547 



result, determined by but a very slight indication, was, as stated 

 above, tbat longitudinal magnetization augmented the resistance. 



The magnetization of the small piece of metal between the poles 

 of the Ruhmkorff electro-magnet being obviously much more in- 

 tense than that of the larger piece under the influence merely of the 

 smaller helix, I recurred to the plan of experiment ( 175) by 

 which the effect of transverse magnetization on the little rectangular 

 piece of nickel was first tested, and I had an equal and similar piece 

 of iron, and another of brass, all prepared to be tested, as well as 

 the nickel, with either longitudinal or transverse magnetic force. 



To each of the little rectangles of metal to be tested, a thin slip 

 of copper (instead of lead, as in the experiment of 1 75), of the 

 same breadth ('52 of an inch), to serve as a reference conductor, 

 was soldered longitudinally, and to the other end of the metal tested, 

 a piece of copper to serve as an electrode, for the principal current, 

 was soldered. The ends of a testing conductor, 6 feet of No. 18 

 copper wire, were soldered respectively to the last-mentioned end of 

 the tested metal, and to a point in the reference-conductor found, so 

 that the resistance between it and the junction of the reference-con- 

 ductor with the tested conductor, should be about equal to the re- 

 sistance in the latter. 



A single element, consisting of four large double cells of Daniell's 

 ( 63), exposing in all 10 square feet of zinc surface to 17 square 

 feet of copper, was used to send the testing current through the con- 

 ducting system thus composed, by electrodes clamped to the ends of 

 the principal conducting channel, just outside the points of attach- 

 ment of the testing conductor. 



The electro-magnet was excited by various battery arrangements, 

 in different experiments, at best by 52 cells of Daniell's, each ex- 

 posing 54 square inches of zinc surface to 90 square inches of copper, 

 and arranged in a double battery* equivalent to one battery of 26 

 elements each of double surface. By accident, only a single battery of 

 26 elements was used in obtaining the numerical results stated below. 



* This arrangement was found to give about the same strength of current 

 through the coils of the electro-magnet, as a single battery of 52 of the same cells 

 in series, and was therefore preferred as involving only half the amount of chemi- 

 cal action in each cell, and consequently maintaining its effect more constantly 

 during many successive hours of use. 



