i 4 4 



A RESPIRATION CALORIMETER. 



BATTEFV. 



or that in the ventilating air current. It will be noticed that systems 

 No. i and No. 2 unite at the left of T in system No. i , and thus one 

 wire serves for the return from both systems. 



The connections between the several thermal junction systems and 

 the galvanometer are relatively simple ; it is with the bridge system for 

 temperature measurements that the electrical connections are the most 

 complicated. For these measurements a type of Wheatstone bridge, 

 illustrated by the diagram in figure 44, is used. 



In the simple form of Wheatstone bridge shown in figure 44 there 

 are in fact four parts two 2o-ohm resistance coils, a standard resist- 

 ance, and the coil of copper wire whose temperature fluctuations (varia- 

 tions in resistance) are to be measured. 

 The battery is connected at the two 

 points B' and B". The galvanometer 

 connections are made at G' and G". 



The two 2o-ohm resistance coils A 

 and B are made of a form of wire that 

 has no temperature coefficient, /. e. , 

 there are no changes in electrical con- 

 ductivity due to changes in temper- 

 ature. These two coils are calibrated 

 with the greatest accuracy, so that the 

 resistance of the coils and connections 

 between B' and G' is exactly equal to 

 those between B' and G"; thus A and B 

 correspond to the proportional arms of 

 a Wheatstone bridge. These two coils 

 are shown in the diagram, figure 43, 

 being marked 2o-ohm res. Their position on figure 40 is likewise clearly 

 seen. The iron post between these two coils corresponds to the point B' 

 of figure 44. The points B" and G' and G" are not so readily discerned 

 on the drawing in figure 43, owing to the complex nature of the elec- 

 trical connections. 



The other two arms of the Wheatstone bridge (fig. 44) are com- 

 posed of a standard coil, D, made of wire similar to that used in coils 

 A and B and having approximately 20 ohms resistance. This coil is 

 used for comparison with the unknown resistance of the copper ther- 

 mometer coil and connections, which correspond to coil C in figure 44. 

 As this coil (for example, the copper thermometer used for measuring 

 the temperature fluctuations of the air in the calorimeter chamber 

 changes in resistance, and obviously may rarely be exactly equal in 

 resistance to coil D, there is a disturbance of the equilibrium of resist- 



FiG. 44. Diagram of simple form of 

 Wheatstone Bridge. A current from a 

 battery passes through an adjustable 

 resistance and connects with the bridge 

 at points B' and B". When all arms of 

 the bridge are proportional, no current 

 flows through the galvanometer G. If 

 C is greater or less in resistance than D, 

 the current passes through G. 



