1884.] on Volta-Eleciric and Magneto-Electric Induction. 



123 



duration that when the speed of the reverser is at all rapid, the cur- 

 rent not having time to exhaust itself before the galvanometer is 

 reversed, tends to produce a lower deflection. If the speed of the 



100 



500 1000 1500 



REVERSALS PER MINrTE. 



2000 



reverser is further increased, the induced current is received on the 

 opposite terminal of the galvanometer, and thus a negative result is 

 obtained. 



My next object was to verify, if possible, by a different system of 

 experiment, the correctness of this theory, and I could think of no 

 better arrangements than those used by Faraday in some of his 

 experiments on Magneto-Electric Induction. I was not, however, 

 encouraged to proceed in that direction ; for if my theory were 

 correct, the results published by Faraday could not be so ; and know- 

 ing what a careful experimentalist he was, I could not doubt that 

 he was right. After long and careful thought on the subject, I 

 ventured, however, to repeat some of his experiments, and I will 

 again repeat them before you presently. 



About sixty years ago Arago made the discovery in Electrical 

 Science, that if a plate of copper be revolved close to a magnetic needle, 

 or magnet suspended in such a way that the latter may rotate in a plane 

 parallel to the former, the magnet tends to follow the motion of the 

 plate ; or if the magnet be revolved the plate tends to follow its motion. 

 This simple apparatus will better illustrate the experiment. Here 

 is a copper plate one-tenth of an inch thick, and seven and a half 

 inches in diameter, fixed to a vertical spindle and enclosed in a 

 wooden case having a glass cover; beneath the copper plate is a 

 small grooved pulley around which passes an endless band ; the band 

 also passing round this horizontal wheel to which a handle is fixed, 

 so that it may be conveniently revolved. A small brass disc is here 

 provided, in the centre of which is fixed a pointed steel pin, and on 



