MEASUREMENT OF TEMPERATURE. 
203 
Testing the observations in pairs by this condition, the value of the ratio is found 
nearly constant ; a small deviation from the curve produces a large variation in the 
ratio, which is a ratio of small differences, and is of course immediately deducible 
from the differential equation corresponding to Formula (e). 
The comparison gives the values of the ratios a : ct! : (/3— /3'), but not the absolute 
values of the coefficients. Its accuracy is not therefore affected by that of the 
particular values assumed for the standard wire constants a and /3. 
If we take the observations on the comparison of two platinum wires and treat 
them in the same way, we shall find, from observations (l) and (5), the mean value 
of the ratio 
/ 
- = -97295. 
a 
So that, if we assume a=’0034259, 
From this we obtain 
a' = -0033332. 
(3~P = -00000343, 
so that, if we take the value of ft previously found, namely, /3= *0015290, we shall find 
P = -0015256. 
So that P is nearly equal to (3. 
If fi=P, we have evidently p/ ~ 
at the same temperature. 
For iron, assuming the same values of 
R 
a 
and R' being the values of the resistances 
and (3, we shall find 
a'= -0045657, p = -0007767. 
The chief difficulty of the comparison is that of protecting the iron wire from 
alteration when it is maintained at a red heat for some time. In Table XL the 
observations given in Series VI. are compared with the above exponential formula (e). 
The observations were mostly taken on different days, and the resistances observed at 
the air temperature in the intervals show the direction in which the correction for 
zero variation should be applied, but not its amount. It is noteworthy that in all 
cases the correction for this would tend to reduce the small differences between the 
formula ( e ) and the observations. As it is, the mean difference is only 0°'l C. 
2 D 2 
