The air thermometers are designed with a special view to taking quickly
the temperature of the air. Five thermometers, each having a resistance
of not far from 4 ohms, are connected in series and suspended 3.5
centimeters from the wall on hooks inside the chamber. They are
surrounded for protection, first, with a perforated metal cylinder, and
outside this with a wire guard.
[Illustration: FIG. 15.--Detail of air-resistance thermometer, showing
method of mounting and wiring the thermometer. Parts of the wire guard
and brass guard are shown, cut away so that interior structure can be
seen.]
The details of construction and method of installation are shown in fig.
15. Four strips of mica are inserted into four slots in a hard maple rod
12.5 centimeters long and 12 millimeters in diameter, and around each
strip is wound 5 meters of double silk-covered pure copper wire
(wire-gage No. 30). By means of heavy connecting wires, five of these
thermometers are connected in series, giving a total resistance of the
system of not far from 20 ohms. The thermometer proper is suspended
between two hooks by rubber bands and these two hooks are in turn
fastened to a wire guard which is attached to threaded rods soldered to
the inner surface of the copper wall, thus bringing the center of the
thermometer 3.4 centimeters from the copper wall. Two of these
thermometers are placed in the dome of the calorimeter immediately over
the shoulders of the subject, and the other three are distributed around
the sides and front of the chamber. This type of construction gives
maximum sensibility to the temperature fluctuations of the air itself
and yet insures thorough protection. The two terminals are carried
outside of the respiration chamber to the observer's table, where the
temperature fluctuations are measured on a Wheatstone bridge.