Henry then (1842) carried his investigations to considerable dis- 

 tances, producing high frequency oscillating currents in a primary 

 circuit by the discharge of a battery of Leyden jars, and detecting the 

 result in secondary circuits at a distance first of 30 feet and then of 

 several hundred feet. He concluded that every spark of electricity 

 in motion exerts inductive effects at distances indefinitely great and 

 likened the propagation of this effect to the propagation of light which 

 he surmised was merely another kind of wave in the same medium. 

 It is an extraordinary fact that in these experiments Henry had what 

 would now be considered a form of radio transmitter and receiver, and 

 that he believed the results to be produced by an electromagnetic 

 radiation comparable with light. This was 25 years before the work 

 of Maxwell and more than 40 years before the work of Hertz. 



Henry is sometimes credited with having discovered the oscillatory 

 nature of the discharge of the Leyden jar (1842). Felix Savary, 

 however, in 1827 had advanced the hypothesis that this discharge is 

 oscillatory in order to explain his experimental results. Henry re- 

 peated Savary's investigations and brought about the acceptance of 

 Savary's hypothesis as a result of his authoritative affirmation that 

 this hypothesis was correct. 



The above record of the achievements of Joseph Henry, partial 

 though it be, is the more impressive in consideration of the difficulties 

 under which Joseph Henry labored. He worked in a frontier country 

 far from the current of European scientific thought and with only 

 occasional information regarding the results achieved by others. 

 Furthermore, in his early years his work was limited largely to summer 

 vacations, his time being fully occupied by teaching during the school 

 year. His financial^limitations were serious. His results were achieved 

 by great industry in the application of his genius for practical experi- 

 mentation, his inventive insight, his power for exact observation and 

 his sense of perspective and proportion. 



APPENDIX 



References Bearing on Henry's Electrical Discoveries 

 1 . Construction of Electromagnets 



a. Use of Insulated Wire 



Insulated wire was used before Henry's work for various purposes 

 including electrostatic telegraphs and galvanometers. For example, 

 the wire of the coil in Schweigger's galvanometer as made by Oersted 

 was: 



