40 H. A. Rowland and L. Bell — Explanation of the 



One of Prof. Eenisems experiments illustrates this most 

 beautifully. He places jfieces 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 hour-glass form, and are furthermore pit- 

 ted on the ends in a remarkable manner. On Prof. Eemsen'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 l""" in diameter and 10 mm 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 led to a reflecting galvan- 

 ometer, 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 -galvanome- 

 ter 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 sharp pointed pole had been replaced by copper and the other 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 toward 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 ap- 

 parent " protective throw," since the reversal was so long de- 

 layed 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 minute- as a 

 temporary deflection, the time varying from this down to a 

 few seconds. The throw was independent of direction of 

 current through the magnet, and apparently varied in amount 

 with the strength of acid and with the amount of deflection 

 due to the original difference between the poles. This latter 

 fact simply means that the effect produced by the magnet is 

 more noticeable as the action on the iron becomes freer. 



When a pair of little plates exposed in the middle were sub- 

 stituted for the wires, or when the exposed point of the latter was 

 filed to a flat surface, the protective throw disappeared, though 



