440 
MR. HARRIS ON THE ELEMENTARY LAWS OF ELECTRICITY. 
in my former inquiries*, that their electrical reactions or capacities are precisely the 
same as those of the plane areas, into which we may suppose them to be expanded. 
Thus the electrical capacity of the hollow cylinder A', fig. 19, open at both ends, is 
precisely the same as if it were cut through in the line a b, and expanded into a 
plane A, fig. 20, and conversely the capacity of the plate A, of inconsiderable thick- 
ness, is the same as that of an open cylinder A', fig. 19, or A", fig. 21, into which we 
may suppose it to be turned without doubling over the edges, whether rolled in the 
direction of its length m n or breadth n o ; now such could not possibly happen if 
their capacities were not the same, since the same quantity expanded on a double 
surface has a very different electrical reaction. 
29. In comparing the capacities of a sphere and plate of equal area by the method 
just given (28.), we may place any charged body in connexion with the fixed ball of 
the balance, — such as a common cylindrical conductor, — since we have merely to 
discover the respective quantities abstracted. In every case of this kind the effect of 
contact with a plate and sphere of equal area will be found the same. We may also 
reverse the former experiment, and connect the plate with the fixed disc instead of 
one of the spheres, and so examine the decreased intensity of the plate after contact 
with the sphere or with a similar plate. 
On referring to columns N ; b, M ; b\ M', &c. of Table VII., we observe one of those 
cases in which the force is nearly in an inverse ratio of the distance, the distance 23 
and 48 being as 1:2 nearly, whilst the corresponding forces 23 and 12 are nearly 
as 2 : 1 . 
30. This simple induction of facts appears sufficiently conclusive ; it clearly shows 
that a spherical conductor, either hollow or solid, and a plate of equal area, have the 
same electrical capacity, and that Coulombe’s experiments are not opposed to such a 
conclusion. 
31. We may now proceed to consider the case of an insulated body plunged within 
an electrified sphere, and this will necessarily lead to some further inquiries into the 
action of the proof plane, and to the conditions under which one substance receives 
electricity from another. 
The curious fact that we do not abstract any portion of the charge accumulated on 
a hollow sphere by touching its interior surface with an insulated neutral disc placed 
wholly within it, will, on inquiry, be found little conclusive of the non-existence of 
electricity upon that surface ; it may be experimentally shown that any insulated body 
plunged within an electrified shell, could not possibly take up a particle of electricity, 
even although a powerful accumulation actually existed there. 
Experiment L. — Let a small sphere of glass a a!, fig. 22, made clean and dry, having 
a projecting neck at a! varnished with shell lac, be nearly filled with dry mercury, 
and let the whole be placed in a vessel b b', also containing mercury, so as to give 
the glass an outer and inner coating ; charge this system, and remove the charging 
* Philosophical Transactions for 1834, pp. 232, 233. 
