104 ELEMENTS OF STATIC ELECTRICITY. 



plate will have the same charge as the prime conductor. 

 If the charge be positive, electricity is repelled by in- 

 duction from the lower plate to the earth, through the 

 chain ; if negative, it is attracted through the same 

 medium ; and, in either case, the plates are oppositely 

 charged to the same potential, the air being the dielec- 

 tric. 



When the space is properly adjusted, pith balls or 

 images, placed on the lower plate, are alternately at- 

 tracted and repelled, dancing up and down between 

 the plates in a manner which is often quite amusing. 



If electric connection 

 with the earth be sev- 

 ered by removing the 

 chain, this effect will 

 cease : which proves 

 that the opposite poten- 

 tials of the plates was 

 caused by the transfer 

 Fig. 33-The Eteoric Whirl. of electricity to or from 



the earth, as stated. 



THE ELECTRIC WHIRL. This little instrument, 

 shown in Fig. 33, consists of a set of pointed brass arms 

 attached to a common center, which is pivoted on the 

 point of a vertical rod connected with the prime con- 

 ductor; the arms being bent, so that when passing a 

 given point each shall turn in the same direction. 



When the machine is put in operation, the air in 

 front of each point becomes electrified, either positively 

 or negatively, by the passage of electricity either from 

 or to the point; while that back of it is oppositely 

 electrified by induction. This causes repulsion from 

 the air in front, and attraction toward that at the back, 



