the variations made in the interior; and these were the changes I was in search of, the negation or establishment of such differences being the great object of my inquiry.  I considered that these differences, if they existed, would be most distinctly set forth by having two apparatus of the kind described, precisely similar in every respect; and then, different insulating media being within, to charge one and measure it, and after dividing the charge with the other, to observe what the ultimate conditions of both were.  If insulating media really had any specific differences in favouring or opposing inductive action through them, such differences, I conceived, could not fail of being developed by such a process.

1196.  I will wind up this description of the apparatus, and explain the precautions necessary to their use, by describing the form and order of the experiments made to prove their equality when both contained common air.  In order to facilitate reference I will distinguish the two by the terms App. i. and App. ii.

1197.  The electrometer is first to be adjusted and examined (1184.), and the app. i. and ii. are to be perfectly discharged.  A Leyden phial is to be charged to such a degree that it would give a spark of about one-sixteenth or one-twentieth 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.