Electrical Resistance of Nickel in Magnetic Fields* 651 



diameter '00206 cm., was passed through fine holes at the 

 ends o£ copper leads attached to an ebonite stem of: square 

 cross-section. The solder was applied to the outer sides 

 of the leads, and finally carefully filed down in order to 

 remove any end portions of the specimen which might not be 

 longitudinal. Great care was exercised during this operation 

 and the whole finally examined under a microscope in order 

 to ensure that the wire was perfectly straight. Flexible wires 

 soldered to the ends of the copper leads were twisted together 

 as far as possible and connected to the terminals shown. In 

 the figure they are separated for the sake of clearness. 

 Terminals at the back of: the instrument served to connect up 

 the specimen to a thick twisted cable suspended by silk cords 

 from the beams of the laboratory and leading to one of the 

 gaps of a slide-wire bridge 30 feet from the electromagnet. 

 The whole instrument was carried by a metal carriage sliding 

 on a rigid framework above the poles of the magnet in such 

 a manner that the specimen could be brought out of the field 

 in a direction perpendicular to the axis of the pole-pieces. 

 The whole arrangement was made as rigid as possible, to 

 ensure the return of the specimen to exactly the same place 

 in the field. By means of screws provided on the instrument 

 the specimen could be rotated in either a horizontal or a 

 vertical plane, the amount of rotation being measured on the 

 scales shown in the figure. An ebonite oil-bath with mica 

 sides was sprung on to a circular collar fixed to the stem, and 

 could be kept in position between the pole-pieces when the 

 specimen was rotated in a horizontal plane. It was thus 

 possible, while investigating the effects for various orienta- 

 tions, to place the pole-pieces much nearer together than 

 could be done with an oil-bath fixed rigidly to the stem. 



The instrument was first adjusted by means of nuts attached 

 to the sliding carriage until the specimen was as nearly as 

 possible at the right height between the pole-pieces, a 

 travelling microscope being employed for the purpose. Any 

 small inclination of the wire to the common axis of the cones 

 was corrected by slightly rotating the lower portion of the 

 instrument. A final adjustment was made by altering the 

 position of the brass stop used in connexion with the sliding 

 carriage, in order that the specimen should be stopped as 

 accurately as possible at the right position in the field, an 

 inclined mirror placed under the pole-pieces being used during 

 this operation. 



The changes of resistance were measured by means of a 

 slide-wire bridge similar to that described by Owen (Phil. 

 Mag. xxi. p. 122, 1911). The sensitiveness of the arrange- 

 ment was, however, largely increased, the maximum step 



