520 Prof. H. L. Callendar on a 



(i The arguments in favour of the adoption of the platinum 

 resistance-thermometer as a practical standard were given by 

 Professor H. L. Callendar, in a paper ' On the Practical 

 Measurement of Temperature,' communicated to the Royal 

 Society in June 1886, and published in the ' Phil. Trans.' in 

 the following year. These arguments have since been con- 

 firmed and strengthened by the work of many independent 

 observers. 



" The Electrical Standards Committee of the British 

 Association has done so much in the past with reference to 

 the adoption of the present electrical standards, and more 

 recently in connexion with the adoption of the joule as the 

 absolute unit of heat, that it would appear to be the most 

 appropriate authority for the discussion and approval in the 

 first instance of proposals relating to an electrical standard of 

 thermometry. 



" The suggestions for the standard scale of temperature here 

 proposed may be embodied in the following resolutions : — 



" (1) That a particular sample of platinum wire be selected, 

 and platinum resistance-thermometers constructed to serve as 

 standards of the platinum scale of temperature. 



" (Note. — A degree centigrade of temperature on the scale 

 of a platinum resistance-thermometer corresponds to an 

 increase of resistance equal to the hundredth part of the 

 change of resistance between 0°and 100° C. In other words, 

 temperature pt on the platinum scale is defined by the formula 



pt = 100 (R-R°)/(R'-R C ), 



in which the letters R, R°, and R' stand for the resistances of 

 the thermometer at the temperatures pt, 0°, and 100 c C, 

 respectively. The melting-point of ice is taken as the zero of 

 this scale in accordance with common usage.) 



" (2) That the scale of temperature t deduced from the 

 standard platinum scale by means of the parabolic difference 

 formula 



*-p£ = ^/100-l>/100, 



which has been proved to give a very close approximation 

 to the true or thermodynamic scale, be recommended for 

 adoption as a practical standard of reference, and be called 

 the British Association Scale of Temperature. 



u (Note. — The gas-thermometer would still remain the ulti- 

 mate or theoretical standard, and the exact relation of the 

 British Association scale to the absolute scale would be the 

 subject of future investigation. In the present state of 

 experimental science, the difference between the two scales 

 over the greater part of the range is less than the probable 



