122 The Smithsonian Institution 



in. 



HENRY'S experimental work was done, for the most part, 

 between 1826 and 1847. Many of his broader generalizations 

 were published later, though these were largely based upon 

 the work of early years. 



His studies in electricity began in 1827, while he was a 

 teacher in the Albany Academy, and it was not long before 

 Sir David Brewster was moved to say: "On the shoulders 

 of young Henry has fallen the mantle of Franklin ! " His 

 laboratory work in Albany included the only continuous series 

 of physical investigations which any one had up to that time 

 attempted in America. 



In the course of these researches he transformed an ineffi- 

 cient piece of electrical apparatus the significance of which 

 had been but partially understood by Ampere, Arago, and even 

 Sturgeon, by whom it had been greatly improved into the 

 powerful electro-magnet, and laid the foundation for the most 

 important discoveries of the century, not only his own, but 

 those of the great masters of Europe. The electro-magnet 

 in 1828 was still an ineffective instrument. Barlow had tested 

 its capabilities in London three years before, and had found 

 its effect so diminished at the distance of only two hundred 

 feet that he pronounced telegraphy by its use impossible. 



In Henry's hands the feeble toy of Sturgeon was converted 

 into instruments of infinite possibilities. He made two dis- 

 tinct forms of magnets, one capable of excitation at a distance, 

 which he named the " intensity magnet " ; another having 

 possibilities of infinite development of strength, to which he 

 gave the name of " quantity magnet." 



He so named the magnets because he had discovered that 

 with the one, in order to overcome the resistance opposed to 

 the passage of electricity by the long, fine wire of which it 



