56 APPENDIX. 



and the galvanometer moved. This experiment 

 says his biographer, is " the discovery by which 

 he will be forever known." 



Henry's account of his own discovery exhibits 

 the very same apparatus. He used his electro- 

 magnet, capable of lifting 600 or 700 Ibs., and 

 united its poles by an iron bar or "keeper," firmly 

 fixed, so as to form a complete circuit the same 

 as the iron ring in Faraday's experiment. Around 

 this " keeper" he wound about 30 feet of insulated 

 wire, in many layers, occupying about one inch in 

 the length of the keeper, and placed a galvanom- 

 eter in the circuit of the coil. When the battery 

 circuit was closed and broken on the coil of the 

 magnet, the galvanometer moved, and the great 

 discovery was made (Sillimari's Journal, July, 

 1832). 



Faraday had been working over it for seven 

 years. Henry never touched the question till 1827. 



NOTE D, PAGE 21. 



In another arrangement of this same invention, 

 a heavy wire is laid between the tracks, and large 

 inductive coils, near the floor of the car, are 

 affected by the current in the line wire. Henry 

 exhibited the principle of this apparatus in 

 Princeton, when in the cellar of the Philosophical 

 Hall, induced currents were set up in a wire lead- 

 ing around the apartment, induced by passing a 

 battery current through a similar wire in the 

 upper story, thirty feet above, and with two floors 

 between. 



