$2 MEMORIAL OF .JOSEPH HENRY. 



years later entered upon that scries of investigations which, con- 

 tinued !'<>r many years, gave to science, as embodied in his well- 

 known "Researches in electricity," those varied and brilliant dis- 

 coveries which have placed him in the first rank of the philosophers 

 of modern times. 



About the same period our countryman, Dr. Robert Hare, gave 

 a new interest to the study of electric currents in another aspect, 

 that 'if their heating energy, by his invention of the calorimotor 

 ami deflagrator, the earlv products of his untiring ingenuity, which 

 in the laboratories of former years so dazzled us by their exhibi- 

 tion of transformed electric power. 



Allusion has already been made to the observation of Arago 

 in 1820, that an iron wire, surrounded by a helix conducting a 

 voltaic current, became a temporary magnet. In the same year 

 Schweigger, of Halle, conceived- the idea of greatly augmenting 

 the deviating effect of an electric current on a magnetic needle 

 by causing it to traverse successive parallel closely adjacent coils of 

 the conducting wire, in which the needle was suspended, and in 

 this way constructed the well-known galvanometer; an instrument 

 which, as improved by NoBILI, became indispensable in the meas- 

 urement of current electricity, and which through the recent refined 

 improvements given to it by Sir William THOMSON, the first of 

 living electricians, has been made one of the most perfect and deli- 

 cate of all known means of measuring force. 



At length, in 1825, an English electrician, Sturgeon, w ' k) had 

 done much in the contrivance of electro-dynamic apparatus, 

 improved upon Arago's experiment by using an iron wire bent 

 in horse-shoe form covered with non-conducting varnish, around 

 which was wound in an open helix the conducting wire. As long 

 as the voltaic current was allowed to pass through the conductor 

 the inclosed iron wire was made magnetic with poles like those of 

 a horse-shoe magnet. When the current ceased, the magnetic force 



