EXPERIMENTAL EXAMINATION OF THE INDICATIONS OF THE PROOF PLANE. 445 
at the centre c or at the extremity a; as also from the fact already noticed (36.), that 
whatever part of a charged conductor be connected with the fixed disc of the balance, 
or with any other species of electrometer, the instrument is equally affected. The 
proof plane, therefore, if it really exhibits the ratios of the quantities of electri- 
city disposed in different points of a charged body, must depend materially on the 
resistance to an equal participation in the charge, arising from its little inductive 
susceptibility; it is hence very desirable to investigate experimentally the various 
effects which increased thickness, or extension of any kind, position, intensity of 
charge, and the like, may have on its indications. An experimental inquiry of this kind 
is, however, extremely difficult : we are, for example, open to all the sources of fallacy 
above described (15.), and illustrated by Table IV. Thus, if we examine the electrical 
reactions, having previously charged the disc of the needle with some given quantity 
of the same electricity, the subsequent quantities taken up by proof planes of various 
thicknesses, &c., may bear all sorts of proportions to this quantity * and since the 
law of action is under these circumstances not always regular (15.), we might have, 
in examining the respective forces at some given distance, d, very obscure and un- 
certain results. If, on the other hand, we neutralize the disc of the needle at each 
experiment, and diffuse the electricity taken up by the proof plane equally over each, 
we are still open to fallacy ; since at small distances the force is sometimes in an 
inverse ratio of the simple distance, whilst at others it varies in an inverse ratio of 
the square of the distance ( 17 -) ; and we have in some instances the electrical re- 
action so little that the discs do not separate, in consequence of the slight attractive 
force exerted between them at the point of contact, and which in some degree vitiates 
the result at a given distance, d. 
41. Experiment P. — Notwithstanding these obstacles, I have endeavoured to obtain 
a good series of observations on the indications of insulated tangent planes of various 
thicknesses, applied to different points of a charged cylinder of about four feet in 
length, and 2 b inches in diameter, terminating in plane circular faces. The experi- 
ments were conducted in the following way. The charged cylinder c, fig. 18, was 
placed on two slight insulators, I, I', fixed on a small platform, N, supported beneath 
on four small rollers, so as to be moveable between the guide pieces g' g' of the fixed 
base B. The extremity of the cylinder could be by this method brought immediately 
under the suspended plane p of the electrometer E*. When it was required to convey 
to the cylinder a charge of any given magnitude, the index of the instrument was ad- 
justed from zero to a given number of degrees in the direction o y , by means of small 
weights placed in the cup at q. Electricity was then communicated to the cylinder c 
until the index came again to zero ; and thus the distance p a, at which the attrac- 
tive force operated on the suspended plane p, was always constant for any given 
charge. Now as the attractive force is as the square of the quantity of electricity 
on the charged body'f~, we have only to make the degrees at which we adjust the 
* Philosophical Transactions for 1834, p. 215. t Ibid. p. 221. 
