CHANGE OF ELECTRIC RESISTANCE OF NICKEL. 



543 



values. As the magnetising current and field are diminished this disturbance rapidly 

 becomes insignificant, since the heating effect is as the square of the current. The 

 only way of obviating this source of error would be to leave the current on until 

 temperatures were perfectly steady in both wires, then reverse the current suddenly in 

 one of the two coils round one of the wires so as to produce magnetisation or destroy 

 it as the case might be, the heating effect remaining unchanged. This method would 

 have entailed a great expenditure of electric energy in the higher fields, and was not 

 thought of until the insulation in the anchor ring N broke down. It is clear that too 

 much stress must not be laid upon the readings in the highest fields. Probably the 

 best average results will be obtained by combining the readings for the two wires, as 

 in the following table, in which, corresponding to their appropriate fields, the means of 

 the numbers for the two wires are tabulated in columns headed by the mean of the 

 nearly equal temperatures. 



CHANGE OF RESISTANCE OF 100,000 OHMS OF NICKEL WIRE 

 (MEAN FOR THE TWO WIRES). 

















ddR/dt. 



Field. 



At 12 



'•7. 



At 57 



'5. 



At 93 



•5. 



















At 35°. 



At 75°. 







Diff. 





Diff. 





Diff. 







69-5 



755 



39 

 41 

 44 

 45 

 49 

 46 

 60 

 57 

 62 

 63 

 67 

 74 



800 



51 

 41 

 42 

 48 

 50 

 54 

 51 

 58 

 65 

 69 

 75 

 76 



806 



32 

 46 

 41 

 43 

 49 

 50 

 56 

 56 

 63 

 70 

 79 

 82 



1-100 



0-017 



65 



716 



749 



774 



074 



•70 



60-3 



675 



708 



728 



•74 



•56 



55-7 



631 



666 



687 



•78 



•58 



51 



586 



618 



644 



■71 



■72 



464 



537 



568 



595 



•69 



•75 



41-6 



491 



514 



545 



•51 



•86 



37-1 



431 



463 



489 



•71 



•72 



32-5 



374 



405 



433 



•69 



•78 



27-8 



312 



340 



370 



•63 



•83 



23-2 



249 



271 



300 



•49 



■81 



18-6 



182 



196 



221 



•31 



•69 



13-9 



108 



120 



139 



•27 



•53 



11-6 



72 





83 





94 





•25 



1 



•31 



In addition to the resistance changes with their differences, two columns are given 

 showing the average rate of change, due to rise of temperature, in the value of the 

 resistance change in given fields. These average rates of change may be taken to be 

 the rates of change at temperatures 35° C. and 75° C, which were the means of 

 12°7 and 57°'5 and of 57°'5 and 93°'5 respectively. 



Distinguishing the rate of change of resistance per unit increase of field at constant 

 temperature, and the rate of change, per unit increase of temperature, of the increase of 

 resistance in a given magnetic field by the somewhat elliptic terms "magnetic change 

 rate," and " thermal change rate," we may state the results embodied in these tables in 

 the following words : — 



TRANS. ROY. SOC. EDIN., VOL. XL. PART III. (NO. 23). 4 m 



