paratus were such as to call for a registering
recorder that would indicate the differences in temperature between the
ingoing and outcoming water to within 0.5 per cent and to record these
differences in a permanent ink line on coordinate paper. Furthermore,
the apparatus must be installed in a fixed position in the laboratory,
and connections should be such as to make it interchangeable with any
one of five calorimeters.

After a great deal of preliminary experimenting, in which the Leeds &
Northrup Company have most generously interpreted our specifications,
they have furnished us with an apparatus which meets to a high degree of
satisfaction the conditions imposed. The thermometers themselves have
already been discussed. (See page 30.) The recording apparatus consists
of three parts: (1) the galvanometer; (2) the creeper or automatic
sliding-contact; (3) the clockwork for the forward movement of the roll
of coordinate paper and to control the periodic movement of the creeper.

Under ordinary conditions with rest experiments in the chair calorimeter
or bed calorimeter, the temperature differences run not far from 2 deg. to
4 deg. Thus, it is seen that if the apparatus is to meet the conditions of
the specifications it must measure differences of 2 deg. C. to within 0.01
deg. C. Provision has also been made to extend the measurement of
temperature differences with the apparatus so that a difference of 8 deg.
can be measured with the same percentage accuracy.

FUNDAMENTAL PRINCIPLE OF THE APPARATUS.

The apparatus depends fundamentally upon the perfect balancing of the
two sides of a differential electric circuit. A conventional diagram,
fig. 19, gives a schematic outline of the connections. The two
galvanometer coils, _fl_ and _fr_, are wound differentially and both
coils most carefully balanced so that the two windings have equal
temperature coefficients. This is done by inserting a small shunt _y_,
parallel with the coil _fl_, and thus the temperature coefficient of
_fl_ and _fr_ are made absolutely equal. The two thermometers are
indicated as T_{1} and T_{2} and are inserted in the ingoing and
outgoing water respectively. A slide-wire resistance is indicated by J,
and _r_ is the resistance for the zero adjustment. Ba, Z, and Z_{1} are
the battery and its variable series resistances. If T_{1} and T_{2} are
exactly of the same temperature, _i. e._, if the temperature difference
of the ingoing and outcoming water