18 
PROF. H. L. CALLENDAR ON THE VARIATION OF THE SPECIFIC 
15 cm. At the head of each thermometer the silver leads were soldered to flexible 
copper leads, 3 m. long, with amalgamated copper terminals fitting the mercury cups 
of the compensated box described in a previous paper (Callendae, loc. cit., p. 90). 
This was essential to permit of rapid interchange of the thermometers in taking 
readings. 
The ice- and steam-points were observed on several occasions. The sulphur-point 
was also observed on one occasion, when the following readings were taken:— 
Thermometer. 
Ti. 
T 2 . 
T 3 . 
I 4 , 
Steam at 100°‘262 C. 
1785-39 
1783-70 
1785-38 
1782-39 
Ice at 0 ° C. 
1284-07 
1282-84 
1284-1] 
1281-85 
Fundamental interval. 
500-03 
499-57 
499-98 
499-25 
Sulphur reading. 
3394-67 
3391-26 
3394-38 
3389-10 
^4 deduced. 
422-09 
422-05 
422-07 
422-08 
4 from barometer. 
445-22 
445-19 
445-21 
445-21 
4-i?4. 
23ffi3 
23-14 
23-14 
23-13 
Difference-coefficient x 10^ . . . 
1-505 
1-506 
1-506 
1 - 505 
Readings at the fixed points were taken and corrected for box-temperature and 
coil-errors to one figure beyond that given in the above table (corresponding to 
0°‘0002 C.), but the last figure has been rejected as not being fully significant even at 
the fixed points, and as being beyond the limit of accuracy at the sulphur-point. The 
fundamental intervals of the thermometers were approximately 5 ohms each, or 
500 cm. of the bridge-wire. Thermometers T^ and Tg had been made at the same 
time, by my assistant Mr. W. J. Colebrook, for use as a difierential pair, and had 
been adjusted with special care so that their uncorrected readings agreed to 1 in 
10,000 throughout the scale. Tg and T 4 were precisely similar thermometers made at 
different times, with resistances adjusted to about 1 in 1 , 000 . The readings at the 
sulphur-point were taken in succession on the same day, when the barometer was 
falling slightly, in the order T^, T 3 , T 4 , Tg. The corresponding temperatures, on the 
gas-scale are calculated from the observed barometer readings by assuming the normal 
boiling-point to be 444°'53 C., and the pressure variation to be 0°’090 C. per mm., as 
found by Chree. The results show tliat all four thermometers agreed in their 
temperature scales to 0°’01 C. at the sulphur-point, which is nearly the limit of error 
of the readings. An iron-tube apparatus was employed for boiling the sulphur and 
the thermometers were fitted with a single screen. Experience has shown that this 
apparatus gives consistently a temperature nearly 0°’10 C. lower than the glass-tube 
apparatus with two screens, as originally described. Allowing for this, the difterence 
coefficients of all four thermometers woifid be within 1 in 1,500 of the standard 
value, lAOxlO-^, for pure platinum when the S.B.P. is taken as 444°‘53 C. 
In any case, this correction (from 1'506 to lAOO) would not affect the scale of 
the thermometers by so much as 0°‘002 C. at 50° C. In my experience, platinum 
