NOTES ON RESISTANCE MEASUREMENTS IN 

 PLATINUM THERMOMETRY. 



By IIahoi.d Edwards. 



Presented by John Trowbridge, June 8, 1904. Received October 10, 1904. 



Several years ago an investigation was begun in the Jefferson 

 Physical Laboratory, the object of which was to compare the scale of 

 the constant-volume gas thermometer with that of the platinum-resist- 

 ance thermometer, more especially at temperatures above 100° C. The 

 intention was to extend the work of M. Chappuis, of the Bureau Inter- 

 national des Poids et Mesures, and also to obtain results on the constant- 

 volume thermometer for comparison with those of Messrs. Callendar 

 and Griffiths on the constant-pressure instrument. 



Before this was completed the publication of other investigations, 

 particularly those of the Physikalsch Technische Reichsanstalt, made 

 such a research hardly necessary. During the preliminary work, how- 

 ever, problems came up leading to experimental methods which in them- 

 selves may be of some interest. 



In the standardization of the platinum thermometer the greatest pos- 

 sible refinements are called for in the resistance measurements. Errors 

 peculiar to this instrument arise in correcting for the lead resistance, in 

 avoiding possible cooling effects on the platinum on account of the pres- 

 ence of the comparatively large leads, and also in expressing in terms of 

 some selected coil or unit the value of the platinum resistance. 



The elimination of lead resistance is usually accomplished by subtract- 

 ing the values obtained by direct measurement, or by placing in the stem 

 of the thermometer dummy leads which are connected in the opposite 

 side of a Wheatstone's bridge, the ratio coils of which are equal. The 

 first method necessitates duplicate measurements of great absolute accu- 

 racy ; the second involves the assumption that the two sets of leads change 

 their resistance equally under all possible conditions, an assumption that 

 can never be proved in any particular case. Moreover, alterations in 

 the leads result in a new thermometer, and prevent to a considerable 

 extent the verification of old measurements. 



