12 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XI.) 



prove their equality when both contained common air. In order to facilitate refer- 

 ence I will distinguish the two by the terms App. i. and App. ii. 



1 197. The electrometer is first to be adjusted and examined (1184.), and the app. i. 

 and ii. are to be perfectly discharged. A Ley den 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° by the motion of the torsion index, and 

 observe the force in degrees required for this purpose ; this force will in future ex- 

 periments be called repulsion of the halls. 



1 198. 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 (A:, fig. 1.), then introduced into the electrometer, and the repulsive force (at 

 the distance of 30°) 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, ap- 

 plied 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 di- 

 electrics. 



1 1 99. 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 tin foil, connected with an extensive discharging train (292.), is what 1 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 elec- 

 tricity from the apparatus, should always be applied at the same part of the ball, 

 as, for instance, the summit /r, and always in the same way ; variable induction from 

 the vicinity of the head, hands, &c. being avoided, and the ball after contact being 

 withdrawn upwards in a regular and constant manner. 



1200. As the stem had occasionally to be changed (1190.), and the change might 

 occasion slight variations in the position of the ball within, I made such a variation 



