The effect of lightning on an electroscope had been observed prior 

 to Henry's work. In extending this work, Henry connected a wire to 

 the metallic roof of his study, carried this wire through a magnetizing 

 spiral and to ground, by means of which arrangement needles were 

 magnetized in his study "even when the flash is at a distance of seven 

 or eight miles, and when the thunder is scarcely audible."-^ William 

 J. Gibson, in 1844, in his notes on Henry's lectures, states that needles 

 were magnetized by lightning 20 miles away. 



c. Oscillatory Nature of Discharge of Leyden Jar 



Anomalies in the polarity of small steel needles magnetized by the 

 discharge from the Leyden jar had been observed prior to Henry's 

 work notably by Felix Savary, who in 1827 advanced in the following 

 words the hypothesis that these results were due to the oscillatory 

 discharge of the Leyden jar: 



"An electric discharge is a phenomerron of movement. Is this movement a 

 continuous translation of matter in a determined direction? Then the opposite 

 polarity of magnetism observed at different distances from a straight conductor, or m 

 a helix' with graduallv increasing discharges, would be due entirely to the mutual 

 reactions of the magnetic particles in the steel needles. The manner in which the 

 action of a wire changes with its length appears to me to exclude this supposition. 



" Is the electric movement during the discharge, on the other hand, a series of os- 

 cillations transmitted from the wire to the surrounding medium and soon attenuated 

 by resistances which increase rapidly with the absolute velocity of the moving 

 particles? , . • 



"All the phenomena lead to this hypothesis which makes not only the intensity 

 but the polarity of the magnetism depend on the laws in accordance with which the 

 small movements die out in the wire in the surrounding medium and in the substance 

 which receives and conserv-es the magnetism." *^ 



Henry repeated and extended Savary's investigations and confirmed 

 Savary's hypothesis by a clear and authoritative statement of the 

 phenomenon as follows: 



"The discharge, whatever may be its nature, is not correctly represented (employ- 

 ing for simplicitv the theory of Franklin) by the single transfer of an imponderable 

 fluid from one side of the jar to the other; the phenomena require us to admit the 

 existence of a principal discharge in one direction, and then several reflex actions back- 

 ward and forward, each more feeble than the preceding, until the equilibrium is obtained. 

 All the facts are shown to be in accordance with this hypothesis, and a ready explana- 

 tion is afforded by it of a number of phenomena which are to be found in the older 

 works on electricity, but which have until this time remained unexplained." *" 



References 



For convenience there is given below a list of the principal publica- 

 tions referred to in connection with the study as original sources. 

 Many of the references cover the general field of electrical discovery 

 contemporary with Henry; others relate to specific items of the work. 



« SW, Vol. 1, p. 203— Paper presented in 1842 and published the following year. 



** Annales de Chimie et de Physique, 1827. 

 «S\V, Vol. 1, p. 201, 1842. 



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