LESSONS IN ELECTRICITY. 



299 



FIQ. 12. 



FIG. 11. 



RflTKl particles descend as a continuous 

 si i cam, s s', fig. 12, but at every stroke 

 of die rubber they fly asunder, as in fig. 

 13, through self-repulsion. f 



Or let three or four fine fillets of water 

 issue from three or four pin-holos in the 

 bottom of a vessel close to each other. 

 Connect the water of the vessel with 

 your glass tube, and rub as before. The 

 liquid veins are scattered into spray by 

 every stroke of the rubber. 



These experiments are best made with 

 " CottrelPs rubber," described in 24. 



And now you must learu to determine 

 with certainty the quality cf the elec- 

 tricity with which any body presented to 

 you may be charged. You see immedi- 

 ately that attraction is no sure test, be- 

 cause unelectrified bodies are attracted. 

 Further on ( 14) you will be able to 

 grapple with another possible source of 

 error in the employment of attraction. 



In determining quality, you must 

 ascertain, by trial, the kind of electricity 

 by which the charged body is repelled ; 

 if, for example, any electrified body re- 

 pel, or is repelled by, sealing-wax rubbed 

 with flannel, the electricity of the body 



f For these, and also for experiments with 

 thu electroscope, the teacher of a large class 

 will find the lime light shadows upon awhile 

 screen (or better still, those of the electric 

 light) exceedingly useful. The effects arc 

 thus rendered visible to all at once. 



is negative ; if it repel, or is repelled by, 

 glass, rubbed with silk, its electricity is 

 positive. Du Fay had the sagacity to 

 propose this mode of testing quality. 



Apply this test to the strips of fools- 

 cap paper excited by the india-rubber. 

 Bring a rubbed gutta-percha tube near 

 the electrified strips, you have strong at- 

 traction. Bring a rubbed glass tube 

 between the strips, you have strong re- 

 pulsion and augmented divergence. 

 Hence, the electricity, being repelled by 

 the positive glass, is itself positive. 



11. Electricity of the Rubber. Doubh 

 or " Polar" Character of the Electric 

 Force. 



We have examined the action of each 

 kind of electricity upon itself, and upon 

 the other kind ; but hitherto we have 

 kept the rubber out of view. One of 

 the questions which inevitably occur to 

 the inquiring scientific mind would be, 

 How is the rubber affected by the act of 

 friction ? Here, as elsewhere, you must 

 examine the subject for yourself, and 

 base your conclusions on the facts you 

 establish. 



Test your rubber, then, by your bal- 

 anced lath. The lath is attracted by the 

 flannel which has rubbed against gutta- 

 percha ; and it is attracted by the silk, 

 which has rubbed against glass. 



