ON THE MEASUREMENT OF TEMPERATURE. 
119 
scale was sent to the National Physical Lahoratoi'y, and was there compared with 
a standard brass metre scale (Societe Genevoise, No. 59); the comparison was 
guaranteed to O'Ol millim. For this we wish to express our thanks to the Director, 
Mr. E. T. Glazebrook. 
The results of the two sets of comparison are veiy concordant, except so far as 
the distance between scale 1. and scale III. is concerned. With regard to the 
discrepancy of O'Ol5 millim. which exists here, we wish to state that the values of 
the pressure-coefficients first obtained for hydrogen at initial pressures of 350 and 
500 millims., indicated that there was probalfiy a small error in the scale at this 
point. This has been corrected. 
In the following table column A gives the distances between the line 9 on the 
lowest scale (I.) and the middle points on the four remainiiig scales, as determined 
by us ; column 0 gives the value stated in the certificate furnished by the National 
Physical Laboratory, while the numbers in column B are calculated from those in 
column G by multiplying the latter 1)y the factor 962'395/962'28. 
A. 
R. 
C. 
Line 9, scale L, 
to line -50, scale II. . . . 
.369-895 
369-895 
369-85 
„ III. . . 
521-505 
521-52 
521-46 
„ IV. . . 
684-644 
684 - 64 
684-56 
)) 
„ V. . . . 
962-.395 
962-395 
962-28 
It need not be pointed out here, that in the measurements of the pressure- 
coefficient it is only necessary to know the absolute length of the scale approximately. 
We have accordingly taken the figures in the second column (B) as representing the 
distances between the standard points on our scale. 
14. 21'te Temperature of the Mercury Column. 
Since in these experiments we were attempting to measure the pressure with a 
degree of accuracy approaching O'Ol millim., it was necessary to take special 
precaution with regard to the temperature of the mercury column. In our first 
experiments we attempted to measure the temperature of the column by means of 
four thermometers, the bulbs of which were enclosed in^ glass tubes, of the same 
diameter as the barometer column, filled with mercury. We found, however, that the 
temperature could not be read with certainty to 0'2 C., nor even with that degree of 
accuracy when the steam point was being determined. As an error of O'Ol in the 
temperature of the column involves a corresponding error of O'Ol6 millim. in P^qq 
( 950 millims.), we enclosed the dead-space and mercury columns in a water jacket. 
