Induction Apparatus. — Precautions in its use. 295 



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 di- 

 stance of 30° 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 rcpulsioji of the halls. 



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. 1.), then introduced into the electrome- 

 ter, 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, 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 pre- 

 cisely 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 re- 

 sults 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 dis- 

 charging 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 re- 

 ceiving its portion of electricity from the apparatus, should 

 always be applied at the same part of the ball, as, for instance, 

 the summit ^, 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 purposely, to the 



