40 PROFESSOR HENRY AND THE 



and let that fact answer the envious suggestion 

 that Henry's achievement involved no great 

 amount of analytic and inventive power. 



When Barlow's demonstration was published in 

 1824, Henry had never seen an electro-magnet, nor 

 tried an experiment in electricity. When, how- 

 ever, two years later he took up the subject, and 

 began the first set of regular scientific investiga- 

 tions ever attempted in the United States, he de- 

 duced from Ampere's law the principle that the 

 voltaic currents, carried on wires around the iron 

 core of the electro-magnet, should move in planes 

 at right angles to the axis of that core which 

 they could not do even approximately if the core 

 itself were insulated, as in Sturgeon's small 

 magnet, having only one coil of naked wire 

 wound spirally around it, necessarily leaving open 

 spaces between the successive spirals, and so lead- 

 ing the current like a cork screw around the core. 

 He also reasoned that, as the current must be led 

 through a spiral circuit, which theoretically should 

 be circular, the departure from its true course 

 might be counteracted by winding the wire on a 

 second spiral outside of the first, but with its 

 spiral angle opposed, so that the resultant of the 

 current from the two spirals would be the same as 

 if it revolved in planes at right angles to the axis 

 of the core. 



He brought his reasoning to the test of experi- 



