80 Mr. Williams on Magnetic Change of Resistance in Iron, 



Another difficulty arises from the impossibility of keeping 

 the heating-current quite constant. In order to obtain the 

 highest temperatures, it was necessary to pass a current of 

 about 15 amperes through the heating-coil. This was 



Fig. 1. 



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-Wax 



/Rom and Platinum /V//?es 



Po/TCELAIW TO BE 



Apparatus for Measuring Change of Resistance at High Temperatures. 



obtained from a secondary battery of thirty cells, and as it is 

 necessary to keep the current on for a considerable time, the 

 battery is run down and the current steadily decreases. As 

 the heating effect is proportional to the square of the current, 

 a comparatively small variation in the current causes a very 

 considerable change in the temperature, making it impossible 

 to obtain satisfactory measurements. This difficulty was 



