158 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 2 



The relation between resistance and temperature above 0° is 

 given by the Callendar equation. 1 



a\i? / V100 7100 v y 



R t — E n { t \ t 



1 - ^r=i, x 10 ° + 'few - Vioo < 36) 



where £ is the temperature in degrees centigrade, _K <5 i? , and R 100 are 

 the resistance values at the temperatures t°, 0°, and 100°C, respec- 

 tively. For very pure platinum, a = 0.00392 and d is between 1.49 

 and 1.50. For temperatures below down to — 190°C, a further 

 correction is furnished by Van Dusen. 2 A discussion of the accuracy 

 of the Callendar and Van Dusen equations is furnished by Mueller. 3 

 The resistance thermometer is not used as a standard below 

 — 190°C because in this range the resistance-temperature charac- 

 teristic is greatly affected by impurities and each resistor requires a 

 separate calibration. The National Bureau of Standards maintains 

 a group of platinum resistance thermometers calibrated in the range 

 from 1 1 to 90°K. Other thermometers may be calibrated against 

 these standards. 4 



technical, details. The diameter of the wire varies from 0.02 to 0.6 mm, 

 the latter for temperatures of 1000°C ; for general use, the preferred type of wire 

 is about 0. 1 mm in diameter. For thermometers that closely duplicate those 

 of the National Bureau of Standards, the wire should be smooth, free of 

 defects, and drawn from an ingot which has been completely fused (not from 

 a forged sponge). It should conform to the purity test, that is, the ratio of the 

 resistance at 100°C to the resistance at C C should be larger than 1.391. The 

 character of the metal may be tested by melting the end of the wire in a flame; 

 it should melt quietly, without sputtering, scintillating, or evolving volatile 

 material. In general, for stability the wire should be heated to incandescence 

 for some time (E. F. Mueller, loc. cit.). The wire is then formed into a coil and 

 mounted on a framework such as a mica cross. For methods of mounting, see 

 J. A. Beattie et al., Proc. Am. Acad. Arts Sci., 66, 167 (1930-1931); also C. H. 

 Meyers, J. Research Natl. Bur. Standards, 9, 807 (1932). For calorimetric work, 

 the form described by T. S. Sligh, Natl. Bur. Standards Sci. Papers, 17, 49 

 (1922), is commonly used. A very compact form of a resistance thermometer 

 is described by Meyers. 



X H. L. Callendar, Phil. Trans. Roy. Soc. {London), 178, 160 (1887). 



2 M. S. Van Dusen, J. Am. Chem. Soc, 47, 327 (1925). 



3 E. F. Mueller, in "Temperature," pp. 162ff., Reinhold Publishing Corpora- 

 tion, New York, 1941. 



4 H. J. Hoge, in "Temperature," p. 141, Reinhold Publishing Corporation, 

 New York, 1941; H. J. Hoge and F. G. Brickwedde, J. Research Natl. Bur. 

 Standards, 22, 351 (1939). 



