HEAT OF EVAPORATION OF WATER. 289 
of the formula on p. 286) gives the change in temperature (on the N scale) corre- 
sponding to a change of 0°1 box ohm in thermometer AB*. 



TABLE LV. 
: = a i 
| Temp. | AR (box ohms). | A pt. AO. 
| | 
| ° | ° ° 
| 20 | 0-1 | 14518 14387 
| 30 | 01 14518 14430 
| 40 | O-1 | 14518 14474 
50 0-1 | 1-4518 14518 


Now the value of the mean millimetre of the bridge-wire (at temperature 15°) was 
00062875 box ohm, hence the following Table which (col. 2) gives the difference in 
@ indicated by a difference of 1 mean bridge-wire millim. from the bridge-wire null- 
point at the given temperature, and (col. 3) the difference in temperature indicated 
by a swing of 10 when the contact-maker is on the null-point. 




TABLE) Vic 
] i } 
| . 
| ‘Temp. DOOD IE rence Ona meet |) “Noltorlswing of 10: 
| _ baw. 
o ° O° 
20 0:009046 0-001865 
| 30 0-009073 0:001930 
| 40 0:009101 0-002022 
| 50 0:009128 0-002074 



Before an experiment, the contact-maker was set, with the aid of a magnifying 
glass, to the exact null-point corresponding to the temperature at which the experi- 
ment was to be conducted, and it was left untouched throughout the experiment.t 
The initial and the tinal swing rarely exceeded 30 or 40, and could certainly be read 
with a limit of error of 2, when only one observation was taken, and as, at these 
times, three observations were meaned (they were, by the way, usually identical), the 
* As AB was the thermometer immersed in the calorimeter it is with changes in its. temperature 
that we have chiefly to do. 
+ An error of ‘05 millim. in the setting of the bridge-wire contact-maker would correspond to a 
difference of 000455" at 40°, and the radiation, &c., due to such a difference would be negligible. The 
radiation, &c., coefficient of this calorimeter when full was about ‘00009 per l° per sec., or the loss in 
temperature due to a difference of 1° would be about ‘324° = 100-4 thermal grams per hour (assuming 
the capacity as 310), thus a difference of ‘01° throughout an experiment would have caused a loss or 
gain of 1 thermal gram. Of course, the differences in temperature rarely attained to ‘01°, and again, the 
differences were alternately + and —, so that the radiation, é&c., loss or gain was evidently negligible. 
The radiation, &c., coefficient can be deduced from the experiments on the rate of rise made when 
determining the value of Cs, (Appendix II.). 
MDCCCXCV.—A. 24 1 
a 
