1899.] on Measuring Extreme Temperatures, 101 



the custom at present for different observers to reduce their results 

 indirectly to the scale of the gas-thermometer by reference to certain 

 assumed values of the boiling and freezing points of various substances. 

 They generally assume different values for these fixed points, and 

 adopt different methods of calibration, which are undoubtedly respon- 

 sible for many of the discrepancies at present existing. 



To take an illustration from the experiments already quoted, the re- 

 markable discrepancy between the experiments of Bottomley, Paschen 

 and Petavel, on the one hand, and those of Wilson and Gray and 

 Schleierrnacher, on the other, in the determination of the intensity 

 of radiation from polished platinum, may be traced primarily to 

 differences in the methods of measurement adopted. Bottomley and 

 Petavel measured the electrical resistance of the radiating wire itself, 

 and deduced the temperature by the usual formula for the platinum 

 scale. Paschen calibrated his thermo-couple by reference to numerous 

 fusing and boiling points. Wilson and Gray adopted the meldometer 

 methods based on the expansion of platinum, which they found to be 

 uniform. The vacuum in Schleiermacher's experiments could not be 

 measured, and was probably vitiated by gas evolved from the heated 

 platinum. 



These and similar discrepancies might be in a great measure 

 removed, so far as they depend on the measurement of temperature, 

 by the direct comparison of the various methods of measurement. 

 The " platinum " methods are among the most important and the most 

 easily comparable by direct experiment. These methods are founded 

 on the characteristic stability and infusibility of the metals of the 

 platinum group, properties which are accompanied by an even more 

 remarkable degree of constancy in their less obvious electrical 

 attributes. The two older methods, based on (1) the expansion and 

 (2) the specific heat of platinum, are of comparatively limited appli- 

 cation, but have given very good results in the able hands of Joly 

 and Violle. The more modern electrical methods have the advantage 

 of much wider applicability and convenience. They are of two 

 distinct kinds : (3) the thermo-electric method, represented by the 

 Pt-Pd thermo-couple of Becquerel the Pt-Ir thermo-couple of Barus, 

 and the Pt-Rh thermo-couple of Le Chatelier, and (4) the platinum 

 resistance pyrometer of Siemens. The third method has been natu- 

 ralised in this country, and brought to great perfection by the work 

 of Sir William "Roberts- Austen. The fourth method was that adopted 

 by Bottomley, Schleiermacher, and Petavel in the experiments above 

 mentioned, and has been applied with great success by Heycock an 1 

 Neville at high temperatures, and by Dewar and Fleming at the 

 other extremity of the scale. 



The usual or indirect comparison of the foregoing methods by 

 means of the fusing points of various metals is illustrated in the 

 annexed table, which contains several of the most recent results. 

 The numbers given in brackets are now published for the first time, 

 and should be regarded as preliminary. 



