106 Messrs. Rowland and Bell on the 



rate of variation of the square of the magnetic force in that 

 direction. This rate of variation is greatest near the edges 

 and points of a magnetic pole, and more work will be required 

 to tear away a particle of iron or steel from such an edge or 

 point than from a hollow. This follows whether the tearing 

 away is done mechanically or chemically. Hence the points 

 and edges of a magnetic pole, either of a permanent or induced 

 magnet, are protected from chemical action. 



One of Prof. Remsen's experiments illustrates this most 

 beautifully. He places pieces of iron wire in a strong mag- 

 netic field, with their axes along the lines of force. On 

 attacking them with dilute nitric acid they are eaten away 

 until they assume an hourglass form, and are furthermore 

 pitted on the ends in a remarkable manner. On Prof. Remsen's 

 signifying that he had abandoned the field for the present, we 

 set to work to illustrate the matter in another manner, by means 

 of the electric currents produced from the change in the elec- 

 trochemical nature of the points and hollows of the iron. 



The first experiments were conducted as follows : — Two bits 

 of iron or steel wire about 1 millim. in diameter and 10 millim. 

 long were imbedded side by side in insulating material, and 

 each was attached to an insulated wire. One of them was 

 filed to a sharp point, which was exposed by cutting away a 

 little of the insulation, while the other was laid bare on a 

 portion of the side. The connecting wires were laid to a re- 

 flecting-galvanometer, and the whole arrangement was placed 

 in a small beaker held closely between the poles of a large 

 electromagnet, the iron wires being in the direction of the 

 lines of force. When there was acid or any other substance 

 acting upon iron in the beaker, there was always a deflection 

 of the galvanometer due to the slightly different action on the 

 two poles. When the magnet was excited the phenomena 

 were various. When dilute nitric acid was placed in the beaker 

 and the magnet excited, there was always a strong throw of the 

 needle at the moment of making circuit, in the same direction 

 as if the sharp-pointed pole had been replaced by copper and 

 the other by zinc. This throw did not usually result in a 

 permanent deflection, but the needle slowly returned towards 

 its starting point, and nearly always passed it and produced a 

 reversed deflection. This latter effect was disregarded for 

 the time being, and attention was directed to the laws that 

 governed the apparent " protective throw," since the reversal 

 was so long delayed as to be quite evidently due to after 

 effects and not to the immediate action of the magnet. 



With nitric acid this throw was always present in greater or 

 less degree, and sometimes remained for some minutes as a 



