28 



CALORIMETERS FOR STUDYING RESPIRATORY EXCHANGE, ETC. 



eter bulb from the center of the tube to a point near the edge, the water, 

 which at the edge may be somewhat warmer than at the center, immediately 

 affects the thermometer. By use of the vacuum jacket mentioned above, 

 this warming of the water has been avoided, and in electric-resistance ther- 

 mometers special precautions are taken not only with regard to the relative 

 position of the bulb of the mercury thermometer and the resistance ther- 

 mometer, but also with regard to the hard-rubber insulation, to avoid errors 

 of this nature. 



ELECTRIC-RESISTANCE THERMOMETERS. 



Electric-resistance thermometers are used in connection with the respira- 

 tion calorimeter for several purposes : first, to determine the fluctuations in 

 the temperature of the air inside the chamber; second, to measure the 

 fluctuations of the temperature of the copper wall of the respiration cham- 

 ber; third, for determining the variations in body tem- 

 perature ; finally, for recording the differences in tempera- 

 ture of the incoming and outgoing water. While these 

 thermometers are all built on the same principle, their 

 installation is very different, and a word regarding the 

 method of using each is necessary. 



AIR THERMOMETERS. 



The air thermometers are designed with a special view 

 to taking quickly the temperature of the air. Five ther- 

 mometers, each having a resistance of not far from 4 ohms, 

 are connected in series and suspended 3.5 centimeters from 

 the wall on hooks inside the chamber. They are surrounded 

 for protection, first, with a perforated metal cylinder, and 

 outside this with a wire guard. 



The details of construction and method of installation 

 are shown in fig. 15. Four strips of mica are inserted into 

 four slots in a hard maple rod 12.5 centimeters long and 

 12 millimeters in diameter, and around each strip is wound 

 5 meters of double silk-covered pure copper wire (wire-gage 

 No. 30). By means of heavy connecting wires, five of these 

 thermometers are connected in series, giving a total resist- 

 ance of the system of not far from 20 ohms. The ther- 

 mometer proper is suspended between two hooks by rubber 

 bands and these two hooks are in turn fastened to a wire 

 guard which is attached to threaded rods soldered to the inner surface of the 

 copper wall, thus bringing the center of the thermometer 3.4 centimeters 

 from the copper wall. Two of these thermometers are placed in the dome 

 of the calorimeter immediately over the shoulders of the subject, and the 

 other three are distributed around the sides and front of the chamber. This 



Fig. 15.— Detail of 

 air-resistanae 

 thermometer, 

 showing method 

 of mounting and 

 wiring the ther- 

 mometer. Parts 

 of the wire guard 

 and brass guard 

 are shown, cut 

 away so that in- 

 terior structure 

 can be seen. 



