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190 



Mr. G. J. Stoney on the Experiment 



everywhere perpendicular. The charge on the ball of a Leyden 

 jar is evidently most intense on the top of it, and gradually 

 feebler down its sides. Hence the force exerted on a molecule 

 of free electricity close to the knob is greater if the molecule of 

 electricity be above the ball than if it be at the same distance 

 beside it. And accordingly, if the equipotential surfaces corre- 

 sponding to the potentials V, V — «, V — 2a, &c. be drawn, V 

 being the potential of the inside coating of the Leyden jar, and 

 u being a small quantity, the first of these surfaces will be the 

 surface of the ball and stem, and the others of the form repre- 

 sented by the unbroken lines in the diagram, since it is a pro- 

 perty of these surfaces that 

 the interval between them 

 at any point of space is in- 

 versely as the intensity of 

 the force at that point/' so 

 that they lie closest to one 

 another above the ball, 

 where, as we have seen, elec- 

 trical attraction and repul- 

 sion are most intense, and 

 further asunder at the sides, 

 where the electrical force is 

 feebler. Having now as- 

 certained the general form 

 of the equipotential surfaces, we have only to draw perpendiculars 

 to them everywhere in order to have the directions of the lines 

 of force. It thus appears that these latter are curved upwards, 

 as represented by the dotted lines in the figure, and that the re- 

 pulsive force acting on the further end of the gold-leaf is accord- 

 ingly more steeply inclined to the horizontal than the attractive 

 force acting on its nearer end. Hence they have a resultant 

 acting upwards, which may balance the weight of the gold-leaf. 

 If this resultant be too strong, it will draw the gold-leaf up 

 into a higher position, in which the lines are less curved. If, on 

 the other hand, the weight preponderate, it brings the gold-leaf 

 into a lower position, in which the electrical forces acting on it 

 become more inclined to one another. Now the effect of the in- 

 creased inclination of these forces in the lower position, which 

 tends to increase their upward component, may or may not pre- 

 ponderate over the reverse tendency of their diminished strength. 

 This will depend on the size of the knob, and on its elevation 

 above the outer coating of the Leyden jar; for the smaller the 

 knob, the more intense will the accumulation of electricity on its 

 top be as compared with that on its sides, and, accordingly, the 

 more will its lines of force be bent upwards. And, again, since 





