his dot made with the lines of the
two graduations just referred to, to ascertain accurately the position of
the ball. The upper end of the glass thread was attached, as in Coulomb's
original electrometer, to an index, which had its appropriate graduated
circle, upon which the degree of torsion was ultimately to be read off.

1184. After the levelling of the instrument and adjustment of the glass
thread, the blocks which determine the place of the _carrier ball_ are to
be regulated (1181.) so that, when the carrier arrangement is placed
against them, the centre of the ball may be in the radius of the instrument
corresponding to 0 deg. on the lower graduation or that on the side of the
electrometer, and at the same level and distance from the centre as the
_repelled ball_ on the suspended torsion lever. Then the torsion index is
to be turned until the ball connected with it (the repelled ball) is
accurately at 30 deg., and finally the graduated arc belonging to the torsion
index is to be adjusted so as to bring 0 deg. upon it to the index. This state
of the instrument was adopted as that which gave the most direct expression
of the experimental results, and in the form having fewest variable errors;
the angular distance of 30 deg. being always retained as the standard distance
to which the balls were in every case to be brought, and the whole of the
torsion being read off at once on the graduated circle above. Under these
circumstances the distance of the balls from each other was not merely the
same in degree, but their position in the instrument, and in relation to
every part of it, was actually the same every time that a measurement was
made; so that all irregularities arising from slight difference of form and
action in the instrument and the bodies around were avoided. The only
difference which could occur in the position of anything within, consisted
in the deflexion of the torsion thread from a vertical position, more or
less, according to the force of repulsion of the balls; but this was so
slight as to cause no interfering difference in the symmetry of form within
the instrument, and gave no error in the amount of torsion force indicated
on the graduation above.

1185. Although the constant angular distance of 30 deg. between the centres of
the balls was adopted, and found abundantly sensible, for all ordinary
purposes, yet the facility of rendering the instrument far more sensible by
diminishing this distance wa