Coulomb's torsion balance has been adapted by M. Baille to the measurement
of low electromotive forces in a very successful manner, and has been found
preferable by him to the delicate electrometers of Sir W. Thomson. It
is necessary to guard it from disturbances due to extraneous electric
influences and the trembling of the ground. These can be eliminated
completely by encircling the instrument in a metal case connected to
earth, and mounting it on solid pillars in a still place. Heat also has a
disturbing effect, and makes itself felt in the torsion of the fiber and
the cage surrounding the lever. These effects are warded off by inclosing
the instrument in a non-conducting jacket of wood shavings.
The apparatus of M. Baille consists of an annealed silver torsion wire of
2.70 meters long, and a lever 0.50 meter long, carrying at each extremity
a ball of copper, gilded, and three centimeters in diameter. Similar balls
are fixed at the corners of a square 20.5 meters in the side, and connected
in diagonal pairs by fine wire. The lever placed at equal distances from
the fixed balls communicates, by the medium of the torsion wire, with the
positive pole of a battery, P, the other pole being to earth.
Owing to some unaccountable variations in the change of the lever or
needle, M. Baille was obliged to measure the change at each observation.
This was done by joining the + pole of the battery to the needle, and one
pair of the fixed balls, and observing the deflection; then the deflection
produced by the other balls was observed. This operation was repeated
several times.
The battery, X, to be measured consisted of ten similar elements, and one
pole of it was connected to the fixed balls, while the other pole was
connected to the earth. The needle, of course, remained in contact with the
+ pole of the charging battery, P.
The deflections were read from a clear glass scale, placed at a distance
of 3.30 meters from the needle, and the results worked out from Coulomb's
static formula,