity ir 

 is very small. It is shown in Part II of this paper that for most 

 substances tt has a value different from zero, being positive in some 

 cases, negative in others. 



Part II. 



Application to the Theory of the Origin of Electrification by Friction. 



The rubbing together of two bodies is the most ancient means 

 known of obtaining electricity. The absence of any accepted 

 explanation of this historical mode of rendering a body electrified 

 does not need to be enlarged upon. 



I have ventured to entertain the hope that the general theorem 

 proved above, together with the experimental results obtained by Dr. 

 Kerr in his memorable researches in the region of " electro-optics," 

 may be found to prove adequate to the explanation of the fundamental 

 and important subject of electrification by friction. 



As is well known, Dr. Kerr has proved that transparent dielectrics 

 become as a rule doubly refracting when subjected to electric force. 

 Under the action of electric stress, a dielectric becomes strained. 

 With the electric stress different at different parts of the field of 

 force, the strain varies from point to point. This space-variation of 

 strain manifests itself optically by the material assuming the property 

 of converting plane polarised into elliptically polarised light, when 

 the incident light is passed transversely across the direction of the 

 electric induction in the dielectric, and the plane of polarisation is 

 inclined at an angle to this direction. 



Moreover, as has been pointed out and proved experimentally by 

 Prof. Quincke,f the electrically-induced strain — the effect of which 

 Dr. Kerr observed as double refraction — produces a change in the 

 index of refraction. When the strain is uniform, Quincke has shown 

 that no double refraction ensues. Doubly refracting properties are 



* "Phil. Trans.," vol. 172, p. 355 (1881). 



t Quincke, " On Electrical Expansion," " Phil. Mag.," Ser. 5, vol. x, p. 30. 



