HEAT OF EVAPORATION OF WATER. 295 
working with a D.P. of four cells at least sixteen were really in use. Throughout 
these experiments they were maintained at a temperature close to 15° C., by a 
regulator which warmed the tank when below 15° C. and turned tap-water through 
the tank when above 15°C. As full particulars are given in the pages of Paper J., 
I think the above summary will be sufficient. 
There is one alteration in the electrical connections to which I wish to draw 
attention. In former experiments some time always elapsed, after the current was 
established, before any observations were taken. It, however, appeared probable that 
in this work I should occasionally have to cut off and put on the current during an 
experiment. For some time after the establishing of the current, a constant 
re-adjustment of the rheochord was required, as the temperature of the wires, &c., 
forming the external circuit was gradually raised, and the external resistance in 
consequence increased. During this period of adjustment the balance could not be 
as accurately maintained as when the temperature of the external wires had become 
steady. In order to avoid this preliminary adjustment, a coil of the same wire and 
resistance as that in the calorimeter was constructed and placed in a tube containing 
the same oil as that in the calorimeter, and this tube was fixed in the tank at F 
(Plate 6, fig. 3). 
At k, a key was so arranged that one movement brought the storages into 
connection with the calorimeter coil by means of leads 2 and 4, while another move- 
ment caused the current to leave the calorimeter circuit and pass through F. A 
second key (not shown in fig. 8) enabled the Clark cell to be also connected to F. 
The current was sent through F for 10 minutes or so and the balance adjusted, before 
an experiment commenced. On moving the key at ,, the current was switched on 
to the calorimeter, the Clark cell circuit was then also shifted and the balance once 
more made perfect. Thus the current through the externals was kept constant 
whether it was passing through the calorimeter or not, and therefore the rheochord 
required far less manipulation. 
Again, the key k, was so arranged that whenever the current was shifted, a 
connection in a chronograph circuit was established, and thus the times of “making 
and breaking” the calorimetric current were automatically registered. True, the 
marked time was in each case a fraction of a second before the “ make and break,” but 
the actual duration of the intervals was truly recorded. 
Measurement of Ry. 
The holes in the paraffin blocks P, and P, (Plate 6, fig. 3) contained mercury. 
Connection could be made by two cross pieces of copper rod, between either of the 
slots in P, and any of the holes in P,. These copper rods, when in position, rested on 
the wires which passed through the bottom of the holes. Denoting the resistance of 
the coil by Ry, that of the leads from P, to the lid of the calorimeter by 7, 7, 73, 4; 
