406 A. L. Day and J. K. Clement — Gas Thermometer. 



worthy temperature scale of sufficient accuracy and range can 

 be satisfied. 



It is no disparagement of the present system of temperature 

 definition to say that the gas thermometer itself is a compli- 

 cated and cumbersome instrument to use in any of the forms 

 which have hitherto been devised, and possesses limitations, 

 both of range and of accuracy, which are very difficult to 

 overcome. One consequence of this, particularly in the region 

 of high temperature measurements, where errors of consider- 

 able magnitude still exist in the fundamental scale, is that 

 temperature measurements easily come to receive a somewhat 

 fictitious value in the hands of those who have never acquired 

 a firsthand knowledge of these limiting conditions. This has 

 been further facilitated by the comparative ease with which 

 relative measurements of temperature can be made, even in 

 the more inaccessible parts of the scale, with the thermoelement 

 and the resistance thermometer. These devices are sensitive 

 to temperature differences of the order of magnitude of '01° 

 throughout their entire range, and yet depend absolutely upon 

 fundamental measurements with the gas thermometer for their 

 evaluation in terms of the generally accepted degree centigrade. 

 It is sufficiently obvious, though often carelessly overlooked, 

 that no method of temperature measurement, however sensitive 

 or adaptable it may be, can yield temperatures of greater abso- 

 lute accuracy than the system in terms of which those temper- 

 atures are defined. With the gas thermometer as our basis of 

 definition, therefore, we shall know our temperatures with just 

 the certainty which it is able to furnish and no more. There 

 is, to be sure, some justification for expressing the results of 

 thermoelectric or resistance measurements in units smaller than 

 the errors of the fundamental scale, provided they are so 

 recorded that they can be translated in terms of some more 

 accurate fundamental system when future investigations shall 

 have provided one, or where only comparative measurement is 

 involved ; but this is also frequently overlooked. 



The fundamental temperature scale now used is the centi- 

 grade scale of the Bureau International des Poids et 

 Mesures, determined with the constant volume hydrogen ther- 

 mometer from an initial pressure of l m of mercury. Its inter- 

 polation between the melting point of pure ice and the boiling 

 point of pure water (0°-100°) is probably accurate to the 

 nearest -005°* ; the extrapolation from 100°-300° may be in 



* The fundamental subdivision of the interval between the melting point 

 of ice and the boiling point of water by means of the expansion of hydrogen 

 was undertaken with great care by the Bureau International des Poids et 

 Mesures (P. Chappuis, Trav. et Mem. d. Bur. Int., vol. vi) and is a work of 

 such extraordinarily painstaking character in most particulars that no inves- 

 tigator has found it necessary to repeat it. It is commonly accorded an 



