juiy 3 ii9i& Respiration Calorimeter 709 



In the computation of the quantities of water vapor, carbon dioxid, 

 and oxygen in the atmosphere of the chamber the actual volume of air in 

 the chamber must be known, and this depends upon the capacity of the 

 chamber under standard conditions of temperature (o°C.) and of baro- 

 metric pressure (760 mm. of mercury) and the actual temperature and 

 pressure of the air at the time the samples were taken. The barometric 

 pressure of the air of the chamber, because of the tension equalizer, is 

 always the same as that of the laboratory, which is determined to an 

 accuracy of 0.01 mm. by means of a standardized barometer. The tem- 

 perature of the air of the chamber is measured by means of an electric- 

 resistance thermometer with the sensitive portion in the chamber and a 

 temperature indicator outside. Either of two thermometers is available, 

 one consisting of a single unit and the other of three units in series, which 

 are modifications of the type of thermometer developed by Dickinson and 

 Miiller. 1 They are very sensitive and follow temperature changes 

 rapidly. The single unit consists of a coil of nickel wire having a re- 

 sistance of about 20 ohms at 20 C, wound on a very thin strip of mica, 

 placed between two similar strips, and inclosed in a flat case of thin 

 copper pressed firmly against the mica. The portion of the case which 

 incloses the coil is about 15 cm. in length, 13 mm. in width, and less than 

 1.5 mm. in thickness. The case terminates at the top in a short tube, 

 through which the leads are extended to the resistance wire, being sealed 

 in the tube with a hard wax to exclude moisture from the interior of the 

 case. Each of the three units in series is constructed like the one just 

 described, except that it has only one-third the total amount of resistance 

 wire; hence, the unit is shorter, the other dimensions being the same. 



The leads from the resistance thermometer coils pass through the 

 "outlet" mentioned on page 704 and extend to a multiple-switch (PI. 

 XCIV), by which either the single or the triple thermometer may be con- 

 nected with the temperature indicator, which does not appear in any of 

 the views shown. The latter device consists of a Wheatstone bridge 

 having a slide wire by which the bridge circuit may be kept in balance 

 with the thermometer coils at any temperature between o° and 50 C. 

 The readings of the bridge scale, when translated into temperature by 

 means of a calibration curve, show changes to o. i°. The effect of the 

 resistance of the thermometer leads and of their change in resistance, due 

 to change in temperature, is neutralized by compensating leads from the 

 opposite side of the bridge, so that the measurements by means of the 

 bridge are of a high order of accuracy; although, because of the small 

 volume of air in the chamber, absolute accuracy of these determinations 

 is of less significance than in experiments with the larger respiration 

 apparatus. 2 



1 Dickinson, H. C, and Mueller, E. F. New ealorimetric resistance thermometers. In U. S, Dept 

 Com., Bur. Standards Bui., v. 9, no. 4, p. 483-492, 2 fig. 1913. 



2 I.angworthy, C. F., and Milner, R. D. Op. cit., p. 312. 



42721°— 16 4 



