96 DR. FARADAY'S EXPERIMENTAL, RESEARCHES IN ELECTRICITY. (SERIES XII.) 



limit of the influence which the dielectric exerts in resisting discharge ; it is a mea- 

 sure, consequently, of the conservative power of the dielectric, which in its turn may 

 be considered as becoming a measure, and therefore a representative of the intensity 

 of the electric forces in activity. 



1363. Many philosophers have examined the circumstances of this limiting action 

 in air, but, as far as I know, none have come near Mr. Harris as to the accuracy 

 with, and the extent to, which he has carried on his investigations*. Some of his 

 results I must very briefly notice, premising that they are all obtained with the use 

 of air as the dielectric between the conducting surfaces. 



1364. First as to the distance between the two balls used, or in other words, the 

 thickness of the dielectric across which the induction was sustained. The quantity 

 of electricity, measured by a unit jar or otherwise on the same principle with the unit 

 jar. In the charged or inductive ball, necessary to produce spark discharge, was found 

 to vary exactly with the distance between the balls, or between the discharging 

 points, and that under very varied and exact forms of experiment-f-. 



1365. Then with respect to variation in the pressure or density of the air. The 

 quantities of electricity required to produce discharge across a constant interval varied 

 exactly with variations of the density ; the quantity of electricity and density of the 

 air being in the same simple ratio. Or, if the quantity was retained the same, whilst 

 the interval and density of the air were varied, then these were found in the inverse 

 simple ratio of each other, the same quantity passing across twice the distance with 

 air rarefied to one half J. 



1366. It must be remembered that these effects take place without any variation of 

 the inductive force by condensation or rarefaction of the air. That force remains 

 the same in air§, and in all gases (1284. 1292.), whatever their rarefaction may be. 



1367- Variation of the temperature of the air produced no variation of the quantity 

 of electricity required to cause discharge across a given interval ||. 



Such are the general results, which I have occasion for at present, obtained by 

 Mr. Harris, and they appear to me to be unexceptionable. 



1368. In the theory of induction founded upon a molecular action of the dielectric, 

 we have to look to the state of that body principally for the cause and determination 

 of the above effects. Whilst the induction continues, it is assumed that the particles 

 of the dielectric are in a certain polarized state, the tension of this state rising higher 

 in each particle as the induction is raised to a higher degree, either by approximation 

 of the inducing surfaces, variations of form, increase of the original force, or other 

 means ; until at last, the tension of the particles having reached the utmost degree 

 which they can sustain without subversion of the whole arrangement, discharge im- 

 mediately after takes place. 



1369. The theory does not assume, however, that all the particles of the dielectric 



* Philosophical Transactions, 1834, p. 225. f Ibid. % Ibid. p. 229. 



§ Ibid. pp. 237. 244. |1 Ibid. p. 230. 



