MAGNETISM AND ELECTRICITY
397
Quantitative measurement is more difficult, but Prof.
Mayer has lately used a pendulum electroscope in a form that
makes a most sensitive electro¬
meter. He suspends a gilt
pith-ball 1 cm. radius (built up
of small pieces cemented) a
distance of 364 cm. from the
ceiling, by two silk fibres
fastened 52 cm. apart and
meeting at the ball, behind
which a scale is arranged. A
brass ball of the same size is
mounted on a glass rod, var¬
nished while warm with
paraffin wax. A force of only
one dyne acting on the sus¬
pended ball deflects it 13*3
mm. ; and charging both balls
in contact so as to give the
same charge to each, the
charge c on either was found
in absolute electrostatic units
by the formula
C = D
where d is the distance in
cm. between the centres of
the balls, and cl the deflection
from the vertical in cm.
When d is over 5j cm. the
law of inverse squares was
demonstrated within 1 per
cent ; and by using a proof-
plane, Coulomb’s law of distribution on the pay
drical body was verified with close approxima/ioh> Shell
LJ # /