ELECTRICAL MEASUREMENTS. 271 



have been calculated by reference to which the proportion in 

 which the electricity divides can generally be easily ascertained. 

 Such facts, however, are applicable only to the case of the 

 partition of electricity between spherical conductors ; but we 

 may make any conductor virtually spherical by putting it 

 inside a hollow sphere. If I charge this ball and then hold 

 it inside this jar by an insulating fibre, the outside of the 

 jar, although the ball does not touch it at all, and there is 

 perfect insulation between them, assumes a charge which is 

 just equal to the charge of the ball ; the inner surface of the 

 jar assumes a charge which is equal but opposite to the charge 

 of the ball, and the outer surface gets a charge which is equal 

 and similar to that of the ball ; so that even without any actual 

 contact we get a charge outside the jar equal to the charge of 

 any body which is inside. It does not matter whether contact 

 takes place or not. A charged body inside a perfectly closed 

 conductor causes on the outside of that conductor a charge 

 exactly equal to its own. If we had better insulation it 

 would be easy by a simple experiment to prove this, but I am 

 afraid I could not in the present state of the atmosphere, and 

 with the probable dampness of the supports, make the 

 experiment in a satisfactory way, and therefore I will not 

 attempt it. The experiment is this : You charge the ball 

 and put it inside the jar without letting it come in contact. 

 Then you touch the outside of the jar, and so render it 

 neutral. If you now take out the ball without allowing 

 contact to occur, although the whole apparatus is neutral as 

 long as the ball is inside, it is not neutral when the ball is 

 taken out ; but if before removing the ball you let it touch 

 the jar which should be done without interfering with 

 the insulation of the jar and then take it out, you will 

 find the apparatus is still neutral. This shews that the 

 electricity which escapes to the ground, when the jar is 

 touched while the ball hangs inside, is equal in quantity 

 to, and of the same kind as, the charge of the ball ; for, 

 after its escape, the jar is rendered permanently neutral by 

 receiving the electricity of the ball. On the other hand, if 

 the jar remains insulated while the ball is inside it, its outer 

 surface retains the electricity which, when it is uninsulated, 

 passes away to the earth, that is, it retains a charge equal 

 and similar to that of the ball. However, independently of 

 any particular experiment to prove the point, this is a prin- 



