74 ELEMENTS OF LABORATORY WORK 



an electrified body, e.g. a piece of ebonite or sealing-wax which 

 has been rubbed with flannel, is brought near the top of this 

 instrument, the gold leaves move simultaneously. If the disc 

 is now touched with the hand, the leaves return to their 

 original position ; but if the hand first, and then the electri- 

 fied body, be removed, the leaves again simultaneously diverge 

 during the removal of the electrified body, and remain apart for 

 some time. 



It may be noted in the first observation, that the nearer 

 the electrified body approaches the instrument the more the 

 leaves diverge ; and in the second observation, that the leaves 

 diverge the more widely the further away the electrified body 

 is removed, after the disc has been touched by the hand. 



This instrument, which exhibits very conveniently the con- 

 dition of the electric field, and may be used for purposes of 

 comparison, is called an electroscope. It is essential that the 

 metal rod and leaves be well insulated. 



We have observed from previous experiments that the 

 effects produced by an electrified body depend upon the rela- 

 tive position, nature, and quantity of matter acted upon. The 

 use of the electroscope, however, discovers another very impor- 

 tant condition essential to the manifestation of electric stress, 

 and this is a change in the configuration of the system con- 

 cerned. However great the stress may be, it would be unob- 

 served so long as the system is unaltered in configuration. It 

 is not manifested except by certain movements taking place 

 in a neighbouring body, or bodies, concurrently with a dis- 

 placement in the relative position of the electrified body. 



We have so far made use of the term, ' electrified body,' but 

 it is now clear that it would be more correct to include in our 

 observations all the bodies concerned in the mutual changes 

 which are said to be due to electricity, and speak of them as 

 c the electrified system.' There is no indication of such a thing 

 as an electrified body existing entirely by itself, or of electric 

 changes in which only one body is concerned. 



51. The Quadrant Electrometer. The electric stress which 

 has been produced by friction, or any other cause, may be 

 further investigated by the use of an electrometer. The thin 



