642 HENRY A. KOWLAND 



it; indeed I am sorry for it; it is putting new arms into the hands of the 

 incendiary." This occurred a short time after the papers had been 

 filled with the doings of the hayrick burners. 



Now, after more than fifty years, the spark of Faraday blazes at every 

 street corner, but it has never been found more efficient than an ordinary 

 lucifer match in the burning of hayricks. 



Faraday's attention was now called to the explanation of a curious 

 action discovered by Arago, who found that a rotating disk of copper 

 carried a magnetic needle with it when the latter was suspended over 

 it. The explanation had never been obtained, but Faraday now saw 

 that it was but an instance of his newly discovered action. In order 

 to show that currents were induced in the revolving plate, he mounted 

 it between the poles of a magnet and connected the centre with one 

 pole of a galvanometer; on pressing a wire from the other pole to the 

 edge, Faraday obtained a continuous current of electricity. This was 

 the first continuous current dynamo ever constructed. 



But he rested not until he had obtained the laws of induced currents 

 and expressed them in such simple language that they have ever since 

 been the admiration of the scientific world. 



In giving the law of the production of these induced currents, Fara- 

 day for the first time made use of his famous " lines of force," although 

 he here calls them magnetic curves. 



He showed that a wire must cut these lines in order to have a current 

 induced in it. In order to account for the induction in neighboring 

 wires on making and breaking an electric current, he pictured in his 

 mind the lines of force moving. The current could only start gradually 

 after contact was made, and while it was increasing the lines of force 

 always closed on themselves in rings, were expanding outwards cutting 

 any wires near it, and inducing currents in them. When the current 

 was broken, the lines contracted and produced contrary induced cur- 

 rents. 



In after years he made his law quantitative, and proved that the 

 integral induced current was in proportion to the number of lines of 

 force cut by the wire. 



In his papers of 1831-2 I find these lines always called magnetic 

 curves, and his laws of induced currents are given in terms of these 

 curves. This idea of lines of force was ever after one of the principal 

 points around which the mind of Faraday revolved. He applied it to 

 electrical action as well as to magnetic, and we see him in after years 

 striving to do away with action at a distance, and substitute for it a 

 medium filled with these lines of force. 



