428 WRITINGS OF JOSEPH HENRY. [1857 



his celebrated theory that magnetism consists merely in the 

 attraction of electrical currents revolving at right angles to 

 the line joining the two poles of the magnet. The magnet- 

 ization of a bar of steel or iron, according to this theory, 

 consists in establishing within the metal by induction — a 

 series of electrical currents, all revolving in the same direc- 

 tion at right angles to the axis or length of the bar. 



It was this theory which led Arago, as he states, to adopt 

 the method of magnetizing sewing needles and pieces of 

 steel wire shown in figure 3. This method consists in trans- 



mitting a current of electricity through a helix surrounding 

 the needle or wire to be magnetized. For the purpose of 

 insulation the needle was inclosed in a glass tube, and the 

 several turns of the helix were at a distance from each other 

 to insure the passage of electricity through the whole length 

 of the wire, or in other words, to prevent it from seeking a 

 shorter passage by cutting across from one spire to another. 

 The helix employed by Arago obviously approximates the 

 arrangement required by the theory of Ampere in order to 

 develop by induction the magnetism of the iron. By an 

 attentive perusal of the original account of the experiments 

 of Arago (given in the Annates de Chimie ct Physique, 1820, 

 vol. XV, pages 93-95) it will be seen that properly speaking he 

 made no electro-magnet, as has been often stated. His ex- 

 periments were confined to the magnetizing of iron filings, 

 sewing needles, and pieces of steel wire of the diameter of a 

 millimetre, or of about the thickness of a small knitting 

 needle. 



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 wire 

 into the form of a horseshoe, covered it with varnish to insu- 

 late it, and surrounded it with a helix, of which the spires 

 were at a distance. When a current of galvanism was 



