SOME SPECIAL MUTUAL CHAGENS 77 



this instrument should be compared with those made with the 

 gold-leaf electroscope. It will be found much less sensitive, on 

 account of the greater quantity of matter displaced under the 

 action of the stress. 



52. Exploration of the Electric Field by Two Discs. The 

 neighbourhood of an electrified " body is in such a condition 

 that not only do conductors, whether electrified or not, tend 

 to move, but also an electric change is induced in them, 

 whether previously electrified or not. 



Two similar metal discs with non-conducting handles are 

 brought, with their faces in contact, into the electric field. If 

 they are now separated in such a manner that they are placed 

 at different linear distances from the electrified body, and then 

 removed from the field, they will be found to possess the same 

 properties as two bodies which have been together electrified 

 by friction. This is proved by the behaviour of a small 

 electrified body (a gilt pith ball), or by use of the electroscope 

 or quadrant electrometer. If not separated before removal, 

 no change takes place ; or if brought together after separation 

 and removal from the field, no electrification can be further 

 obtained. 



The same condition of the field may be shown by electrically 

 connecting a conductor on a non-conducting support to a dis- 

 tant electroscope or electrometer by means of a wire. On 

 bringing the electrified body near the conductor, the leaves 

 of the electroscope, or vane of the electrometer, will move and 

 take up a fresh position so long as the electrified body is near. 



If the conductor, while in the field, be touched by the hand 

 and then removed, it may be shown, by applying any of the 

 previous tests, to be electrified. 



All the actions observed above will be found to diminish 

 very rapidly as the distance from the electrified body increases, 

 i.e. the nearer the body originating the changes, the greater 

 will these changes be. 



We may have a series of these phenomena caused by the 

 same body, either simultaneously, in case there are several 

 suitable bodies near, or successively, in case the body induced 

 is removed from the system and then treated as the originating 



