Professor Joseph Henry's Invention 



thus was produced the first temporary soft iron 

 magnet. 



The electro-magnet of Sturgeon is shown in 

 Fig. 4. By comparing Figs. 3 and 4 it will be 

 seen that the helix employed by Sturgeon was 

 of the same kind as that used by Arago ; instead 

 however, of a straight steel wire inclosed in a tube 

 of glass, the former employed a bent wire of soft 

 iron. The difference in 

 the arrangement at first 

 sight might appear to 

 be small, but the differ- 

 ence in the results pro- 

 duced was important, 

 since the temporary mag- 

 netism developed in the 

 arrangement of Sturgeon 

 was sufficient to support 

 a weight of several 



pounds, and an instrument was thus produced 

 of value in future research. 



The next improvement was made by myself. 

 After reading an account of the galvanometer of 

 Schweigger, the idea occurred to me that a 

 much nearer approximation to the requirements 

 of the theory of Ampere could be attained by 

 insulating the conducting wire itself, instead of 

 the rod to be magnetized, and by covering the 

 whole surface of the iron with a series of coils 

 in close contact. This was effected by insulating 

 a long wire with silk thread, and winding this 

 around the rod of iron in close coils from one end 

 27 



