-1859] 



WRITINGS OP JOSEPH HENRY. 



323 



touch the outside of the globe, it 

 will carry off with it a charge 

 which will cause the leaves of the 

 electroscope C to diverge in pro- 

 portion to the original quantity- 

 imparted to the sphere. A simi- 

 lar effect will be exhibited if the 

 ball B be lowered into an insulated 

 cylinder of wire gauze A, Fig. 4, 

 which has been charged with elec- 

 tricity. Not the least sign of ex- 

 citement will be found on the in- 

 side, while a spark may perhaps 

 be drawn from the exterior. The 

 same result is produced, (as will 

 be seen,) whether the globe be 

 charged negatively or positively. 

 On the hypothesis that the attrac- 

 tion and repulsion both observe 

 the law of diminution with the 

 square of the distance, this curious 

 phenomenon is readily explained. 



Newton has demonstrated the following propositions 

 relative to the action of gravitation; and these principles 

 are equally applicable to electrical attraction and repulsion, 

 or to any other action which varies inversely as the square 

 of the distance: 



1. A particle of matter placed outside of a hollow sphere 

 of attracting or repelling matter of uniform thickness, is 

 acted upon as if all the matter were concentrated at the 

 centre of the sphere. 



2. A particle of matter (or of free electricity) placed at any 

 point within a hollow sphere of uniform attracting or repel- 

 ling matter, will be acted upon in every direction by an 

 equal force, and will consequently be in equilibrium. 



The form of the demonstration of the first of these propo- 

 sitions may be easily understood by a reference to Fig- 

 ure 5, and the accompanying considerations. 



Fig. 4. 



