HEAT OF EVAPORATION OF WATER. 285 
final differences between the temperature of the calorimeter, 6,, and that of the 
surrounding walls, 4,). 
As differences of temperature had now to be dealt with, the use of differential 
thermometers naturally suggested itself. 
The following extract is taken from a full description of these thermometers given 
in my Paper A. (see also Plate 6, fig. 2) :— 
“Two platinum thermometers (labelled AB and CD) were constructed with great 
care; four stout platinum leads passed down the stem of each, supported and 
insulated in the usual manner by small disks of mica, and the resistance of all these 
leads was made as equal as possible before attaching the coils. Great attention was 
given to this matter, and it is probable that the leads in no case differed amongst 
themselves by 1 in 10,000. The coils, consisting of a particularly pure sample 
of platinum wire, were then attached, and several days were devoted to securing 
their equality Their resistance in ice was about 18 ohms, thus ygoo of their 
resistance could be directly determined on the box. The galvanometer swing was 
about 500 for a change of ‘01 in the box; and such equality was secured that when 
both thermometers were placed in ice (the necessary precautions being taken with 
regard to exterior connections, &c.) no readable difference in the swing of the 
galvanometer could be observed ; thus they differed by a quantity certainly less than 
1 in 100,000. This equality, although not a necessity, was a great convenience. 
“ Although cut from the same length of wire, and insulated in a precisely similar 
manner, the coils did not possess exactly the same coefficients. The resistances in 
steam and sulphur were repeatedly determined and checked by observations in the 
vapour of aniline. Both thermometers were on several occasions heated to a red 
heat, the hard glass tubes containing them becoming slightly bent in the process ; 
but, since this annealing, no further change has been observable in them. The method 
of completely standardising such instruments has been fully described by Professor 
CALLENDAR and myself in ‘ Phil. ['rans.,’ 1891, A, and I need not, therefore, here 
dwell upon it. The values of 6 differed slightly, viz, 1°513 and 1°511; but such a 
difference, even if not allowed for, would, over the range 0° C. to 100° C., in no case 
cause an error exceeding about 3,55 C. in elevation. These thermometers were so 
connected that the compensating leads of AB were placed in series with the coil of 
CD, and vice versd. Any heating of the stem of AB or CD, therefore, added an 
equal resistance to each arm of the bridge; and, as the leads were everywhere bound 
together, the indications were absolutely independent of all changes in temperature 
except those of the bulbs. 
“The two thermometers, with their leads connected as described, were placed at 
opposite ends of a bridge-wire of platinum-silver. During the spring of this 
year this wire was subjected to a most careful calibration by what was practically 
Carey Fosrer’s method, and it proved to be more unequal than I had expected. It 
was therefore re-calibrated by a different method in which a resistance-box was used 
