430 MR. HARRIS ON THE ELEMENTARY LAWS OF ELECTRICITY. 
his results were sometimes uncertain. We may also avoid this by exposing the insu- 
lators to the influence of small irons heated beyond redness, and prepared of various 
forms for the occasion, which at once deprives them of any electricity they may have 
taken up, and renders their insulating power sufficiently perfect. I have found slender 
glass rods covered with a varnish of coarse shell lac laid on warm, or otherwise good 
sealing-wax, become extremely perfect insulators when treated in this way. 
14. The relation of the repulsive force between two bodies, as p , m, figs. 3 and 14, 
to the quantity of electricity with which each is charged, and to their distance apart, 
is of vital consequence to electrical investigations depending on the principle of 
repulsion. According to the experiments of M. Coulombe, the total force between 
two insulated charged bodies, is as the quantity of electricity in each directly, and as 
the squares of the distances inversely*. Hence he supposes the action of each of the 
repelling bodies to enter into the composition of the result in such way, that the total 
force increases or diminishes with the electricity contained in either. 
Thus the total force with which two bodies considered as points repel each other 
F 
at any distance, D, is represented by jy 2 , or considering F as the product of two 
constants R R', that is to say, of the force of each body, this expression becomes — 
Biot terms these constants R R' the electrical reaction of the bodies to which they 
apply. Although this expression coincides in many cases with the results of experi- 
ment, it does not seem upon the whole sufficiently general. Having found many 
important exceptions to it, I was led to institute a series of experiments with a view 
of discovering the cases in which it might possibly fail. 
Experiment C. — The results given in the following Table were deduced by re- 
peatedly examining the repulsive force exerted between the electrified discs p, m, fig. 3, 
placed in a perfectly insulating atmosphere (13.) at various distances apart, and 
charged with equal and unequal quantities of electricity. The relative charges were 
obtained by the simple method resorted to by Coulombe, viz. by touching one of the 
bodies with an insulated neutral body of the same dimensions, so as to abstract at 
any time one half the charge. In these experiments the quantity requisite to 
produce a repulsive force of 24° when equally divided between the two discs was 
taken as a unit of charge ; the reactive force of the instrument being about the 
-i-jVo-dth of a grain for each degree. I then proceeded to reduce the charge upon one 
of the discs in such given proportions as could be obtained by continually abstracting 
one half the quantity remaining on either disc. In deducing the results each disc 
was in turn made constant, and a mean result taken ; this did not, however, upon 
the whole, greatly differ from that arrived at by one disc alone. The length of the 
threads of suspension v/as taken at 20 inches, and their distance apart ‘25 of an inch. 
* Hauy’s Philosophy, by Gregory, p. 356 and 364. 
f BioT, Traite de Physique, tom. ii. p. 242. 
