While the temperature control of the calorimeter is such that in general
the average temperature varies but a few hundredths of a degree between
the beginning and the end of an experimental period, in extremely
accurate work it is necessary to know the amount of heat which is
absorbed with any increase in temperature. In other words, the
determination of the hydrothermal equivalent is essential.
The large majority of the methods for determining the hydrothermal
equivalent of materials are at once eliminated when the nature of the
calorimeter here used is taken into consideration. Obviously, in warming
up the chamber there are two sources of heat: first, the heat inside of
the chamber; second, the heat in the outer walls. As has been previously
described, the zinc wall is arbitrarily heated so that its temperature
fluctuations will follow exactly those of the inner wall, hence it is
impossible to compute from the weight of the metal the hydrothermal
equivalent. By means of the electrical check experiments, however, a
method for determining the hydrothermal equivalent is at hand. The
general scheme is as follows.
During an electrical check experiment, when thermal equilibrium has been
thoroughly established and the heat brought away by the water-current
exactly counterbalances the heat generated in the resistance-coil inside
the chamber, the temperature of the calorimeter is allowed to rise
slowly by raising the temperature of the ingoing water and thus bringing
away less heat. At the same time the utmost pains are taken to maintain
the adiabatic condition of the metal walls. Since the temperature is
rising during this period, it is necessary to warm the air in the outer
spaces by the electric current. By this method it is possible to raise
the temperature of the calorimeter 1 degree or more in 2 hours and
establish thermal equilibrium at the higher level. The experiment is
then continued for 2 hours at this level, and the next 2 hours the
temperature is gradually allowed to fall by lowering the temperature of
the ingoing water so that more heat is brought away than is generated,
care being taken likewise to keep the walls adiabatic. Under these
conditions the heat brought away by the water-current during the period
of rising temperature is considerably less than that actually developed
by the electric current and the difference represents the amount of heat
absorbed by the calorimeter in the period of the temperature rise.
Conversely, during the period when the temperature is falling, there is
a considerable increase in the amount of heat brought away by the
water-current over that generated in the resistance-coil and the
difference represents exactly the amount of heat given up by the
calorimeter during the fall in temperature. It is thus possible to
measure the capacity of the calorimeter for absorbing heat during a rise
in temperature and the amount of heat lost by it during cooling. A
number of such experiments have been made with both calorimeters and it
has been found that the hydrothermal equivalent of the bed calorimeter
is not far from 21 kilograms. For the chair calorimeter a somewhat lower
figure has been found, _i. e._, 19.5 kilograms.