454 PROFESSOR A. SCHUSTER AND MR. W. GANNON ON A 
- A weighed and measured quantity of water having been placed in the calorimeter, 
the platinum dish (after being weighed several times) was filled with 120 cub. centims. 
of the silver nitrate solution. The water in the jacket of the calorimeter was raised 
about 1° C. above the temperature of the water in the calorimeter, so as to secure a 
convenient rise before the heating of the coil commenced. The observations are 
conveniently divided into three periods. The reference to the connections will be 
understood with the help of fig. 5. 
Preliminary Period. 
A, being the main battery, the key Q was open at first, and the connections in the 
other keys were as follows: k, (a), k, (a), k, (a). The observer (A) compared the 
Clark cells with the help of the key &,, and at the conclusion of the comparisons 
which took about 8 minutes—the temporary bridges of k, were removed. (A) then 
closed the main circuit at @, and the connections were such that the current was 

flowing through the temporary coil and could be adjusted till a balance was obtained 
against the 20 Clarks C.B. The observer (B) in the meantime observed every minute 
the thermometer which was plunged into the calorimeter, The rise was roughly about 
0°-002 per minute, and so regular that more frequent observations would have been of 
no value. (B) also observed the auxiliary thermometers. 
Everything being ready, a signal was given 10 seconds before the current was to be 
sent through the principal coil. (B) now broke connections at k, and joined k; (a). 
Main Period. 
(B) changed the connection /, (a) to k, (b), and the main current thus passed through 
the coil in the calorimeter. A signal being given that this was done, (A) makes 
connections k, (b), and thereby opposed the Clark battery to the electromotive force 
at the terminals of the principal coil. If the preliminary adjustment was perfect (as 
it was on two occasions), the spot of light remained at rest ; but generally it moved 
slightly to one side, and then a slight readjustment of the resistances brought it back 
to its position of rest. (A) called out when he had obtained a satisfactory balance, and 
this was generally three seconds after the main current was complete. As the 
experiment lasted nine or ten minutes, and as the electromotive force was never as 
much as two in a thousand out during the first three seconds, no error in the final 
result can be introduced in this short period. During the heating of the coil (B) did 
not take any temperature observations except when half the proposed time of heating 
had elapsed. The object of this observation is to be able to point the telescope very 
near the division of the thermometer which the mercury thread would reach at the 
conclusion of the experiment. 
Final Period. 
Ten seconds before the main current was interrupted, the connection at k, was 
broken, the balance being always so steady that it could be trusted to be maintained 
during that interval. (B) now broke &, (a), and three seconds after connected ks (0). 
