INFLUENCE OF POSITION ON THE INDICATIONS OF THE PROOF PLANE. 443 
face of action, as respects the plate p; for, as already observed, if such really existed, 
the plate could not possibly evince the same intensity with the same quantity, as is 
shown by disposing the electricity on two plates instead of one, in which case the 
attractive force, as evinced by the electrometer, is in an inverse ratio of the square of 
the surface * ; a result also demonstrable, in many instances, by means of the repul- 
sive force communicated to the discs of the balance, fig. 3. Under these circum- 
stances it is not illogical to conclude, that so far as surface is concerned, the electricity 
is upon the whole circumstanced much in the same way in each. Let a small proof 
plane be now applied to these charged bodies successively, in the similar points p,p',p", 
and the respective electrical reactions observed. The greatest electrical reaction will 
be obtained on the convexity p', the next on the plane p, and the least on the con- 
cavity p". 
If in these experiments p, p', p" represent the situation of the proof plane, we ob- 
serve that in the concavity p" it is most completely enveloped in the electrical mole- 
cules of the charged body ; on the plane it is much less enveloped ; on the convexity 
of the segment p' the particles of electricity are bent away from it, as it were, in all 
directions. Position alone therefore in respect of the other parts of the charged body 
may possibly influence the quantity taken up by the insulated plane. Now whatever 
tends to increase the inductive susceptibility of the proof plane, will bring the elec- 
trical reactions nearer a ratio of equality. Thus in giving the proof plane some con- 
siderable extension in the direction of its thickness, or otherwise in holding it by an 
insulated metallic wire, w, fig. 27, we increase its inductive force ; in this case the dif- 
ferences in the electrical reactions with a given charge become less. 
36. It follows from this, that could we employ a proof plane of perfect inductive 
susceptibility, we should actually arrive at equality in the reactions of the three bodies 
above mentioned. One method of effecting this is to make the proof plate a portion 
of a small coated element of glass, as represented in figs. 5 and 8. If a compound 
element such as this, and which has been already described (8, &.), be substituted for the 
small insulated disc above mentioned, it will, after contact with the charged bodies 
p, p', p" , fig. 24, have an equal reaction imparted by each. Another method consists 
in extending the limits of the wire w, fig. 27 ; but this we do in connecting the bodies 
with the fixed disc of the balance in the way above mentioned (8, v.) ; and we accord- 
ingly find that the repulsive forces imparted to the discs of the balance are equal. 
37- Experiment O. — Should any doubts remain concerning the actual disposition of 
the electrical charge on the three bodies p, p ' , p", figs. 24, 25, &c., we may substitute 
coated glass for these bodies, and charge them each equally by means of a small unit 
jar-f~. We have then, on removing the coatings from the charged side, or otherwise 
both the coatings, three strata of free electricity of the forms above mentioned. If 
any difficulty occur in obtaining two spherical segments of glass sufficiently similar, 
we may charge the opposite sides of the same segment successively. The temporary 
* Philosophical Transactions for 1834, p. 219. t Ibid. p. 217. 
3 L 2 
