128 LEADING AMERICAN MEN OF SCIENCE 



is it possible to determine invisible motions back and forth in a 

 millionth of a second. 



Henry's method was very simple. He passed the electric dis- 

 charge through a wire round a needle. This object being of 

 highly tempered steel retained the magnetism communicated to it 

 by the current. Henry found that, when the needle was examined 

 after the current had flashed around it, its north and south poles 

 were not always at the ends which should have been produced 

 by the discharge, but were often in the opposite direction, the north 

 pole being the one that should have been south. He immediately 

 saw what was the cause. The electricity must have flashed first in 

 one direction and then in the opposite one. In perhaps the mil- 

 lionth of a second it not only destroyed the magnetism which had 

 first been produced by the current but induced a magnetism of 

 the opposite kind. 



Henry's active and fertile mind was by no means confined to 

 electricity. Everything he could find in the heavens or on the 

 earth to investigate, he was ready to actively take hold of. He 

 delighted in experimenting on the properties of matter, and left 

 behind voluminous notes of his results in this field. 



p 



About 1832 Professor Henry was called to the chair of Natural 

 Philosophy in Princeton College. Although the duties of an Amer- 

 ican college professor seldom allow much time for original investi- 

 gation, he soon resumed his electrical researches, and the first of a 

 regular series was communicated to the American Philosophical 

 Society in 1835. On February 6 of that year he continued the 

 subject of the self-induction of the electric current with especial 

 reference to the influence of a spiral conductor upon it. The 

 series of experiments on this subject are very elaborate, but can- 

 not be fully described without going into details too minute for 

 the present sketch. 



Among the little known works of Professor Henry during this 

 period are his researches upon solar radiation and the heat of the 

 solar spots. In connection with his relative, Professor Stephen 

 Alexander, he may be said to have commenced a branch of modern 

 solar physics which has since grown to large proportions, by com- 



