Pager 4 
XX. Researches in Statical Electricity. 
By Sir W. Snow Harnris, F.R.S. 
[Continued from p. 100.] 
Charging of the proof-plane and other insulated conductors. 
Coulomb’s experiment with a hollow globe and circular plate of twice 
the diameter. 
Expression representing electrical charge. 
23. bi QUESTION has not unfrequently arisen in regard to 
the charging of the proof-plane and simple electrified 
conductors, which admits of an easy solution upon the element- 
ary principles and subsequent experimental inquiries we have been 
considering. It has been doubted whether these bodies take up 
electricity upon all their surfaces, or upon one only. Now it is 
evident, (3) and Exp. 4, that neither the proof-plane nor any other 
conductor can take up electricity upon either face, except we dis- 
place from the face of contact a portion of its own electricity (3). 
The charge virtually consists of its own displaced electricity (9), 
consequently the charge it receives will be entirely dependent on 
this induced change, as we have already seen (17). In the case of 
an insulated conducting surface exposed to the operation of sur- 
rounding matter, a stratum of accumulated electricity must always 
be found upon all its surfaces upon the principles already exposed 
(10). Let, for example, a, b,c, fig. 18, be three concentric hollow 
spheres placed one within the other, and so sustained as not 
anywhere to touch ; let the interior middle sphere be perfectly 
insulated, whilst the spheres a@ and ¢ communicate with the 
ground; then in communicating electricity to the insulated 
sphere 4, we find it charge on both its surfaces, and so produce 
an electrical stratum, bc, on each side of it. If we cut off the 
influence of the sphere a or c, then it charges on one surface 
only ; at least the charge on the opposite surface is so small as 
to admit of being neglected in such an experiment. In the case 
of the double induction, twice the quantity of electricity may be 
accumulated under the same degree of the electrometer. The 
following experiments are instructive, and very conclusive. 
Ezp.14, Let a light circular disc of gilded or silvered wood, c, 
fig. 19, about a foot in diameter and something less than *25 of 
an inch thick, be suspended from a varnished glass arm by an in- 
sulating thread of varnished silk gut (2); connect it with the 
hydrostatic electrometer E, or with the fixed ball of the balance, 
as in fig. 17 (22). Let two other perfectly similar plates, a, 6, be 
suspended in like manner at an equal distance upon each side of 
the plate c. 
Connect one of the external plates, a, with the ground, and 
proceed to charge the centre plate ¢ with a given number of 
