BALANCE APPLIED TO THE PURPOSES OF AN ELECTROMETER. 
429 
from less than a single degree up to 360 degrees if requisite (/.) (Table III.), by 
turning the circle A 1 1 either in a direction contrary to that in which the needle is 
repulsed, or otherwise in the same direction. We may in either case always estimate 
the distance of the repelling bodies independently of the vertical index ty , by adding 
or subtracting the angular turning of the circle A t I, to or from the angular deflec- 
tion of the needle, according to the direction in which it has been moved. 
If the disc m be connected with any external body, s, figs. 3 and 9, then the relative 
distances and forces maybe varied by turning the micrometer index x a given number 
of degrees, either in the same or in a direction contrary to that in which the needle 
is repelled, which must be either added to, or subtracted from the total angular de- 
viation of the index v v' , according as the direction is + or — in respect of the di- 
rection of the force of repulsion. Thus if, as in fig. 14, the deviation of the needle as 
expressed by the index v v amounted at first to 20°, and by subsequently turning the 
index x, figs. 3 and 15, by a quantity equal to 70°, in a direction contrary to that 
in which the needle was repelled, we had brought the index v v back to a distance 
of 10°, then the total force at 10° would be expressed by 10° 4- 70°, that is, 80°. If, 
on the contrary, we had turned the micrometer index x 30° in the same direction as 
that of the repulsion, and had by this advanced the index, say to a distance of 40°, 
then the total force at 40° would be expressed by 40° — 30°, that is, 10° only. 
With respect to the distances of the electrified discs p m, they are always measured 
with sufficient accuracy by the arc contained between the vertical index ty and hori- 
zontal index v v\ or by adding or subtracting the angular turning of the circle A 1 1. 
13. There are one or two circumstances connected with the investigation of the 
laws of electrical action by means of this instrument, or the balance of torsion, re- 
quiring some attention. 1°. If any considerable time be employed in a particular 
experiment, the charged bodies will, under ordinary circumstances, lose some portion 
of their electricity. Coulombe endeavoured to avoid the error necessarily arising' 
from this, by computing the amount of the electrical dissipation and applying a cor- 
rection, and by some other methods incidental to the nature of the given inquiry. 
This process, however, is not always easy, or even safe ; the dissipation of a charge 
being frequently a most uncertain operation ; and a little more or a little less al- 
lowance for the assumed decrease of the charge makes an astonishing difference in 
delicate researches, the force by which a body loses its charge being as the square of 
the quantity of electricity. The only sure protection from this source of fallacy is 
to avoid it altogether, which can be always effected if we choose a proper season for 
experiment, and operate in a dry room moderately warmed by a common drying 
stove. It is surprising under these circumstances to see how completely the repel- 
ling bodies retain the charge ; this, together with the facility of manipulating ob- 
tained by the instrument above mentioned, will be found to reduce indefinitely any 
error arising from dissipation. 2°. Coulombe found that some insulators did not 
insulate sufficiently, and were liable to become electrified ; in consequence of this 
