EXPERIMENTAL EXAMINATION OF THE INDICATIONS OF THE PROOF PLANE. 449 
electricity ; so that the same quantity may possibly exist in two different points, and 
yet the proof plane become charged in a different ratio, the inductive power of the 
plate being different in these points. M. Biot, in the second volume of his Traite de 
Physique*, has given, from Coulombe’s manuscripts, an account of the application of 
the proof plane to an electrified lamina of steel plate, eleven inches in length, one inch 
in width, and half a line thick. He states, that when the proof plane, which was an 
inch long and about a quarter of an inch wide, was applied beyond the extremity of 
the charged lamina, so as to touch its two opposite surfaces, the plate abstracted the 
electricity of both, and exhibited a reaction double of that which it showed when only 
applied at the extremity of one of the surfaces. Now it will be immediately perceived 
that in this application of the proof plane it was placed entirely without the charged 
body, the most favourable position possible for taking up the electricity of the plate ; 
a circumstance which would greatly influence the result (35.). Should this ingenious 
experiment really prove the diffusion of a stratum of an invisible subtle fluid over the 
two surfaces of the plate, it would at the same time equally well demonstrate the 
existence of electricity upon the interior surface of a hollow body, as, for example, 
upon a hollow cylinder, into which we may suppose the steel plate in question to be 
formed, since we have shown (28.) that the intensities evinced by a given quantity 
of electricity are the same in both cases, and that consequently the distribution, so 
far as respects these two surfaces, must be similar. 
47. Should the inductive susceptibility of the tangent disc be at any time, by its 
position in respect of the electrical particles, reduced to zero, or nearly so, it would 
then fail to become in any degree charged, and would be as inefficient as a plate of 
varnished glass or any other non-conductor, the inductive susceptibility of which is 
so little, that it will not, under ordinary circumstances, take up the least electricity 
on being applied to a charged body. Now it is not improbable that a small insulated 
plate plunged within a spherical charged shell is thus circumstanced : it may hence 
fail to become charged, even although electricity should really exist there, and which 
fact we have experimentally shown (31.). Similar effects would ensue in placing a 
neutral body under any other circumstances involving similar conditions, as in placing 
a very small conducting ball immediately between two large electrified globes. The 
small globe does not, under these circumstances, according to Coulombe, take up any 
free electricity. 
48. It would be difficult, without the aid of induction, to explain in what way the 
mere position of a neutral body, in respect of the electrical particles by which it is 
surrounded, effects its power of absorbing electricity ; and even with this we require 
a more extensive investigation of the phenomena than has yet appeared. That a 
change of position of the electrical particles of a charged surface in respect of each 
other, and of the body charged, is attended by important consequences, has been 
already shown by Volta, who observed the curious fact that the intensity of a charged 
MDCCCXXXVI. 
* p. 275. 
3 M 
