448 
MR. HARRIS ON THE ELEMENTARY LAWS OF ELECTRICITY. 
vary, as seen in the same Table. When, however, the two columns c and e again vary 
together, or nearly so, the ratio of the quantities, as given by each of the discs, is 
about the same as before, until at last, where the induction upon the tangent plate 
is the most perfect, the ratio becomes one of equality, or nearly so. The same re- 
marks apply in comparing the horizontal columns c' and e or c and c'. 
43. In comparing Tables X. and XI. there is one important fact to be observed ; 
viz. that we do not get the exact ratio of the respective quantities of electricity on the 
charged cylinder until we arrive at the most perfect inductions, as in the action of 
the plates p and q : here the reactions are as 2 : 1, or nearly so, whereas in comparing 
the similar plates and parts touched with the less powerful plates deg the discre- 
pancies are considerable, evidently showing that we have not really arrived at the 
true ratio of the quantities by means of these plates. 
44. It is worthy of further inquiry, whether the proof plane be really identical with 
an element of the charged surface to which it is applied, or whether it be not in the 
condition of a neutral insulated body placed within an extremely small distance of a 
charged body, and subject to the same laws as subsist between two such bodies when 
placed under similar conditions, at more sensible distances, and at which electricity 
may be communicated. A rigorous examination of this question would probably 
elucidate many phenomena of electrical action at present involved in doubt : in the 
mean time it may not be unimportant to review such facts connected with this point 
as are already known. 
45. It has been found, for example, that the attractive force between charged and 
insulated neutral bodies is less than when the latter are uninsulated ; that perfect in- 
sulators are not sensibly attracted by electrified substances ; and that, in every case of 
electrical attraction, the force is (as already observed,) proportionate to the previous 
induction of which the bodies are susceptible. In accordance with these facts a per- 
fectly insulating disc reposes on a charged surface without becoming itself electrified ; 
an insulated neutral conducting disc more or less so, in proportion to its thickness 
(42.); whilst an insulated plate whose inductive power is nearly perfect, is charged to 
equality with the point touched (36.). 
I have found in the course of some recent inquiries that the attractive force 
between an electrified plane surface and an insulated disc of inconsiderable thick- 
ness, in a neutral state, is frequently in an inverse ratio of the distance between the 
two planes ; the induction of which such a disc is susceptible being extremely limited ; 
that on increasing the thickness the force also increases up to a certain point, where, 
under the given conditions of distance, quantity of electricity, and the like, the induc- 
tion on the opposed surface remains nearly the same. 
46. These facts, together with those already mentioned in the course of these inquiries, 
render it highly probable that the quantity of electricity taken up from the surface of 
a charged body by a small insulated disc of inconsiderable thickness may be greatly 
influenced by the position of the point of application, independently of the quantity of 
