Faraday's Discoveries 



before the Royal Society on the 24th of 

 November, 1831. 



On January 12, 1832, he communicated to the 

 Royal Society a second paper on "Terrestrial 

 Magneto-electric Induction," which was chosen 

 as the Bakerian Lecture for the year. He placed 

 a bar of iron in a coil of wire, and lifting the bar 

 into the direction of the dipping needle, he ex- 

 cited by this action a current in the coil. On 

 reversing the bar, a current in the opposite direc- 

 tion rushed through the wire. The same effect 

 was produced, when, on holding the helix in the 

 line of dip, a bar of iron was thrust into it. Here, 

 however, the earth acted on the coil through 

 the intermediation of the bar of iron. He 

 abandoned the bar and simply set a copper-plate 

 spinning in a horizontal plane ; he knew that the 

 earth's lines of magnetic force then crossed the 

 plate at an angle of about 70. When the plate 

 spun round, the lines of force were intersected 

 and induced currents generated, which produced ( 

 their proper effect when carried from the plate to 

 the galvanometer. "When the plate was in the 

 magnetic meridian, or in any other plane coincid- 

 ing with the magnetic dip, then its rotation pro- 

 duced no effect upon the galvanometer. ' ' 



At the suggestion of a mind fruitful in sugges- 

 tions of a profound and philosophic character 

 1 mean that of Sir John Herschel Mr. Barlow, 

 of Woolwich, had experimented with a rotating 

 iron shell. Mr. Christie had also performed an 

 elaborate series of experiments on a rotating 

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