21 



1820, page 93, it will be seen tliat, properly speaking, lie made no elec- 

 tro-magnet, as has been asserted by jNIorse and others ; his experiments 

 were confined to the magnetism of iron filings, to sewing needles and 

 pieces of steel wire of the diameter of a millimetre, or of about the thick- 

 ness of a small knitting needle. (See Annales de Chimie, vol. 15, page 95.) 



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

 periments of Arago, and produced what is properly known as the electro- 

 magnet. He bent a piece of iron ivire into the form of a horseshoe, cov- 

 ered 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 Fig. 4. 



in figure 4, which is an exact copy from the „^4Mt^ 



drawing in the Transactions of the Society ff^^M. 



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



xliii. By comparing figures 3 and 4 it y^ ^ ,r;^x y 



will be seen that the helix employed by Stur- ^ fflTf ll 11 flrjl 

 geon was of the same kind as that used by I'i 111 



Arago ; instead, however, of a straight steel |Mi|ai|||]iij|i{aB 



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 pro- 

 duced was important, since the temporary magnetism developed in the 

 arrangement of Sturgeon was sufficient to support a Aveight 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 to the other. The same principle fig- s. 



was extended by employing a still longer insulated 

 wire, and winding several strata of this over the 

 first, care being taken to insure the insulation 

 between each stratum by a covering of silk ribbon. 

 By this arrangement the rod was surrounded by a 

 compound hehx formed of a long wire of many coils, 

 instead of a single helix of a few coils, (figure 5.) 



