404 Note 19: determination of law of electric attraction 



I have supposed the planes Aa, &c. to be extended infinitely, because by 

 that means I was enabled to solve the question accurately in the cases where 

 the repulsion is supposed inversely as the square of the distance, which I could 

 not have done otherwise, but it is evident that the phenomena will be nearly 

 of the same kind if the planes are not infinitely extended. 



For if the distance ag be small in respect of the length and breadth of the 

 plane A a, a particle placed at a will be repelled by the plane Aa with very 

 nearly the same force as if the plane was infinitely extended. 



It is plain that these 6 last cases agree very exactly with the laws of elec- 

 tricity laid down in the 3rd and 4th hypotheses [Thoughts... Art. 202]. 



If the lines Bb and Dd touch one another so that 



[Here the MS. ends. ED.] 



NOTE 19, ART. 234. 



[Determination of the Law of Electric Attraction.} 

 Cavendish's Experiment on the Charge of a Globe between two Hemispheres. 



This experiment has recently been repeated * at the Cavendish Laboratory 

 in a somewhat different manner. 



The hemispheres were fixed on an insulating stand, so as to form a spherical 

 shell concentric with the globe, which stood inside the shell upon a short piece 

 of a wide ebonite tube. 



By this arrangement, since during the whole experiment the potentials of 

 the globe and sphere remained sensibly equal, the insulating support of the 

 globe was never exposed to the action of any sensible electromotive force, and 

 therefore had no tendency to become charged. 



If the other end of the insulator supporting the globe had been connected 

 to earth, then, when the potential of the globe was high, electricity would have 

 crept from it along the insulator, and would have crept back again when, in 

 the second part of the experiment, the potential of the globe was sensibly zero. 

 In fact this was the chief source of disturbance in Cavendish's experiment. 

 See Art. 512. 



Instead of removing the hemispheres before testing the potential of the 

 globe, they were left in their position, but discharged to earth. The effect on 

 the electrometer of a given charge of the globe was less than if the hemispheres 

 had been removed, but this disadvantage was more than compensated by the 

 perfect security from all external electric disturbances afforded by the con- 

 ducting shell. 



The short wire which formed the communication between the shell and the 

 globe was fastened to a small metal disk hinged to the shell, and acting as a 

 lid to a small hole in it, so that when the lid and its wire were lifted up by 



* [By Sir Donald M<=Alister in 1878.] 



