MEASUREMENT OE TEMPERATURE. 
195 
the limit ‘00252, and does not vanish or give a maximum value for R. Siemens’ 
formula makes R negative when 0 = 0. 
Siemens found for platinum 
R =-0393690*+-002164070—’2413, 
which happens to agree nearly with (s), except that (s) is straighter and steeper. 
It has been usual to use a formula of the type 
R/R 0 = 1 + at + fit' 2 
for extrapolation, which corresponds to Formula (d), pp. 163, 165. 
For instance, Benoit gives R/R 0 = 1 + ‘00245R — ‘000000594+ and recently 
Humpidge* R/R 0 = 1 + '00271 —‘00000019£ 3 ; with these we may compare my 
formula. 
From these we deduce, as on p. 165 : 
Benoit . . 
. 8 = 2‘48 
Rjl/Rq = 1‘24 (roughly). 
Humpidge . . 
o o 
n 
o 
At 
i—* 
Bi/Ro = 1 '27 ,, 
Callendar . 
. 8=1‘57 
Ri/Ro = 1 ’ 8 4 ,, 
Benoit took the boiling point of cadmium to be 860° C., instead of about 770° C. 
But the discrepancies in the values of 8 seem too large to be entirely attributable to 
errors of observation. 
Formulae of extrapolation, unless very accurately verified through a wide range, are 
unsatisfactory, since they disagree so widely at high temperatures, and a very small 
change in the data from which they are calculated makes a great difference in the 
value of the coefficients ; whereas, if instead a formula of the type R/R 0 = 1 + at were 
used, it seems probable that different platinum wires would give almost identical 
results. 
In support of this, the reader is referred to the “ Comparison of platinum wires ” 
(p. 200 ). For the value of a in this formula we substitute the mean coefficient 
between 0 ° and 100 ° C. For instance, in the present paper we use the formula (a), 
pt = 
I'—ffi 
I-h-R 0 
100 , 
(«) 
and call the temperature so calculated “ temperature by platinum wire,” just as we 
speak of “ temperature by mercurial thermometer,” although mercurial thermometers 
differ much more widely and irregularly from each other.! Temperatures so calculated 
will be strictly comparable, and the actual relation between platinum and absolute 
temperature may be subsequently determined by a more enlightened and skilful 
* ‘ Roy. Soc. Proc.,’ vol. 39, No. 239, Feb. 1885. 
f See Regnault, ‘ Memoires cle l’lnstitut,’ tome 21. 
2 c 2 
