[Illustration: FIG. 25.--Section of calorimeter walls and part of
ventilating air-circuit, showing part of pipes for ingoing air and
outgoing air. On the ingoing air-pipe at the right is the lamp for
heating the ingoing air. Just above it, H is the quick-throw valve for
shutting off the tension equalizer IJ. I is the copper portion of the
tension equalizer, while J is the rubber diaphragm; K, the pet-cock for
admitting oxygen; F, E, G, the lead pipe conducting the cold water for
the ingoing air; and C, the hair-felt insulation. N, N are brass ferules
soldered into the copper and zinc walls through which air-pipes pass; M,
a rubber stopper for insulating the air-pipe from the calorimeter; O,
the thermal junctions for indicating differences of temperature of
ingoing and outgoing air and U, the connection to the outside; QQ, exits
for the air-pipes from the box in which thermal junctions are placed; P,
the dividing plate separating the ingoing and outgoing air; R, the
section of piping conducting the air inside the calorimeter; S, a
section of piping through which the air passes from the calorimeter; A,
a section of the copper wall; Y, a bolt fastening the copper wall to the
2-1/2 inch angle W; B, a portion of zinc wall; C, hair-felt lining of
asbestos wall D; T-J, a thermal junction in the walls.]
In passing the current of air through the calorimeter, temperature
conditions may easily be such that the air entering is warmer than the
outcoming air, in which case heat will be imparted to the calorimeter,
or the reverse conditions may obtain and then heat will be brought away.
To avoid this difficulty, arrangements are made for arbitrarily
controlling the temperature of the air as it enters the calorimeter.
This temperature control is based upon the fact that the air leaving the
chamber is caused to pass over the ends of a series of thermal junctions
shown as O in fig. 25. These thermal junctions have one terminal in the
outgoing air and the other in the ingoing air, and consequently any
difference in the temperature of the two air-currents is instantly
detected by connecting the circuit with the galvanometer. Formerly the
temperature control was made a varying one, by providing for either
cooling or heating the ingoing air as the situation called for. The
heating was done by passing the current through an electric lamp placed
in the cross immediately below the tension equalizer J. Cooling was
effected by means of a current of water through the lead pipe E closely
wrapped around the air-pipe, water entering at F and leaving at G. This
lead pipe is insulated by hair-felt pipe-covering, C. More recently, we
have adopted the procedure of passing a continuous current of water,
usually at a very slow rate, through the lead pipe E and always heating
the air somewhat by means of the lamp, the exact temperature control
being obtained by varying the heating effect of the lamp itself. This
has been found much more satisfactory than by alternating from the
cooling system to the heating system. In the case of the air-current,
however, it is unnecessary to have the drop-sight feed-valve as used for
the wall control, shown in fig. 13.