LESSORS IN ELECTRICITY 



33 



mental law. Repeat, for example, Otlo von Guericke's experiment. 

 Hang- a leather by a silk thread, and bring your rubbed glass tube 

 near it : the feather is attracted, touches the rod, charges itself with 

 the electricity of the rod, and is then repelled. Cause it to retreat 

 from the rod in various directions. 



Hang your feather by a common thread : if no insulating substance 

 intervenes between the feather and the earth, you can get no repul- 

 sion. Why ? you ought to be able to answer. Obviously it is be- 

 cause the charge of positive electricity communicated by the rod is 

 not retained by the feather, but passes away to the earth. Hence, 

 you have not positive acting against positive at all. Why you should 

 have the attraction of the neutral body by the electrified one will, as 

 already stated, appear by-and-by. 



Attract your straw needle by your rubbed glass rod. Let the 

 straw strike the rod, so that the one shall rub against the other. The 

 straw accepts the electricity of the rod, and repulsion immediately 

 follows attraction, as shown in Fig. 7. 



Fig. 7. 



Mr. Cottrell has devised the simple electroscope represented in 

 Fig. 8 to show repulsion. A is a stem of sealing-wax, with a small 

 circle of tin, T, at the top. Wis a bent wire proceeding from T, with 

 a small disk attached to it by wax. 1 1' is a little straw index, sup- 

 ported by the needle, N, as shown in the figure. The stem, A, is not 

 quite vertical, the object being to cause the bit of paper, I, to rest 

 close to W when the apparatus is not electrified. When electricity 

 is imparted to T, it flows through the wires, TFand w, over both disk 

 and index : immediate repulsion of the straw is the consequence. 



No better experiment can be made to illustrate the self-repulsive 

 character of electricity than the following one : Heat your square 

 board again, and warm, as before, your sheet of foolscap. Spread 

 the paper upon the board, and excite it by the friction of India-rub- 

 ber. Cut from the sheet two long strips with your penknife. Hold 

 the strips together at one end. Separate them from the board, and 



VOL. IX. 3 



