Section III., 1913. [153] Trans. R.S.C. 



A Study of Iron Wire for Electrical Resistance Thermometers. 



By Arthur A. Scott, M.Sc. Demonstrator of Physics, McGill 

 University, Montreal. 



Presented by Prof. H. T. Barnes 

 (Read May 28, 1913). 



The following paper describes an investigation of the exact relation 

 between the resistance and the temperature of pure iron wire; to 

 ascertain if iron can be employed as a resistance thermometer. 



Platinum is the only substance that has been extensively used in 

 this connection. Invented by Siemens, and subsequently modified by 

 Callendar and Griffiths, the platinum thermometer was fully investigated 

 and established as a standard instrument by Callendar. Platinum 

 possesses the following advantages. It can be obtained pure. It 

 will operate between wide limits of temperature. Although its increase 

 of resistance does not strictly follow a linear law, the indicated tempera- 

 tures can be reduced to true temperatures by a simple parabolic formula. 



It has been frequently observed in this laboratory that when a 

 platinum thermometer is allowed to remain out of use for some time, 

 its resistance at any given temperature will not remain constant until 

 it has undergone a number of cyclical changes of temperature. It is 

 possible that this anomalous behaviour is due to the well-known property 

 platinum possesses of occluding gases. 



For precise measurements at ordinary temperatures, it is of 

 interest to determine if a substitute for platinum can be obtained 

 which does not suffer from the disadvantages of a change of zero, and 

 yet answers the requirements of a resistance thermometer. 



The electrical properties of pure iron wire have been investigated 

 for this purpose and the following advantages have been observed; 

 the temperature coefficient is almost twice as great as that of platinum. 

 Iron is very much cheaper, and it is possible to use much greater resist- 

 ances of it. 



If Rq is the resistance of a thermometer at O** C, and R is its 

 resistance at a temperature t, then 



Rt = Ro(l+at) 

 By putting t equal to 100" C, we obtain 



" 100 Ro 

 Sec. Ill, 1913—10 



