L. Page — A Century's Progress in Physics. 309 



these experiments been published promptly, America 

 would have been entitled to credit for the most important 

 discovery of the greatest of England's many great exper- 

 imenters. 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 this 

 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, 

 instead of the short wire, though no spark will be per- 

 ceptible when the connection is made, yet when it is 

 broken by drawing one end of the wire from its cup of 

 mercury a vivid spark is produced. . . . The effect 

 appears somewhat increased by coiling the wire into a 

 helix ; it seems to depend in some measure on the length 

 and thickness of the wire; I can account for these phe- 

 nomena only by supposing the long wire to become 

 charged with electricity which by its reaction on itself 

 projects a spark when the connection is broken." 



Soon after, Henry went to Princeton and there con- 

 tinued his experiments in electromagnetism. No diffi- 

 culty was experienced in inducing currents of the third, 

 fourth and fifth orders by using the first secondary as 

 primarv for vet another secondary circuit, and so on 

 (38, 209, 1840). The directions of these currents of 

 higher orders when the primary is made or broken 

 proved puzzling at first, but were satisfactorily explained 

 a year later (41, 117, 1841). In addition induced cur- 

 rents were obtained from a Leyden jar discharge. Fara- 

 day failed to find any screening effect of a conducting 

 cylinder placed around the primary and inside the 

 secondary. Henry examined the matter, and found that 

 the screening effect exists only when the induced current 

 is due to a make or break of the primary circuit, and not 

 when it is caused by motion of the primary. 



