RELATIONSHIP BETWEEN THE REAL AND IDEAL RESISTIVITY OF PLATINUM

The relationship between the real and ideal resistance functions (RT/R273) has been examined for a wide variety of specimens of thermometric platinum over the range 0–900° K. An attempt was made to relate these two functions by using Matthiessen's rule in addition to Kohler's theoretical expression for the deviation from Matthiessen's rule. It was found that Kohler's relation did not apply for most specimens of thermometric platinum; however, in the restricted range 20–90° K, it appeared to hold fairly well for about 17 of the 65 resistors examined.Values for the ideal resistance function have been determined by extrapolating the measured resistance values of a large number of platinum resistance thermometers. In the range 10–90° K a special method of extrapolation has been used which appears to give greater accuracy than previously attained. Also, methods for estimating the residual resistance ratio (at 0° K) from measurements at higher temperatures are discussed.The results of this investigation have been applied to the practical problem of extending the present platinum resistance temperature scale below 90.19° K. To this end, a somewhat different method for interpolating the real resistance function between a number of fixed calibration temperatures has been outlined and compared with earlier methods.