from charge on a globe enclosed within hemispheres 405 



means of a silk string, the electrode of the electrometer could be made to dip 

 into the hole in the shell and rest on the globe within. 



The electrometer was Thomson's Quadrant Electrometer. 



The case of the electrometer, and one of the electrodes, were permanently 

 connected to earth, and the testing electrode was also kept connected to earth, 

 except when used to test the potential of the globe. 



To estimate the original charge of the shell, a small brass ball was placed 

 on an insulating stand at a distance of about 60 cm. from the centre of the shell. 



The operations were conducted as follows : 



The lid was closed, so that the shell communicated with the globe by the 

 short wire. 



A Leyden jar was charged from a machine in another room, the shell was 

 charged from the jar, and the jar was taken out of the room again. 



The small brass ball was then connected to earth for an instant, so as to 

 give it a negative charge by induction, and was then left insulated. 



The lid was then lifted up by means of the silk string, so as to take away 

 the communication between the shell and the globe. 



The shell was then discharged and kept connected to earth. 



The testing electrode of the electrometer was then disconnected from earth, 

 and made to pass through the hole in the shell so as to touch the globe within 

 without touching the shell. 



Not the slightest deflexion of the electrometer could be observed. 



To test the sensitiveness of the apparatus, the shell was disconnected from 

 earth and connected to the electrometer. The small brass ball was then dis- 

 charged to earth. 



This produced a large positive deflexion of the electrometer. 



Now in the first part of the experiment, when the brass ball was connected 

 to earth, it became charged negatively, the charge being about ^ of the original 

 positive charge of the shell. 



When the shell was afterwards connected to earth the small ball induced 

 on it a positive charge equal to about one-ninth of its own negative charge. 

 When at the end of the experiment the small ball was discharged to earth, this 

 charge remained on the shell, being about t | ff of its original charge. 



Let us suppose that this produces a deflexion D of the electrometer, and 

 let d be the largest deflexion which could escape observation in the first part 

 of the experiment. 



Then we know that the potential of the globe at the end of the first part 

 of the experiment cannot differ from zero by more than 



* v 



fc 486 D 

 where V is the potential of the shell when first charged. 



