470 Roijal Society : — 



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

 above, that longitudinal magnetization augmented the resistance. 



The magnetization of the small piece of metal between the poles 

 of the Rulimkorff 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 au 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 testing conductor should be about equal to the re- 

 sistance in the latter. 



A single element, consisting of four large double cells of 'Darnell'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. 



The nickel was first placed between the flat poles of the electro- 

 magnet, with its length across the lines of force, and, one galvano- 

 meter electrode being kept soldered to the junction of the nickel and 

 the copper reference-conductor, the other galvanometer electrode was 

 applied to the testing conductor till the point (equipotential with 

 that point of junction) which could be touched without giving any 

 deflection of the needle, was found. A multiplying branch, 3 feet 

 of No. 18 wire, was then soldered with its ends ^ths of an inch on 

 each side of this point, and, as soon as the solderings were cool, the 

 corresponding point on this multiplying branch was found. The 

 magnetizing current was after that sent in either direction through 

 the coils of the electro-magnet, and it was found that the moveable 



* This arrangement was found to give about the same strengtli 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. 



