FRICTIONAL ELECTRICITY. 



207 



negatively electrified. The cover is then placed upon the resin 

 and thus polarized by induction. If the cover be provided with 

 a gold-leaf electroscope, the free negative electricity of the cover 

 will cause the leaves to diverge ; the positive electricity of the 

 cover will be "bound" on the under side of the cover by the 

 attraction of the negative of the plate. Remove the plate, and 

 the separated fluids reunite as is shown by the falling together 

 of the lately divergent gold leaves. Place the cover again upon 

 the plate. Polarization is manifested by the divergence of the 

 leaves. Touch the cover with the finger as shown in the figure; 

 the free electricity escapes and the leaves fall. The cover is now 

 charged positively, but its electricity is all '' bound " at the under 

 surface of the plate, and cannot cause the leaves to separate. Re- 

 move the plate by its insulating handle, and the electricity, lately 

 " bound " but now " free," diffuses itself, and the leaves are divergent 

 with + electricity. The charged cover will give a spark to the 

 knuckle or other unelectrified body presented to it. (Fig. 145.) 



343. The Electrophorus Charged by Induc- 

 tion. The cover may be thus charged and discharged 

 an indefinite number of times, in favorable weather, 



without a second elec- 

 trifying of the resinous 

 plate. This could not 

 happen if the electricity 

 of the cover were drawn 

 from the plate. More- 

 over, if the charge of 

 the cover were drawn 

 from the plate, it would 

 be , and not -f. 

 There is no escape from 

 the conclusion that the 

 cover is charged by in- 

 duction, and not by con- 

 duction. (See Append- 

 ix K.) 



FIG 145. 



