jniy 3 i,i9i6 Respiration Calorimeter 713 



MEASURING THE TEMPERATURE INCREASE 



In the passage of the water through the heat absorber its temperature 

 will increase according to its rate of flow and the quantity and activity 

 of the material in the chamber. The accuracy with which the amount of 

 heat carried from the chamber in the water current is measured depends 

 upon that with which the temperature increase is determined. This is 

 accomplished by means of electric-resistance thermometers and an 

 automatic temperature recorder (PI. XCV), in some respects similar to 

 and in others different from that employed in connection with the large 

 respiration calorimeter. 1 



In construction and characteristics the resistance thermometers are 

 identical with those in the large calorimeter. They consist of two coils 

 of platinum wire of equal resistance, which is about 25.5 ohms at 20 C, 

 and have exactly the same coefficient of change in resistance with change 

 in temperature, the resistance change of each being 0.1 ohm per degree. 

 In each the resistance coil is encased in such a way that it is brought into 

 intimate thermal contact with the flowing water and responds instantly 

 and accurately to any change in its temperature. The water channels 

 in which the resistance coils are installed are fitted into openings in the 

 outlet described on page 704, to provide passage through the walls of 

 the chamber for the ingoing and outgoing water, so that one coil acquires 

 the temperature of the water just entering and the other that of the 

 water just leaving the chamber. 



The thermometers comprise two arms of a special Wheatstone bridge 

 on opposite sides of a slide wire by which the bridge may be balanced for 

 any inequality in the resistance of the two coils between 0.00 1 ohm and 

 0.2 ohm, resulting, respectively, from temperature differences of 0.01 

 and 2 between the ingoing and the outgoing water. The wire is cali- 

 brated so that temperature differences may be read directly from the 

 scale. The total range of the instrument may be extended to indicate 

 a difference as large as 5 . By means of resistance coils that may be 

 connected in series with the slide wire as needed, the position of the 

 balancing contact on the lower end of the wire may be made equivalent 

 to a difference of i°, 2 , or 3 between the thermometer coils, and the 

 upper end 2 higher in each case. 



The slide wire is incorporated in a mechanism which automatically 

 balances the bridge for inequalities of resistance in the thermometers, 

 and at the same time makes a graphic record of the balancing operations 

 in terms of temperature difference and of time. The wire is mounted 

 on the edge of a disk which is rotated to balance the bridge, while the 

 balancing contact point remains fixed. The rotation of the disk, which 

 is due to the action of one or the other of two cams on a shaft driven by 



1 Langworthy, C. F., and Milner, R. D. Op. cit., p. 326. 



