323 A Comparison of Platinum and Gas Thermometers. 



which we may call the scale of platinum temperatures, such that if R be 

 the resistance at any temperature T°, this temperature on the platinum 

 R 



scale will be — — - x 100 degrees. For this quantity Callendar 



Ki - Ro 



employs the symbol pt, its value depending on the sample of platinum 

 chosen. 



In order to reduce to the standard scale of temperature the indica- 

 tions of any platinum thermometer, it is necessary to know the law 

 connecting T andpt. These are, of course, identical at 0° and 100°, 

 but the determination of the curve expressing the relationship between 

 them is a matter for experiment. 



The work of Callendar had established for a particular sample of 

 platinum the relation 



1 |_\ioo/ looj 



over the range 0° to 600°, T being measured on the constant pressure 

 air scale. 



Later experiments by Callendar and Griffiths showed that this 

 relation holds for platinum wires generally, provided they are not 

 very impure. They propose that the value of 8, the constant em- 

 ployed in the formula, should be determined by taking the resistance 

 of the thermometer in the vapour of sulphur. A new determination 

 of this point on the air scale made by them gave 444*53°, as the 

 boiling point under 760 mm. pressure. 



The present paper is the outcome of the co-operation of the Kew 

 Observatory Committee and the authorities of the International 

 Bureau of Weights and Measures at Sevres, for the purpose of 

 carrying out a comparison of some platinum thermometers with the 

 recognised international standards. 



A new resistance-box, designed for this work, and special platinum 

 thermometers together with the other accessories needed were con- 

 structed for the Kew Committee, and after their working had been 

 tested at Kew, were set up at the laboratory at Sevres in August, 

 1897. The comparisons executed between these instruments and the 

 standards of the Bureau may be divided into several groups. The 

 first group of experiments covers the range - 23° to 80°, and consists 

 of direct comparisons between each platinum thermometer and the 

 primary mercury standards of the Bureau. Above 80° the mercury 

 thermometers were replaced by a gas-thermometer, constructed for 

 measurements up to high temperatures. The comparisons between 80° 

 and 200° were made in a vertical bath of stirred oil, heated by different 

 liquids boiling under varying pressures. For work above 200° a bath 

 of mixed nitrates of potash and soda was substituted for the oil tank. 

 In this bath comparisons of the two principal platinum thermometers 

 with the gas-thermometer were made up to 460° ; and with a third 



