202 
MR, H. L. CALLENDAR OK THE PRACTICAL 
platinum, 9'886. So that the resistance of the iron was considerably increased, while 
that of the platinum remained practically unaltered. The following observations 
were then taken :— 
Resistance, iron observed .. 1'0803 P0905 1-5256 1 3135 3’5718 2-59600 
Platinum. 1-0582 P0673 1-3462 1 2162 2-1927 1-8517 tSeries Y. 
Temperature, pt .17'0 19'4 100-06 62‘5 345'0 246’4 J 
They show how slightly the temperature coefficient was affected by annealing. 
Curve I in the diagram, Plate 13, fig. 11, is constructed from the following- 
observations, which were taken by heating platinum iron comparison coil in a vacuum 
in a porcelain tube which was continuously exhausted to remove the steam which is 
given oft* by the clay when first heated, and which vitiated the previous series. 
Iron .. 
Platinum 
Temperature, 
pt 
1-5263 6-2667 7-0566 4'8396 3-6715 27692 2-0036 1-17321 
1-3453 2-8840 3-0314 2-5548 2-2283 1-9220 1-5990 1-1225 
9979 544-5 587-1 449'4 355-0 266-5 173-2 35-4 
Many other observations were taken with the same apparatus at different times. 
The resistance of the iron, when once annealed, was not found to be much per¬ 
manently altered if due precautions were taken. 
The curve representing these and other observations belonging to the same series 
at once suggests an exponential formula. If we assume that the resistance variations 
of platinum and iron may both be represented by formulae of the type 
at 
R = e r +>, 
M 
and if R 1? R 3 , be any two observed values of the resistance of the platinum coil, and 
R/, Ro', those of the iron coil observed simultaneously, we shall obtain at once, by 
equating values of the temperature for each observation, the conditions 
Whence the ratio 
log R x log Rfi 
/ 
P-/3' 
log e 
log R 3 log Rf 
= = constant. 
a 
\log Rj log R J/ \log Iff log Rf 
By taking for R 1; R 3 , the extreme values at either end of the series 
R x =1*3453, I R 3 =3'0314, 
R/= 1*5263, J R,' = 7'0566, 
we obtain the mean value of the ratio — for the whole series to be 
a. 
= 75037 * 
* By some oversight I used the value "75034 in the calculations. 
