444 MR. HARRIS ON THE ELEMENTARY LAWS OF ELECTRICITY. 
coatings both of the plate and segments may consist of sulphate of lime, neatly 
moulded to the glass, and subsequently gilded ; they must be each of an equal surface. 
It will be also requisite for greater accuracy to obtain a glass plate of the same thick- 
ness as the spherical segment. 
It is not therefore impossible for a proof plane, such as that employed by Coulombe. 
to exhibit unequal electrical reactions, and yet the distribution on the conductor to 
which it has been applied be uniform. Thus a tangent plane of inconsiderable thick- 
ness applied at the extremity a of a charged cylindrical conductor, c, fig. 18, maybe 
in a more favourable position for the inductive action than if placed at the centre c. 
or in the centre of the circular plane c', terminating either of its extremities. 
38. In treating of the proof plane, philosophers have considered its action in more 
than one point of view. M. Biot states that the proof plane, in becoming assimilated 
with a superficial element of a charged body, will take up as much electricity upon 
each of its surfaces as exists upon the point to which it is applied ; hence, on re- 
moval, it is charged with double that quantity*. M. Pouillet, on the contrary 
considers the proof plane to be, at the instant of contact, in precisely the same state 
as a superficial element of the same dimensions, and to be, on removal under the same 
electrical conditions, as a similar portion of the charged body would be placed if ac- 
tually cut out of its surface ; that is to say, according to his view, the electricity 
would be first collected on one side only, and would subsequently be expanded over 
both ; each surface therefore has only half the quantity which the superficial element 
at first possessed. 
39. Both these views of the state of the proof plane are evidently at variance with 
the phenomena above recorded (28.) ; this, it is true, is of no great consequence 
to Coulombe’s results when he merely employs the proof plane to determine the 
ratios of the quantities of electricity distributed on a charged conductor ; it has, how- 
ever, still a material influence on the theory of electricity. 
40. The experiments with the proof plane just mentioned (31.), (32.), together with 
the phenomena of repulsion so frequently alluded to in this paper (14.), necessarily 
lead us to investigate more rigorously the nature of its indications, in order to dis- 
cover, if possible, the conditions under which it may fail to become charged, either 
with the same quantity as exists in the points touched, or otherwise in the ratio of 
the quantities. 
The first notions which present themselves from our experience of ordinary elec- 
trical actions, would lead us to conclude that an electrified conductor of large di- 
mensions could always charge a small body to saturation from any point of it, 
provided the electrical state of the touching body was such as would enable it to 
become so charged : this is indeed quite evident from the fact that a compound ele- 
ment su chas that already mentioned (8, &.), and represented fig. 8, becomes charged 
equally at whatever point of a long electrified cylinder, c, fig. 18, it is applied, whether 
* Traits de Physique, tom. ii. p. 271. f Physique Experimentale, tom. i. Secoude Partie, p. 579. 
