Change of Resistance of Nickel and Iron W 

 (1) Apparatus. 



123 



A portion o£ the electromagnet by means of which the 

 fields were produced is shown in section in fig. 1. The 



Fig. 1. 



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currents used in the magnet ranged from "1 ampere to about 

 22 amperes. The current was allowed to flow through the 

 magnet for as short a time as possible, only during the few- 

 seconds required for obtaining the balancing-point on the 

 resistance bridge. To prevent rise of temperature of the 

 pole-pieees, water was arranged to flow along the hollow 

 cores to within a few millimetres of the surfaces of the pole- 

 pieces ; the water was previously passed through a spiral 

 tube which was slightly heated to bring the temperature of 

 the water up to that of the room. 



The strengths of the fields were measured directly after 

 each set of readings for the change of resistance. This was 

 done by the ballistic method, — the exploring coil being 

 connected to a ballistic galvanometer provided with a tele- 

 scope and scale. An arrangement was set up by which the 

 exploring coil could be placed in the same position each 

 time between the poles, and by which it could be suddenly 

 removed from the field. The galvanometer was standardized 

 before and after each set of readings by means of a standard 

 solenoid and a secondary coil. 



No attempt was made to measure the magnetization or the 

 field inside the specimen. No form of " Isthmus Method " 

 would in fact be suitable for such a thin specimen. 



The change of resistance was measured by means of a slide 

 wire bridge, the metal parts of which were all mounted on 

 ebonite. In order to obtain a large step on the bridge, a 

 thick ger man-silver wire was used as the bridge-wire, and 

 also two auxiliary coils of gennan-silver wire were placed in 



