100 PEOCEEDINGS OF THE REGENTS, 



1 



The second fact of importance, discovered in 1820, by Arago and 

 ^'^•^* Davy, is illustrated in 



figure 2. It consists 

 in this, that while a 

 current of galvanism is 

 passing through a cop- 

 per wire A B, it is 

 magnetic, it attracts iron filings and not those of copper or brass, and 

 is capable of developing magnetism in soft iron. (See Annales de 

 Chimie, vol. 15, page 94.) 



The next important discovery, also made in IKIO, by Ampere, was 



that two wires through which galvanic currents are passing in the 



same direction attract, and in opposite direction repel, each other. 



On this fact Ampere founded his celebrated theory, that magnetism 



onsists merely in the attraction of electrical currents revolving at 



right angles to the line joining the two poles of the magnet. The 



magnetisation of a bar of steel or iron, according to this theory, con- 



..■sists in establishing within the metal by induction a series of electrical 



.. ©urrents, all revolving in the same direction at right angles to the 



axis or length of the bar. (See Annales de Chimie, vol. 15, page 69.) 



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 



Fig. 3. figure 3. This method 



tricity 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 Annales de Chimie et Physique, vol. XV, 1820, 

 page 93, it will be seen that, properly speaking, he made no electro- 

 magnet, as has been asserted by Morse and others ; his experiments 

 wer*e 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 

 thickness of a small knitting needle. (See Annales de Chimie, vol. 

 15, page 95.) 



