Nov. 22, 1915 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 o.oi mm. 

 The barometer has been standardized by the Weather Bureau. 



MEASUREMENT OP 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. i). 



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 difi'erent vertical and horizontal positions, to 



