1857] WRITINGS OF JOSEPH HENRY. 429 



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 inter- 

 rupted the magnetism disappeared, and thus was produced 

 the first temporary soft iron magnet. 



The electro-magnet of Sturgeon 

 is shown in figure 4, which is a 

 copy from the drawing in the Trans- 

 actions of the Society for the Encourage- 

 ment of Arts, &c., 1825, vol. xliii, pp. 

 38-52. By comparing figures 3 and 

 4, it will be seen that the helix em- 

 ploj'^ed by Sturgeon was of the same Fig. 4. 



kind as that used by Arago; instead of a straight steel wire 

 inclosed in a tube of glass however, Sturgeon employed a 

 bent wire of soft iron. The difference in the arrangement at 

 first sight might appear to be small, but the difierence in the 

 results produced was important, since the temporary magnet- 

 ism 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 read- 

 ing 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 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 be- 

 tween each stratum by a covering of silk 

 ribbon. By this arrangement the rod was sur- 

 FiG. 5. rounded by a compound helix formed of a 

 long wire of many coils, instead of a single helix of a few 

 coils. (Fig. 5.) 



