LESSOXS IX ELECTRICITY. 



169 



turned, as it passes each rubber it is positively electrified. Facing 

 the electrified glass is the row of points midway between the two 

 rubbers. On these points the electrified glass acts by induction, at- 

 tracting the negative and repelling the positive. In accordance with 

 the principles already explained the negative electricity streams from 

 the points against the excited glass, which passes on neutralized to 

 the next rubber, where it is again excited. Thus the prime conductor 

 is charged, not by the direct communication to it of positive elec- 

 tricity, but by depriving it of its negative. 



Fig. 19. 



If, when the prime conductor is charged, you bring the. knuckle 

 near it, the electricity passes from the conductor to the knuckle in 

 the form of a spark. 



Take this spark while the machine is being turned, and then try 

 the effect of presenting the finger-ends, instead of the knuckle, to the 

 conductor. The spark falls exceedingly in brilliancy. Substitute for 

 the finger-ends a needle-point, you fail to get a spark at all. To ob- 

 tain a good spark the electricity upon the prime conductor must reach 

 a sufficient density (or tension, as it is sometimes called). To secure 

 this, no points from which the electricity can stream must exist on the 

 conductor, or be presented to it. All parts of the conductor are 

 therefore carefully rounded off, sharp points and edges being avoided. 



It is usual to attach to the conductor an electroscope, consisting 

 of an upright metal stem, A C, Fig. 20, to which a straw with a pith- 

 ball, B, at its free end, is attached. The straw turns loosely upon a 

 pivot at C. The electricity passing from the conductor is diffused 

 over the whole electroscope, and the straw and stem, being both posi- 

 tively electrified, repel each other. The straw, being the movable 

 body, flies away. The amount of the divergence is measured upon a 

 graduated arc. 



