HEAT OF WATEE, WITPI EXPERIMENTS BY A NEW MPITHOD. 
17 
nearly the same temperature as the regulator tank, first circulates downwards 
through the outer jacket J and then upwards through the next inner tube, where it 
comes in contact with the hot current, emerging finally through the thermometer- 
pocket T 4 , whence it passes to the collecting bottle, where the flow is measured from 
time to time. The head being maintained constant, steady currents of suitable 
values are obtained by fitting exit tubes of various bores between and the 
collecting bottle. From the collecting bottle the water is continuously returned to 
the heater on the floor above by means of a small rotary pump. With the exception 
of the heat exchanger itself, the details require considerable modification for different 
ranges of temperature. But the arrangement above described has been found to 
work very well for comparing the mean specific heat from 70° C. to 100 ° C. with the 
mean specific heat from 30° C. to 60° C., and will sufficiently illustrate the general 
nature of the method. 
With a flow of 10 c.c. per second the heat-exchange amounts to about 300 calories 
per second, and the external heat-loss with the arrangement above described can be 
reduced to less than a tenth of a calorie per second, or about 1 in 3,000 of the total 
quantity measured. Owing to the relatively small thermal capacity of the exchanger, 
and to the fact that the distribution of temperature is nearly independent of the 
flow, the conditions become steady to 0°‘002 C. in a few minutes when the flow is 
changed. The accuracy attainable depends chiefly on the limit of accuracy in reading 
the thermometers. 
The Platinum Thermometers. 
The thermometers employed were of my usual pattern in glass tubes, with leads 
partly of silver and partly of platinum, insulated by mica discs, spaced at intervals of 
2 cm. throughout the length of the tube by means of mica crosses. Spacing the discs 
by means of mica crosses appears preferable to spacing the discs by long thin tubes of 
biscuit porcelain, as commonly practised by many makers, because the porcelain tubes 
are more hygroscopic than the mica. They also make the compensation less sensitive 
by shielding the leads. The thermometer coils were of pure standard wire, O’Ol cm. 
diameter (0"'004), and the ends of the compensating leads were connected by fusing 
on a short piece of the same wire to eliminate any conduction effects which might 
exist. Thick platinum leads extended for a distance of 7 cm. from the coil, where 
they were fused to silver leads. The object of this is partly to avoid possible 
contamination* of the fine wire with silver and partly to diminish conduction along 
the tube near the bulb. The immersion of the thermometers in the apparatus was 
* From the first I have always adopted this method of construction in my own thermometers for 
accurate work at high temperatures. It seems likely that many of the small variations of zero and 
difference-coefficient, found by careful observers, are due to contamination of the fine wire with gold or 
silver solder at its junction with the copper or silver leads. Holborx and Henning, in their recent paper 
(he. cit.), attribute some of the small variations of their platinum thermometers to this causa 
VOL. CCXII.—A 
D 
