14 



Professor Tyndall 



[Jan. 26, 



to all the forces capable of acting upon them magnetically ; first, to 

 the magnet alone ; secondly, to the electric current alone ; and, 

 thirdly, to the magnet and current combined. Attention to struc- 

 ture was here found very necessary, and the neglect of it appears to 

 have introduced much error into this portion of science. Powdered 

 bismuth, without the admixture of any foreign ingredient, was 

 placed in a strong metallic mould, and submitted to the action of a 

 hydraulic press ; perfectly compact metallic masses were thus pro- 

 cured, which, suspended in the magnetic field with the line of com- 

 pression horizontal, behaved exactly like magnetic bodies, setting 

 their longest dimensions from pole to pole. This identity of 

 deportment with an ordinary magnetic substance was also exhibited 

 in the case of the electric current, and of the current and the 

 magnet combined. In like manner, by the compression of a mag- 

 netic powder magnetic bars were produced, which, between the two 

 poles of a magnet, set exactly like ordinary dia-magnetic ones ; this 

 identity of deportment is preserved when the bars are submitted to 

 the action of the current, and of the current and magnet combined. 

 Calling those bars which show the ordinary magnetic and dia- 

 magnetic action normal bars, and calling the compressed bars 

 abnormal ones, the law follows, that an abnormal bar of one class 

 of bodies exhibits precisely the same deportment, in all cases, as the 

 normal bar of the other class ; but when we compare normal bars 

 of both classes together, or abnormal bars of both classes, then 

 the antithesis of action is perfect. The experiments prove that, 

 if that which Gauss calls the ideal distribution of magnetism in 

 magnetic bars be inverted, we have a distribution which will pro- 

 duce all the phenomena of dia-magnetic ones. 



The important question of dia-magnetic polarity was submitted to 

 further and stricter examination. A flat helix, whose length was 

 an inch, internal diameter an inch, and external diameter seven 

 inches, was attached firmly to a table, with its coils vertical. A 

 suspension was arranged by means of which a bar of bismuth, five 

 inches long, and 0*4 of an inch in diameter, was permitted to 

 swing freely, while surrounded by the helix. With this arrange- 

 ment, the following experiments were, or might be made: — 1. A 

 voltaic current from twenty of Grove's 

 cells was sent through the helix h, the 

 direction of the current in the upper half 

 of the helix being that denoted by the arrow 

 (Fig. 1). The north pole of a magnet being 

 placed at N, the end a of the suspended 

 bar of bismuth, a b, was attracted towards 

 the pole N. 2. The south pole of a second 

 magnet being placed at S, and the cur- 

 rent being sent through the helix in the h 

 same direction as before, the bar left its 

 central position and approached N with greater force than in the 



Fig. 1. 



a 



