MECHANICAL ELECTRICITY. 283 



tool from the other portion of the lathe, was 

 found to be all that is necessary in these experi- 

 ments. A spring of brass wire was made to press 

 firmly against the turning mandril, so as to ensure 

 metallic contact ; and its other extremity was in 

 communication with the northern extremity of 

 the galvanometer.* 



In the following experiments, the direction of 

 the current is simply stated, as seen at the galva- 

 nometer ^ which will be found, in all cases, to be 

 the reverse of what takes place between the 

 opposing metals. Thus, in the experiments in 

 which the zinc robs the copper, as seen at the 

 galvanometer, the current is progressing towards 

 the copper. — (See 31, &c.) And yet the actual 

 transfer at the surfaces is from the copper to the 

 zinc. 



Exp. 21. On applying the smooth surface of 

 the end of a piece of thick rod copper to the 

 turned surface of the cylinder, producing friction, 

 a current was observed from the copper towards 



* The Galvanometer, iu these experiments, was not of 

 the highest sensibility. It consisted of forty-six turns of 

 copper wire, the -^ of an inch in diameter. The needle was 

 single, and had therefore a northern tendency to counteract. 



