HEAT OF WATER, WITH EXPERIMENTS BY A NEW METHOD. 
19 
thermometers, when carefully constructed of suitable wire, always agree so closely 
with this scale that it is seldom worth while to make a special determination of the 
difference-coefficient by reference to the sulphur boiling-point. It was, however, 
considered desirable to make the test in the present instance because the apparent 
variation of the specific heat of water depends in a great measure on the thermometric 
scale employed. The thermometers were also compared differentially between O'" C. 
and 100° C. and were found to agree so closely that no differences in their scales 
could be detected. 
Method of Reading the Thermometers. 
The galvanometer employed was of the moving coil type, with a resistance of 
about 13 ohms. Readings were generally taken by the constant-current method 
which I first introduced in 1891. In this method the platinum thermometer is 
connected on the compensator side of the box in series with the box coils, and is 
balanced against a compensated resistance,* equal to, or greater than, the resistance 
of the thermometer at the highest point of the range it is desired to cover, in this 
case 17'84 ohms. With this arrangement the current through the thermometer is 
nearly the same at all temperatures, since the resistances on both sides of the bridge 
are kept nearly constant. The current through the thermometer was approximately 
0‘005 ampere, and was adjusted so that the deflection of the galvanometer was 1 mm. 
at 1 metre on reversal for a change of temperature of 0°‘001 C. Readings were 
taken by setting the contact-point on the bridge-wire as exactly as possible with a 
lens to the nearest millimetre, and then observing the small deflection of the 
galvanometer on reversing the battery. The definition was so perfect that it was 
jjossible to read to 01 mm. of the scale, or 0°‘0001 C., by the galvanometer deflection 
when the temperature was steady. But this was the case only at the fixed points. 
No attempt was made to read nearer than 0°'001 C. at other temperatures, but the 
provision of ample sensitiveness greatly facilitated quickness and certainty of reading. 
The constant-current arrangement of the measuring apparatus possessed special 
advantages for the continuous-mixture method, because it was necessary to read four 
* These compensated resistances are very useful in accurate resistance measurements or platinum 
thermometry. They are best constructed of platinum-silver, wound on mica and annealed in situ at a dull 
red heat, compensated by a small resistance of pure platinum, having an equal temperature increment, 
and connected in the opposite arm of the bridge (Callendar, ‘ Brit. Patent,’ No. 14,509, 1887). The 
two coils may conveniently be enclosed in a glass tube with compensated leads like a platinum thermo¬ 
meter, and the point of zero temperature-coefScient may easily be adjusted as near to 20° C. as desired. 
In this case the whole change of resistance of the combination between 0° C. and 40° C. is less than 1 in 
100,000, and the change between 15° C. and 25° C. is only 1 in 2,000,000. The best specimens of 
manganin generally have a temperature-coefficient of 2 or 3 in 100,000 per 1° C. at 20° C., which is more 
than 100 times as great; but even if the point of zero-coefficient for manganin could be adjusted to 
20° C., the changes in the neighbourhood of 20° C. would be more than 10 times as great as with the 
platinum-silver coil compensated with platinum. For a further discussion of this point see Callendar, 
‘Phil. Trans.,’ A, vol. 199, p. 89, 1902. 
