orsion balance electrometer of Coulomb, constructed, generally,
according to his directions[A], but with certain variations and additions,
which I will briefly describe. The lower part was a glass cylinder eight
inches in height and eight inches in diameter; the tube for the torsion
thread was seventeen inches in length. The torsion thread itself was not of
metal, but glass, according to the excellent suggestion of the late Dr.
Ritchie[B]. It was twenty inches in length, and of such tenuity that when
the shell-lac lever and attached ball, &c. were connected with it, they
made about ten vibrations in a minute. It would bear torsion through four
revolutions or 1440 deg., and yet, when released, return accurately to its
position; probably it would have borne considerably more than this without
injury. The repelled ball was of pith, gilt, and was 0.3 of an inch in
diameter. The horizontal stem or lever supporting it was of shell-lac,
according to Coulomb's direction, the arm carrying the ball being 2.4
inches long, and the other only 1.2 inches: to this was attached the vane,
also described by Coulomb, which I found to answer admirably its purpose of
quickly destroying vibrations. That the inductive action within the
electrometer might be uniform in all positions of the repelled ball and in
all states of the apparatus, two bands of tin foil, about an inch wide
each, were attached to the inner surface of the glass cylinder, going
entirely round it, at the distance of 0.4 of an inch from each other, and
at such a height that the intermediate clear surface was in the same
horizontal plane with the lever and ball. These bands were connected with
each other and with the earth, and, being perfect conductors, always
exerted a uniform influence on the electrified balls within, which the
glass surface, from its irregularity of condition at different times, I
found, did not. For the purpose of keeping the air within the electrometer
in a constant state as to dryness, a glass dish, of such size as to enter
easily within the cylinder, had a layer of fused potash placed within it,
and this being covered with a disc of fine wire-gauze to render its
inductive action uniform at all parts, was placed within the instrument at
the bottom and left there.

[A] Memoires de l'Academie, 1785, p. 570.

[B] Philosophical Transactions, 1830.

1181. The moveable ball used to take and measure the portion of electricity
under examination, and which