206 FRICTION AL ELECTRICITY. 



brief, the molecules of conductors discharge their electricities easily 

 into each other ; those of non-conductors do not. 



341. Polarization Precedes Attraction. 



When an electrified glass rod is brought near an insulated 

 uncharged pith ball (electric pendu- 

 lum), the pith ball is polarized as 

 shown in the figure. As the of 

 the ball is nearer the + of the glass 



than is the -f- of the ball, the attrac- 



FIG. 143* 

 tion is greater than the repulsion. If 



the pith ball be suspended, not by a silk thread but by 

 some good conductor, the attraction will be more marked, 

 for the -f- of the ball will escape to the earth, through the 

 support, and the repelling component thus removed. 



Note. Polarization and electrification by induction explain a host 

 of phenomena. Let the pupil apply this principle of influence or 

 induction to pointing out the changes in the positions and conditions 

 of the two fluids that are involved in the phenomena mentioned in 

 323. 



342. The Electrophorus. This simple instru- 

 ment consists generally of a shallow tinned pan filled with 



resin, on which rests a movable 

 metallic cover with a glass or 

 other insulating handle. The 

 resinous plate may be replaced 

 by a piece of vulcanized india- 

 rubber. The metal surface 

 and the resinous surface touch 

 at only a few points; they are 

 practically separated by a thin 

 layer of air. (Appendix, K.) 



(a.) The plate is rubbed or struck 

 FIG. 144. wit k flannel or catskin, and thus 



