CONTRIBUTIONS TO SCIENCE 525 



number of lines of magnetic force may pass through the circuit 

 in the positive direction. 



Since the magnetic field may be produced either by 

 magnets or by electric currents themselves, as above de- 

 scribed, this rule combined with the principle that action 

 and reaction are equal and opposite will serve to determine 

 the character of the action either upon circuits conveying 

 currents or upon magnets in every possible case which may 

 arise, and, in fact, embodies the magnificent results of 

 Ampere's investigations in this subject. 



Previously to the experiments of Faraday the induction 

 of electric currents was unknown. The principal phenome- 

 non depending upon this action, which had been observed, 

 and of which no satisfactory explanation had been offered, 

 was that of Arago's rotating disc. In this experiment a 

 disc of copper was made to rotate rapidly in its own hori- 

 zontal plane above a compass needle, when the needle was 

 observed to follow the disc and rotate on its vertical pin. 

 This experiment was subsequently repeated by Sir John 

 Herschel and Mr. Babbage, who employed discs of various 

 substances, and found that it was only when the discs were 

 good conductors of electricity that Arago's result was 

 obtained. Faraday, in the first series of his Experimental 

 Eesearches, describes an experiment in which a copper disc 

 was made to rotate between the poles of an electro-magnet, 

 while one electrode of a galvanometer was connected with 

 the axis of the disc, and the other with a wire which was 

 held in contact with the edge of the disc, which edge was 

 amalgamated to secure a good connection. On spinning the 

 disc a current was immediately obtained, the direction of 

 which was reversed with that of the rotation. This experi- 

 ment may be regarded as the starting-point of the dynamo 

 machines of Wilde, Gramme, Siemens, and others, which 

 seem destined to play so important a part in the civilised 

 life of the future. 



Faraday also showed that when two circuits are placed 

 near to one another, if a current be started in one circuit 

 there is an instantaneous current produced in the opposite 



