38 



CALORIMETERS FOR STUDYING RESPIRATORY EXCHANGE, ETC. 



5 



FUNDAMENTAL PRINCIPLE OF THE APPARATUS. 



The apparatus depends fundamentally upon the perfect balancing of the 

 two sides of a differential electric circuit. A conventional diagram, fig. 19, 

 gives a schematic outline of the connections. The two galvanometer coils, 

 fl and fr, are wound differentially and both coils most carefully balanced 

 so that the two windings have equal temperature coefficients. This is done 

 by inserting a small shunt y, parallel with the coil fl, and thus the tempera- 

 ture coefficient of fl and fr are made 

 absolutely equal. The two thermome- 

 ters are indicated as T 1 and T, and are 

 inserted in the ingoing and outgoing 

 water respectively. A slide-wire resist- 

 ance is indicated by J, and r is the re- 

 sistance for the zero adjustment. Ba, 

 Z, and Z 1 are the battery and its variable 

 series resistances. If T t and T 2 are 

 exactly of the same temperature, i. e., 

 if the temperature difference of the in- 

 going and outeoming water is zero, the 

 sliding contact q stands at on the 

 slide-wire and thus the resistance of 

 the system from through fl, r, and T x 

 back to the point C is exactly the same 

 as the resistance of the slide-wire J 

 plus the coil fr plus T 2 back to the 

 point C. A rise in temperature of T 2 

 gives an increase of resistance in the 

 circuit and the sliding contact q moves 

 along the slide- wire toward J maximum 

 until a balance is obtained. 



Provision is made for automatically 

 moving the contact q by electrical 

 means and thus the complete balance 

 of the two differential circuits is maintained constant from second to second. 

 As the contact q is moved, it carries with it a stylographic pen which travels 

 in a straight line over a regularly moving roll of coordinate paper, thus pro- 

 ducing a permanently recorded curve indicating the temperature differences. 

 The slide-wire J is calibrated so that any inequalities in the temperature 

 coefficient of the thermometer wires are equalized and also so that any unit- 

 length on the slide-wire taken at any point along the temperature scale rep- 

 resents a resistance equal to the resistance change in the thermometer for 

 that particular change in temperature. With the varying conditions to be 

 met with in this apparatus, it is necessary that varying values should be 



Fig. 19. — Diagram of wiring of differential cir- 

 cuit with its various shunts, used in con- 

 nection with resistance thermometers on 

 water-circuit of bed calorimeter. 



