220 



ELECTRICAL MEASUREMENTS 



In this and similar cases, an empirical relation of the form hero 

 given is frequently employed to represent the results of physical 

 measurements : 



R t = flo(l.+ at + bt* + ct* + . . . ). (21) 



R t is the resistance at t, and R that at 0. The departure from 

 a straight line is determined by the constants b and c. For the 

 special case of copper, the variation of which is linear, between 



VARIATION OF RESISTANCE 



OF PLATINUM 

 WITH TEMPERATURE 



200 400 600 800 1000 1200 



Temperature (Gas Scale) 



FIG. 126. Temperature-resistance curve of platinum. 



10 and 100C., the constants b and c are zero. The constants 

 a, b, c are best determined by applying the method of least squai 

 to a series of measurements of the resistance made at different 

 temperatures. 



The temperature-resistance curve for a coil of very pure plati- 

 num is shown in Fig. 126; it is represented by the followii 

 equation. 



R t = R (l + 0.00392J - 0.00000058& 2 ) ; 



