416 BELL SVSTE.U TECHNICAL JOURNAL 



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

 particles? 



"All the phenomena lead to this h^q^othesis which makes not onl)' 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 conserves the magnetism."^ 



Some fifteen years later Joseph Henry in America was experimenting with 

 the Leyden jar and studying the currents induced in adjoining conductors 

 by the discharge through another conductor. To determine the direction 

 of the induced current he observed the polarity of a small steel needle 

 magnetized by the current. In describing his experiments at a session of 

 the American Philosophical Society he made reference to the work of 

 Savary and stated that he had undertaken to repeat this investigator's 

 experiments before attempting any new advances. He observed the same 

 effect, the occasional reversal of the polarity of the needle after a discharge, 

 and arrived at the same explanation: 



"This anomaly which has remained so long unexplained, and which at first 

 sight appears at variance with all our theoretical ideas of the connection of elec- 

 tricity and magnetism, was after considerable study satisfactorily referred by the 

 author to an action of the discharge of the Leyden jar which had never before been 

 recognized. The discharge, whatever may be its nature, is not correctly repre- 

 sented (employing for simplicity 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 backward 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 explanation 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. "'- 



The published account of Henry's observations is not precisely in his 

 own words but apparently in those of the reporter or secretary of the Society 

 before which he spoke. It would seem from the above quotation, if it 

 correctly represented the author, that Henry had overlooked the conclusions 

 drawn by Savary, for they appear to be the same as his own. 



This was in 1842. At a meeting of the Physical Society of Berlin in 1847 

 Helmholtz read his celebrated paper "On the Conservation of Force" 

 {Uber die Erhaltung der Kraft). Among the many illustrations of the 

 conservation of energy principle in various branches of physics he discussed 

 the case of the Leyden jar discharge, and incidentally noted another bit of 

 evidence in favor of its oscillatory nature, an experiment by Wollaston in 

 electrolysis. Commenting on the energy relations found to hold in this 

 case he said : 



"It is easy to explain this law if wc assume that the discharge of a battery is 

 not a simple motion of the electricit\' in one direction, hut a backward and forward 



