A CENTURY'S PEOGEESS IN PHYSICS 341 



plete account of all experimental methods available for 

 inducing a current in a closed circuit. 



While Faraday is entitled to credit for the discovery of 

 current induction by virtue of the priority of his publica- 

 tion, it must not pass unnoticed that Henry obtained 

 many of the same experimental results independently 

 and some even earlier. Henry was at this time instruc- 

 tor in mathematics at the Albany Academy, and seven 

 hours of teaching a day made it well-nigh impossible to 

 carry on original research except during the vacation 

 month of August. As early as the summer of 1830 he 

 had wound 30 feet of copper wire around the armature 

 of a horseshoe electromagnet and connected it to a gal- 

 vanometer. When the magnet was excited, a momen- 

 tary deflection was observed. "I was, however, much 

 surprised," he says, "to see the needle suddenly 

 deflected from a state of rest to about 20 to the east, or 

 in a contrary direction, when the battery was withdrawn 

 from the acid, and again deflected to the west when 

 it was re-immersed." In addition a deflection was 

 obtained by detaching the armature from the magnet, 

 or by bringing it again into contact. Had the results of 

 these experiments been published promptly, America 

 would have been entitled to credit for the most import- 

 ant discovery of the greatest of England's many great 

 experimenters. But Henry desired first to repeat his 

 experiments on a larger scale, and while new magnets 

 were being constructed, the news of Faraday's discovery 

 arrived. This occasioned hasty publication of the work 

 already done in an appendix to volume 22, 1832, of the 

 Journal. 



At almost the same time Henry made another import- 

 ant discovery and this time he was anticipated by no 

 other investigator in making public his results. In the 

 paper already referred to he describes the phenomenon 

 known to-day as self-induction. "When a small battery 

 is moderately excited by diluted acid and its poles, which 

 must be terminated by cups of mercury, are connected by 

 a copper wire not more than a foot in length, no spark 

 is perceived when the connection is either formed or 

 broken ; but if a wire thirty or forty feet long be used, 



