188 
DR. H. T. BARNES ON THE CAPACITY FOR HEAT OF WATER 
required, which was reduced gradually to 150 ohms for the 640 coil in order to 
j)reserve the same sensitiveness throughout the test, with one accumulator. The 
galvanometer contact was arranged so that it could be held in contact with the 
bridge-wire. Therefore instead of obtaining an exact balance and reading the 
vernier, the contact was placed to the nearest millimetre or half-millimetre mark on 
the scale, with the help of the vernier, and the deflection of the galvanometer 
recorded. Accurate account was always kept, by repeated verification, of the 
sensitiveness of the galvanometer, which never altered as much as one scale division 
due to external disturbances. 
For the sake of convenience, the diagram of the complete thermometer circuit is 
given in fig. 6. This shows the relative position of the resistance and compensating 
coils in the bridge system, the position of the ratio coils and bridge-wire. When 
differential thermometers are used we have them connected on opposite arms of the 
bridge, at P and C, and arranged so that the compensating leads for thermometer P 
are in series with thermometer C, and the compensating leads for C connected with 
P. Where P and C are at the same temperature, and of the same resistance, it is 
evident that the bridge system is in equilibrium with the galvanometer contact at 
the middle of the bridge-wire. For a change in the temperature of either P or C the 
bridge reading shifted either to the right or left, and when too great to be read on 
the wire, was compensated by the resistance coils. A change of temperature in C 
higher than P, however, could not be recorded beyond the bridge-wire. It was 
therefore necessary to arrange that P should always be used for measuring a change 
in temperature higher than C. The resistance coils were brought into the circuit by 
removing the heavy copper contacts from the mercury cups. When these contacts 
