190 
DR. H. T. BARNES ON THE CAPACITY FOR HEAT OF WATER 
6i|- centims. long, and were made, as in thermometer D, of the bare wire wound on a 
mica frame. The first arrangement was with the platinum-wire fused to about No. 18 
copper-wire, which in turn was soldered to the copper leads about 6 cent im s. above 
the bulb. This was changed to having the wire fused to much longer copper-wires, 
which were soldered to the leads at a point considerably beyond the glass-tubes con¬ 
taining the bulbs. This avoided the changing of the temperature of the solder-joins 
in the glass-tube. The final arrangement was to have the wire gold-soldered to heavy 
platinum-wires, which in turn were fused to copper-wires about 6 centims. above the 
bulbs. These wires were then soldered to the main leads at a point sufficiently 
beyond the glass-tube so as to remain unaffected by a change in temperature in the 
interior of the glass-tube. All these changes were made to improve the thermometers, 
although the last one was not really necessary. A very considerable uncertainty was 
introduced with the first arrangement, which was removed on removing the solder- 
joins from the interior of the thermometer-tubes. 
The TO-millim. wire is exceedingly delicate to use for thermometric work, and great 
care had to be exercised in constructing the thermometers and in handling them. 
They gave, however, exceedingly consistent results. As a check, a sample of the 
wire was given to Mr. Tory, who very kindly determined its 8 by comparing it with 
a piece of the original •15-millim. wire. This came out P502, and showed that no 
change had been produced in the 8, due to its having been drawn from the 
larger size. 
The fixed points 0° and 100° upon which the accuracy of the F.I. depends, were 
obtained as usual with a mixture of finely-cracked ice and water, and the usual 
form of hypsometer. In regard to the constancy of these points, the former depends 
on the percentage of ice or water present in the mixture, and its rate of melting, 
while the latter depends on the accuracy of reading of the barometer, accepting in 
both cases the purity of the ice or water. Great care was always taken with the 
freezing-point mixture, to have it compact and firmly placed in a copper vessel, 
heavily lagged, and in which water could be made to circulate through the ice around 
the thermometer bulbs. The thermometers were as far as possible placed side by 
side, separated only by a thin partition of ice. 
After obtaining the balance-point with both thermometers in ice, one, P, was removed 
to the steam-jacket, leaving the other, C, still in ice. The change in resistance in P 
was compensated by the resistance-coils until the reading was brought on to the 
bridge-wire. When a sufficient time was allowed (about 15 minutes was generally 
sufficient) for the attainment of a steady temperature, as shown by the steadiness of 
the balance-point, the reading of the bridge-wire was recorded. Thermometer P was 
then returned to the ice-bath, and the first reading repeated, which gave a measure 
of any change of zero in P. The sensitiveness of the galvanometer changed slightly 
between the two points owing to the increase in resistance in the two arms of the 
bridge system, but this was determined always at both points. 
