NOV. 22 , I 9 IS 
Improved Respiration Calorimeter 
313 
that of the laboratory, is determined by means of an accurate barometer 
mounted on the walls of the laboratory. The height of the mercury 
column in the barometer tube may be read by a vernier to 0.01 mm. 
The barometer has been standardized by the Weather Bureau. 
MEASUREMENT OF TEMPERATURE OF THE AIR 
The temperature of the total mass of air in the chamber is not so 
easily determined as its pressure. Even when the walls of the chamber 
are at uniform temperature and no heat is generated in it, the tempera¬ 
ture of the air may not be uniform in all parts of the space. When heat 
is being generated in the chamber and is being absorbed and removed 
as fast as it is generated, so as to maintain constancy in what is assumed 
to be the average temperature, there is a considerable difference between 
the temperature and the consequent density of the air near the source 
of heat and that of air near the heat absorber. It seems reasonable to 
suppose, however, that with the tendency of warm air to rise and of cold 
air to fall, and particularly with the vigorous agitation of the air of the 
chamber by the electric fan, the warmer and colder volumes of air will 
be very rapidly mixed, and more or less complete uniformity of tempera¬ 
ture quickly established throughout the whole mass of air. 
The temperature of the air of the chamber is measured by means of 
an electric-resistance thermometer. The method of measurement em¬ 
ployed is based upon the fact that the resistance of a wire of pure metal 
to an electric current changes definitely with a change in its temperature 
and also that the resistance of the wire, and particularly its change in 
resistance, whether large or small, due to corresponding changes in tem¬ 
perature, may be measured with extreme accuracy by means of a suita¬ 
ble Wheatstone bridge. The device used in the respiration calorimeter 
comprises specially mounted bare nickel resistance wire in the chamber, 
connected with a special Wheatstone bridge, called the “temperature 
indicator,” on the observer’s table (PI. XXXVI, fig. 1). 
The nickel wire, the resistance of which varies with changes in the 
temperature of the air of the chamber, is in six coils of equal resistance, 
each of which is mounted in a rectangular wooden frame about 10 by 13 
cm. that is suspended in the air about 4 cm. from the wall of the chamber, 
on supports attached to the wall. The wire is stretched across the space 
in the frame between two slender wooden rods about 5 cm. apart, with 
successive strands of the coil about 5 mm. apart. Since very little of 
the wire is in contact with the support, it is but little, if at all; affected 
by the temperature of the frame, the object of the construction being to 
eliminate lag in the action of the thermometer. The exposed wire very 
rapidly acquires the temperature of the air of the chamber, and hence 
responds instantly to any changes in it. The six coils are distributed on 
the walls and ceiling in different vertical and horizontal positions, to 
