of an inch in length between two balls of half an inch
diameter; and the carrier ball of the electrometer being charged by this
phial, is to be introduced into the electrometer, and the lever ball
brought by the motion of the torsion index against it; the charge is thus
divided between the balls, and repulsion ensues. It is useful then to bring
the repelled ball to the standard distance of 30 deg. by the motion of the
torsion index, and observe the force in degrees required for this purpose;
this force will in future experiments be called _repulsion of the balls_.

1198. One of the inductive apparatus, as, for instance, app. i., is now to
be charged from the Leyden phial, the latter being in the state it was in
when used to charge the balls; the carrier ball is to be brought into
contact with the top of its upper ball (_k_, fig. 104.), then introduced
into the electrometer, and the repulsive force (at the distance of 30 deg.)
measured. Again, the carrier should be applied to the app. i. and the
measurement repeated; the apparatus i. and ii. are then to be joined, so as
to _divide_ the charge, and afterwards the force of each measured by the
carrier ball, applied as before, and the results carefully noted. After
this both i. and ii. are to be discharged; then app. ii. charged, measured,
divided with app. i., and the force of each again measured and noted. If in
each case the half charges of app. i. and ii. are equal, and are together
equal to the whole charge before division, then it may be considered as
proved that the two apparatus are precisely equal in power, and fit to be
used in cases of comparison between different insulating media or
_dielectrics_.

1199. But the _precautions_ necessary to obtain accurate results are
numerous. The apparatus i. and ii. must always be placed on a thoroughly
uninsulating medium. A mahogany table, for instance, is far from
satisfactory in this respect, and therefore a sheet of tinfoil, connected
with an extensive discharging train (292.), is what I have used. They must
be so placed also as not to be too near each other, and yet equally exposed
to the inductive influence of surrounding objects; and these objects,
again, should not be disturbed in their position during an experiment, or
else variations of induction upon the external ball B of the apparatus may
occur, and so errors be introduced into the results. The carrier ball, when
receiving its portion of electricity from the appara