MEASUREMENT OF TEMPERATURE. 
163 
therefore possesses in an eminent degree the qualifications necessary for a scientific 
standard. There seems no reason to expect that the temperature coefficient should 
be liable to secular change, seeing that violent treatment of these specimens for some 
months has not produced any measurable alteration. 
III. As it is a very important question to determine the relation between tempera¬ 
ture by platinum wire and temperature by air thermometer, considerable space is 
devoted to the discussion of experiments bearing on the question. The present 
standard of thermometry is Regnault’s normal air thermometer ; this, while pos¬ 
sessing several advantages as an ultimate standard, is, nevertheless, so difficult to use 
that the discrepancies at present existing are bewildering, and it cannot be used at 
all in ordinary work for most purposes ; all measurements have, therefore, to lie 
referred to it indirectly, which is a great disadvantage. But until the thermo¬ 
dynamic thermometer shall have assumed a more practical form, the air thermometer 
gives the best approximation to absolute temperature. It happens that the platinum 
thermometer may be more accurately compared with this standard than any other. 
The practical difficulty of all thermometric comparisons is that of maintaining an 
enclosure • at a constant and uniform temperature. This was to a great extent 
avoided in the present case by enclosing the spiral of fine platinum wire inside the 
bulb of the air thermometer itself. There are, of course, some mechanical difficulties 
in the way, and it was found necessary to devise a modified form of air thermometer 
for the purpose. Full details of the experiments and methods are given in subsequent 
sections. The modified form of air thermometer is also minutely described, as it may 
be found useful in other and similar investigations. Various sources of error are also 
discussed, which may have considerable effect in experiments at high temperatures. 
The investigation of these is still incomplete, but does not present any difficulties 
which may not be overcome with the aid of the platinum thermometer. Among 
these we may mention surface actions between the gas and its enclosure ; the question 
of the possibility of eliminating the capricious changes of volume of the envelope ; 
and of determining the amount of dissociation to which gases are subject at high 
temperatures. The experiments are, therefore, necessarily incomplete ; they are 
published chiefly on account of the great practical importance of settling on a 
universal standard for the more accurate comparison of different measurements of 
temperature. 
[The results of the comparison between the platinum and air thermometers are 
best represented graphically by drawing the curve of difference of temperature. If 
t stand for the temperature Centigrade by air thermometer, and pt for the temperature 
Centigrade by platinum wire, that is to say, for the function 100 (R/R o — l)/(R 1 /R 0 — 1 ); 
the observations (Plate 13) are seen to agree fairly well with the parabola 
d=t — pt = 1 *57 {(i/100) 3 — (if/100)}. (cl) 
Y 2 
