20 



winter of 1819-'20, It is illustrated by figure 1, in which the magnetic 

 needle is deflected by , ^'s- 1- 



the action of a current of 

 galvanism transmitted ^ •*" 

 through the wire A B. 

 (See Annals of Philos- 

 ophy, vol. 16, page 273. 



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

 Fig. 2. is illustrated in figure 2. 



It consists in this, that 

 while a current of gal- 

 vanism is passing through 

 a copper 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 1820, by Ampere, was that 

 two wires through which galvanic currents are passing in the same direc- 

 tion attract, and in the opposite direction, repel, each other. On this fact 

 Ampere founded his celebrated theory, that magnetism consists merely in 

 the attraction of electrical currents revolving at right angles to the line 



o o o 



joining the two poles of the magnet. The magnetisation 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. (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 figure 3. 



Fig- 3. This method consists in 



transmitting a current of 

 electricity through a he- 

 lix 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 dis- 

 tance 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 exper- 

 iments of Arago, given in the Annales de Chimie e.t Physique, vol. XV, 



