226 



ELECTRICAL MEASUREMENTS 



To determine 5, pure platinum being used, the resistance must 

 be measured at three known temperatures, which for high tem- 

 perature measurements are usually taken as 0, 100, and 444.70, 

 the latter being the boiling point of sulphur under carefully speci- 

 fied conditions, this having been determined with great care by 

 many experimenters. With the 5 correction applied, the plati- 

 num thermometer reproduces temperatures on the gas (nitrogen) 

 thermometer scale to within the limits of accuracy of the latter 

 between -80 and +1,100C. 



The above empirical formula does not admit of extrapolation 

 downward below 100, so for measurements of extremely low 

 temperatures the formula must either be modified or the coil 



FIG. 127. Bridge connections for resistance pyrometers. 



resistance taken at such points as the temperature of melting 

 ice, solid CO 2 , and the boiling point of oxygen, in order that the 

 extrapolation may not be so excessive. The platinum thermome- 

 ter, when used for precise work at high temperatures, must 

 be frequently calibrated; the purer the platinum, the less the 

 likelihood of a change in its constants. Also the changes in the 

 coils are minimized if the wire be large and be supported free 

 from strains. Annealing at a temperature above that at which 

 the instrument is to be used contributes to constancy. 



The coil and the bridge connections for measuring the resist- 

 ance are shown in Fig. 127. 





