224 
MR. H. L. CALLENDAR ON THE PRACTICAL 
by 1 mm. (one of which is shown in section, full size, at E), which extend 6 inches 
on either side beyond the tube gas-furnace in which the bulb is heated. The side- 
tuhes AC, BD, are fitted to contain double electrodes of thickish platinum w-ire, to 
the ends of which, at A and B, a single length of fine wire is fused, which extends 
through the bulb from A to B. By means of these double electrodes we can deter¬ 
mine the resistance of the fine wire independently of its connections, and we can also 
determine the mean temperature of the side-tubes AC, BD. One end D is fixed 
and connected with wax to the tube of the pressure-gauge. The free end C is closed 
with wax, and observed by means of a reading microscope. The expansion of the bulb 
itself is thus simultaneously observed, and the electrodes pass air-tight at the points 
C and D, which are never heated. Moreover, the form of the bulb renders it suitable 
for heating in a tube gas-furnace, which is very convenient. 
Comparison of Platinum Wires. 
Owing to the importance, from a thermometric point of view, of the comparison of 
different platinum wires, another comparison coil was wound with greater precautions, 
to ensure perfect symmetry. The wires compared w r ere pieces from the reels 1 and 3 
used in making the platinum spirals for the air thermometers of Series III. and Y. 
respectively. They were fused on to platinum electrodes and wound on terra-cotta 
slips, as previously described (pp. 199, 200). 
The following observations were taken :— 
Table P.-it. 
Time. 
Resistance observations. 
Approximate 
temperature. 
Difference 
Vh ~ Ph 
Number of 
observation. 
Bate. 
Hour. 
R + z 
(H + z)/(R' + z) 
s 
pt 
t 
1886. 
O 
O 
O 
Aug. 21 
1 to 3 P.M. 
5-0410 
•61024 
0363 
0 
0 
0 
(1) 
Aug. 23 
10 to 11.30 A.M. 
6-7874 
•61353 
•0460 
100-06 
100-06 
0 
(2) 
12.30 P.M. 
5-905 
•61217 
•0405 
47 v 
47'4 
-0-06 
(3) 
1.20 to 2.46 p.m. 
14-55 
■61771 
■0807 
547 
593 
+ 1-14 
(4) 
2.50 to 3.40 p.m. 
12-83 
•61706 
•0733 
448 
476 
+ 1-08 
(5) 
3.40 to 7.19 p.m. 
10-70 
-61611 
0632 
326 
339 
-4-0-84 
(3) 
Aug. 24 
10 A.M. 
5-357 
•61103 
•0378 
18'5 
18-3 
-0-03 
CD 
10.30 A.M. to 12 NOON 
15-46 
•61812 
•0870 
601 
659 
4-1-05 
(8) 
An attempt was made to take an observation at a temperature of about 800°, but 
the insulation was not quite good enough ; the temperature was allowed to fall again, 
and the following observations were taken :— 
