A CCUMULA TORS. 91 



surface being a non-conductor, the electricity has to force 

 its way against strong resistance, bursting through at the 

 points where resistance is least, and forming the fringe. 

 The strongest effect is produced where the knob first 

 approaches the surface ; as the jar has then a full charge : 

 and the first action is a disruptive discharge through 

 the air, producing the circular, star-like figure, at that 

 point. But as the knob moves along the surface, after 

 contact, new lines start out at right angles to the line 

 of movement. And as the knob leaves a point where 

 such a line has started, it exerts an inductive action on 

 the original impulse, which tends to turn this line for- 

 ward; the diagonal direction being the resultant of 

 these two forces acting at right angles to each other. 

 And the forked branches are the result of similar 

 inductive action of the main fringe lines on the branch 

 lines. 



We have, in this experiment, a graphic demonstration 

 of the effect of an insulating surface in resisting electric 

 movement : since the figures show the exact location of 

 the electric force ; which, we see, is confined chiefly to 

 the tracings, spreading only to the limited extent 

 represented by the fringes. 



It also shows that the effects produced in different 

 substances, by opposite electric influences, are depend- 

 ent on the electric condition of the substances them- 

 selves: so that a mixture or a compound may, in this 

 way, be separated into its elements. The sulphur in 

 this experiment becoming negative, as claimed, by 

 friction, is attracted to the positively charged lines, 

 while the red lead, becoming positive, is attracted to 

 those negatively charged. This principle has numer- 

 ous useful applications in the arts. 



