HIGH TEMPERATURE MEASUREMENT 499 



points. From the time of Regnault onwards the practice 

 seems to have become general to employ the constant-volume 

 method of working. Callendar, however, introduced a form 

 of constant-pressure thermometer, in which several of the 

 most serious sources of difficulty in gas-thermometer measure- 

 ments were notably diminished. In his instrument the 

 pressure in the working bulb is adjusted by means of a 

 delicate oil-gauge to be equal to that of a quantity of gas at 

 o"" C. confined in a second bulb surrounded by melting ice. A 

 system of capillary tubes similar in all respects to those con- 

 nected with the measuring bulb, and exposed to the same 

 changes of temperature serves to compensate exactly for the 

 uncertain "dead-space" correction. An additional advantage is 

 that the bulb in this method of working is not subjected to any 

 pressure strains internal or external. 



Callendar and Griffiths, using this type of thermometer and 

 employing air as their measuring gas, made a determination 

 of the boiling-point of sulphur and obtained the value 444*5° C. 

 under normal pressure, 760 mm. of mercury. 



This point is of special importance in high-temperature 

 measurement, since it has been adopted as the upper fixed 

 temperature of standardisation for the platinum resistance 

 thermometer to be described later and on its accuracy the 

 whole scale of the resistance-thermometer depends. This value 

 444*5° was exactly confirmed by the later investigations of 

 Chappuis and Harker, who found for the same temperature 

 on the constant volume nitrogen scale the value 444*7°, the 

 difference between the two determinations being almost exactly 

 that demanded by theory. 



Before proceeding to discuss work at higher ranges a few 

 further words regarding the mercury thermometer are necessary. 

 The construction of mercury thermometers for high tempera- 

 tures has undergone considerable modification of recent years. 

 Investigations had shown that with almost all thermometers 

 exposure to temperatures even as low as 100° C. causes zero 

 changes and with some of the glasses in use these were of 

 relatively large and very uncertain amount. In general the 

 change produced was found to be made up of two parts, the 

 one, a rise of zero, being permanent in its character, the other 

 being a temporary depression followed by gradual recovery 

 after a more or less prolonged period at ordinary temperature 



