482 MEMORIAL OF JOSEPH HENRY. 



moments of reverie, he arose and exclaimed, "To-morrow I shall 

 make a famous experiment." For several months he had been 

 brooding over Ampere's electro-dynamic theory of magnetism, and 

 he was then deeply interested in the phenomena of the develop- 

 ment of magnetism in soft iron as shown in the experiments of 

 Arago and Sturgeon. At the moment he had arisen from his chair 

 it had occurred to him that the requirements of the theory of 

 Ampfere were not fulfilled in the electro-magnets of Arago and 

 of Sturgeon, but that he could get those conditions which the 

 theory required by covering the enveloping wire with a non-con- 

 ductor like silk, and then wrapping it closely around the soft iron 

 bar in several layers ; for the successive layers of wire coiling first 

 in one direction and then in the other would tend to produce a 

 resultant action of the current at right angles to the axis of the bar; 

 and furthermore, the great number of convolutions thus obtained 

 would act on a greater number of molecules of the bar and thereby 

 exalt its magnetism. "When this conception," said Henry, "came 

 into my brain I was so pleased with it that I could not help rising 

 to my feet and giving it my hearty approbation," 



Henry did go to work the next day, and to his great delight and 

 encouragement discoveries of the highest interest and importance 

 revealed themselves to him week after week. When he had finished 

 his newly-conceived magnet he found that it supported several times 

 more weight than did Sturgeon's magnet of equal size and weight. 

 This was his first original discovery. 



I will now give, as far as possible, Henry's own words in narrating 

 his subsequent investigations of these very interesting phenomena : 



"The maximum effect however with this arrangement and a 

 single battery was not yet obtained. After a certain length of wire 

 had been coiled upon the iron, the power diminished with a further 

 increase of the number of turns. This was due to the increased 

 resistance which the longer wire offered to the conduction of elec- 

 tricity. Two methods of improvement therefore suggested them- 

 selves. The first consisted, not in increasing the length of the coil, 

 but in using a number of separate coils on the same piece of iron. 

 By this arrangement the resistance to the conduction of the electricity 

 was diminished and a greater quantity made to circulate around the 



