juiy 3 i.igi6 Respiration Calorimeter 717 



resistances of the two parts of the thermometer will differ accordingly, 

 and the pointer of the galvanometer will be deflected, the direction of 

 deflection depending upon whether the outer wall is warmer or cooler 

 than the inner, and the contact point of the rheostat will be shifted so as 

 to increase or decrease the heating of the outer wall and bring it again 

 into thermal equilibrium with the inner wall. 



Thermal equilibrium is maintained in the walls by sections rather than 

 as a whole. The resistance coils on the inner and outer metal walls are 

 grouped so that the top, the sides, and the bottom of the chamber each 

 has its own differential thermometer; and provision is likewise made for 

 heating and cooling each section independently, so that thermal condi- 

 tions in each one may be regulated regardless of those in the others. 

 Furthermore, in order that there shall be no excess of sensible heat car- 

 ried into or out of the chamber in the ventilating current of air, the tem- 

 perature of the air entering the chamber is regulated to accord with that 

 of the air leaving. The units of a differential resistance thermometer 

 are inclosed in the pipes carrying the ingoing and outgoing air through 

 the walls of the chamber. Just before the pipe for incoming air reaches 

 the calorimeter a short section of it is inclosed in an electric heating device 

 to warm the air, while inside the same section of pipe is a small copper 

 tube conducting chilled water to cool the air. As in the control of the 

 temperature of the walls, the water is kept running continuously and the 

 temperature of the air is regulated by varying the electric current in the 

 heater surrounding the air pipe. The four rheostats controlling the cur- 

 rents for heating the top, sides, and bottom outer walls of the chamber, 

 and the ingoing air are adjusted by the same mechanism (PI. XCV), 

 which operates them successively, any changes that are needed in a given 

 rheostat being made once every four minutes. 



The widest difference between the respiration calorimeter described in 

 the present article and the larger one previously described in this journal l 

 is in the method of preventing gain or loss of heat in the chamber. The 

 devices described in the paragraphs above render this apparatus quite 

 largely automatic in its operations as a calorimeter, whereas the other 

 calorimeter is controlled mainly by hand. 



By means of a switch, also operated by the mechanism, the galvan- 

 ometer which governs the action of the regulating mechanism upon the 

 rheostats is connected successively across each of the four Wheatstone 

 bridges of which the differential thermometers are integral parts, each 

 pair of thermometers being combined with its own ratio coils to form a 

 bridge. These four sets of coils are mounted in the same case (PI. XCV) 

 in such a manner that the permanence of resistance of each may be 

 easily tested. The coils in each pair may be transposed by changing the 

 position of two plugs, whereupon the galvanometer deflection will alter 



Langworthy, C. !•'., and Milner, R. D. Op. cit., p. 326. 



