52 CALORIMETERS FOR STUDYING RESPIRATORY EXCHANGE, ETC. 



Under ideal conditions of manipulation, the withdrawal of heat from the 

 calorimeter should be at just such a rate as to exactly compensate for the 

 heat developed by the resistance-coil. Under these conditions, then, there 

 would be no heat abstracted from nor stored by the calorimeter and its 

 temperature should remain constant throughout the whole experiment. 

 Practically this is very difficult to accomplish and there are minor fluctua- 

 tions in temperature above and below the initial temperature during a long 

 experiment and, indeed, during a short experimental period. If a certain 

 amount of heat has been stored up in the calorimeter chamber or has been 

 abstracted from it, there should be corrections made for the variations in 

 the temperature of the chamber. Such corrections are impossible unless a 

 proper determination of the hydrothermal equivalent has been made. A 

 number of experiments to determine this hydrothermal equivalent have 

 been made and the results are recorded beyond, together with a discussion 

 of the nature of the experiments. As a result of these experiments it has 

 been possible to make correction for the slight temperature changes in the 

 calorimeter. 



It is interesting to note that these fluctuations are small and there may 

 therefore be a considerable error in the determination of the hydrothermal 

 equivalent without particularly affecting the corrections applied in the 

 ordinary electrical check-test. The greatest difficulty experienced with the 

 calorimeter as a means of measuring heat has been to secure the average 

 temperature of the ingoing water. The temperature difference between the 

 mass of water flowing through the pipes and the outer wall of the pipe is 

 at best considerable. The use of the vacuum-jacketed glass tubes has min- 

 imized the loss of heat through this tube considerably, but it is advisable 

 that the bulb of the thermometer be placed exactly in the center of the 

 water-tube, as otherwise too high a temperature-reading will be secured. 

 When the proper precautions are taken to secure the correct temperature- 

 reading, the results are most satisfactory. 



In testing both calorimeters a large number of electrical check experi- 

 ments have led to the conclusion that discrepancies in results were inva- 

 riably due, not to the loss of heat through the walls of the calorimeter, but 

 to erroneous measurement of the temperature of the water-current. 



DETERMINATION OF THE HYDROTHERMAL EQUIVALENT OF THE 



CALORIMETER. 



While the temperature control of the calorimeter is such that in general 

 the average temperature varies but a few hundredths of a degree between 

 the beginning and the end of an experimental period, in extremely accurate 

 work it is necessary to know the amount of heat which is absorbed with any 

 increase in temperature. In other words, the determination of the hydro- 

 thermal equivalent is essential. 



