HEAT OF EVAPORATION OF WATER. 287 
difference of temperature between the calorimeter and the surrounding walls, and I 
therefore omit the details of the determination of their fixed points, &c., although I 
must add some other facts which are of importance for the present purpose. 
The bridge-wire scale was so fixed that the mark 60 centims.* fell exactly in the 
middle. Had the bridge-wire been uniform therefore, and had the coefficients of the 
two thermometers been always the same, then when both were at the same tempera- 
ture the bridge-wire reading would necessarily have been always 60 centims. I 
have stated that the thermometer resistances might be regarded as practically equal 
when both were in ice. The bridge-wire reading, however, was found to be 598°35 
millims., when AB and CD were both in ice. At first sight this appeared to indicate 
a difference in the resistances of the two thermometers at 0°, but when the 
calibration of the bridge was concluded, it gave 5984 millims. as the middle point of 
the bridge and thus afforded independent proof of the truth of the calibration and 
of the equality of the coils forming the other two arms of the bridge. Owing to the 
slight difference above referred to in the coefficients of the two thermometers the 
reading of the bridge null-point was found to be 601°4 millims. when both thermo- 
meters were at 100°. Now (as described in Paper A.) the fixed points of these 
thermometers in ice, steam, aniline, and sulphur-vapour, had been repeatedly 
determined with extreme care. I have, since the conclusion of these experiments 
(z.e., on November 4), again taken the values of Ry and R,—ice and steam. They 
remain practically unchanged and are as follows :— 
AB. CD. 
Raine 41) 24558526 2.4°58333t 
Rp oe 4 IAGRVALT 17°69720 
R, — R,= 6°88815 6°88613 
Thus the difference between AB and CD would, when in steam, exceed their 
difference in ice by ‘00198 ohm. Now the value of 1 millim. of the bridge-wire at 
reading 60 was 1:0020 times the mean bridge-wire millim., and the value of the 
mean bridge millim. was ‘00062875 box ohm. Thus 1 millim. of bridge-wire at 
60 = ‘0006300 box ohm. Hence the movement of the two thermometers from 
ice to steam ought, according to the standardisation, to have caused a movement 
of 42° millims. = 3°1 millims. On placing both thermometers in steam at 100° the 
63 
actual bridge-wire reading was found to have altered from 598°35 to 601°40, a 
* The scale was marked from 20 to 100 centims. 
+ Expressed after correction for individual coil errors, &c., in terms of my ‘ mean-box” ohm 
(Paper J., p. 409). The absolute values being of no consequence, I have, in order to save arithmetic, 
expressed the resistances of all my platinum thermometers in ‘‘ mean-box’”’ ohms. 
yP 
