LESSOVS IN ELECTRICITY. 



once excited, is retained, and the attrac- 

 tion of the lath is the consequence. 



In like manner, a brass tube, held in 

 the hand and struck with a fox's brush, 

 shows no attractive power ; but when a 

 stick of sealing-wax, ebonite, or gutta- 

 percha is thrust into the tube as a han- 

 dle, the striking of the tube at once de- 

 velops the power of attraction. 



And now you see more clearly than 

 you did at first the meaning of the ex 

 periment with the heated foolscap anj 

 india-rubber. Paper and wood always 

 imbibe a certain amount of moisture 

 from the air. When the rubber was 

 passed over the cold paper electricity was 

 excited, but the paper, being rendered a 

 conductor by its moisture, allowed the 

 electricity to pass away. 



Prove all things. Lay your coll fools- 

 cap on a cold board supported by dry- 

 tumblers ; pass your india-rubber over 

 the paper ; lift it by a, loop of silk which 

 has been previously attached to it, for if 

 you touch it it will discharge itself. 

 You will find it electric ; and with it you 

 can charge your electroscope, or attract 

 from a distance your balanced lath. 



The human body was ranked among 

 the non-electrics. Make plain to your- 

 self the reason. Stand upon the iloor 

 and permit a friend to strike you briskly 

 with the fox's brush. Present your 

 knuckle to the balanced lath, you will 

 find no attraction. Here, however, yon. 

 stand upon the earth, so that even if 

 electricity had been developed, there is 

 nothing to hinder it from passing away. 



But, place upon the ground four warm 

 glass tumblers, and upon the tumblers a 

 board.* Stand upon the board and pre- 

 sent your knuckle to the lath. A single 

 stroke of the fox's fur, if skilfully given, 

 will produce attraction. If you stand 

 upon a cake of resin, of ebonite, or upon 

 a sheet of good india-rubber, the effect 

 will be the same. You can also charge 

 your electroscope with this electricity. 



Throw a mackintosh over your shoul- 

 ders and let a friend strike it with the 

 fox's brush, the attractive force is greatly 

 augmented. 



After brisk striking, present your 



* Some caution is necessary hero. A large 

 class of cheap glass tumblers conduct s.i 

 freely that they are unfit for this and similar 

 expeiiineiits. See 19. 



knuckle to the knuckle of your friend. 

 A spark will pass between you. 



This experiment with the mackintosh 

 further illustrates what you have already 

 frequently observed namely, that it i> 

 not friction alono, but the friction of 

 special substances against each other, that 

 produces electricity. 



Thus we prove that non-electrics, like 

 electrics, can be excited, the condition of 

 success being, that an insulator shall be 

 interposed between the non-electric and 

 the earth. It is obvious that the old di- 

 vision into electrics and non-electrics, 

 really meant a division into insulators 

 and conductors. 



9. Electric Repulsions. Discovery of 

 two Electricities. 



Wo have hitherto dealt almost exclu- 

 sively with electric attractions, but in an 

 experiment already referred to (2), 

 Otto von Guericke observed the repulsion 

 of a feather by his sulphur globe. I also 

 anticipated mutters in the use of our 

 Dutch metal electroscope ( 7), where 

 the repulsion of the leaves informed us 

 of the arrival of the electricity. 



Du Fay, who was the real discoverer 

 here, found a gold-leaf floating in the air 

 to be first attracted and then repelled by 

 the same excited body. He afterward 

 proved that when -the floating leaf was 

 repelled by rubbed glass, it was attracted 

 by rubbed resin and that when it was 

 repelled by rubbed resin it was attracted 

 by rubbed glass. Hence the important 

 announcement, by Du Fay, that there are 

 two kinds of electricity. 



The electricity excited on glass was for 

 a time called vitreous electricity, while 

 that excited on sealing-wax was called res- 

 inous electricity. These terms are how- 

 ever improper ; because, by changing the 

 rubber, we can obtain the electricity of 

 sealing-wax upon glass, and the electric- 

 ity of glass upon sealing-wax. 



Roughen, for example, the surface of 

 your glass tube with a grindstone, and 

 rub it with flannel, the electricity of seal- 

 ing-wax will be found upon the vitreous 

 surface. Rub your sealing-wax with vul- 

 canized india-rubber, the electricity of 

 glass will be found upon the resinous sur- 

 face. You will be able to prove this im- 

 mediately. 



