PROCEEDINGS OF THE EEGENTS. 101 



Mr. Sturgeon, in 1825, made an important step in advance of the 

 experiments of Arago, and produced what is properly known as the 

 electro-magnet. He bent a piece of iron loire into the form of a horse- 

 shoe, covered it with varnish to insulate it, and surrounded it with a 

 helix, of which the spires were at a distance. When a current of 

 galvanism was passed through the helix from a small battery of a 

 single cup the iron wire became magnetic, and continued so during 

 the passage of the current. When the current was interrupted the 

 magnetism disappeared^ and thus was produced the first temporary 

 soft iron magnet. 



The electro-magnet of Sturgeon is shown pig.4, 



in figure 4, which is an exact copy from the 

 drawing in the Transactions of the Society 

 for the Encouragement of Arts, &c., vol. 

 XLIII. By comparing figures 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 b 

 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 

 difference in the results produced was important, since the temporary 

 magnetism 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 sur- 

 face 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 to the other. The 

 same principle was extended by employing a still Fig. 5. 



longer insulated wire, and winding several strata 

 of this over the first, care being taken to insure 

 the insulation between each stratum by a cover-- 

 ing of silk ribbon. By this arrangement the rod 

 was surrounded by a compound helix formed of a 

 long wire of many coils, instead of a single helix 

 of a few coils, (figure 5.) 



