may be called the _repelling_, or the
_carrier_, ball, was of soft alder wood, well and smoothly gilt. It was
attached to a fine shell-lac stem, and introduced through a hole into the
electrometer according to Coulomb's method: the stem was fixed at its upper
end in a block or vice, supported on three short feet; and on the surface
of the glass cover above was a plate of lead with stops on it, so that when
the carrier ball was adjusted in its right position, with the vice above
bearing at the same time against these stops, it was perfectly easy to
bring away the carrier-ball and restore it to its place again very
accurately, without any loss of time.

1182. It is quite necessary to attend to certain precautions respecting
these balls. If of pith alone they are bad; for when very dry, that
substance is so imperfect a conductor that it neither receives nor gives a
charge freely, and so, after contact with a charged conductor, it is liable
to be in an uncertain condition. Again, it is difficult to turn pith so
smooth as to leave the ball, even when gilt, so free from irregularities of
form, as to retain its charge undiminished for a considerable length of
time. When, therefore, the balls are finally prepared and gilt they should
be examined; and being electrified, unless they can hold their charge with
very little diminution for a considerable time, and yet be discharged
instantly and perfectly by the touch of an uninsulated conductor, they
should be dismissed.

1183. It is, perhaps, unnecessary to refer to the graduation of the
instrument, further than to explain how the observations were made. On a
circle or ring of paper on the outside of the glass cylinder, fixed so as
to cover the internal lower ring of tinfoil, were marked four points
corresponding to angles of 90 deg.; four other points exactly corresponding to
these points being marked on the upper ring of tinfoil within. By these and
the adjusting screws on which the whole instrument stands, the glass
torsion thread could be brought accurately into the centre of the
instrument and of the graduations on it. From one of the four points on the
exterior of the cylinder a graduation of 90 deg. was set off, and a
corresponding graduation was placed upon the upper tinfoil on the opposite
side of the cylinder within; and a dot being marked on that point of the
surface of the repelled ball nearest to the side of the electrometer, it
was easy, by observing the line which t