PHILOSOPHICAL SOCIETY OF WASHINGTON. 269 



A continuation of the memoir was read before the Philoso- 

 phical Society November 20th, 1840, discussing further the 

 theoretical differences between an initial or an increasing gal- 

 vanic current, and a decreasing or a terminal current, in pro- 

 ducing the phenomena of induction. On the same occasion 

 Henry described " an apparatus for producing a reciprocating 

 motion by the repulsion in the consecutive parts of a conductor 

 through which a galvanic current is passing." About ten years 

 before, he had devised the first electro-magnetic engine (ope- 

 rating by intermittent magnetic attractions and repulsions); and 

 now he had contrived the first galvanic engine, operating by the 

 analogous intermittent attractions and repulsions of the electric 

 current.* 



In June 1842, he presented a communication to the Society 

 recounting an investigation of some anomalies in ordinary elec- 

 trical induction. While with the larger needles ("No. 3 and No. 

 4") subjected to the magnetizing helix, the polarity was always 

 conformable to the direction of the discharge, he found that when 

 very fine needles were employed, an increase in the force of the 

 electricity produced changes of polarity. About a thousand 

 needles were magnetized in the testing helices in these researches. 

 This puzzling phenomenon was finally cleared up by the important 

 discovery that an electrical equilibrium was nut instantaneously 

 effected by the spark, but that it was attained only after several 

 oscillations of the flow. "The discharge, whatever may be its 

 nature is not correctly represented by the single transfer 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 backward and forward, each more 

 feeble than the preceding, until the equilibrium is obtained. "f 

 In every case therefore of the electrostatic discharge, the testing 

 needles were really subjected to an oscillating alternation of cur- 

 rents, and consequently to successive partial de-magnetizations 

 and re-magnetizations. The complications produced by this re- 

 sidual action, satisfactorily explained for the first time, the dis- 

 cordant resslts obtained by different investigators. This singular 

 reflux of current was ingeniously applied by Henry to explain the 

 apparent change of inductive current with differing distances. 

 Should the primitive discharge wave be in excess of the magnetic 



* Proceedings Am. Phil. Soc. Nov. 20, 1840, vol. i. p. 301. 



f Proceedings Am. Phil. Soc. .June 17, 1842, vol. ii. pp. 193. — Helniholtz 

 some five years later (iu 1847), but quite independently, suggested "a 

 backward and forward motion between the coatings" when the Leyden 

 jar is discharged. And still five years later (in 185li) Sir William Thom- 

 son made the same independent conjecture. To F. Savary however is 

 due the credit of liaving first thrown out the hypothesisof electrical oscil- 

 lations, as earl^ ao in 1827. 



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