THE CALORIMETER. 51 



system in equilibrium, the exact time is noted and the water-current 

 deflected into the meter. At the end of one hour, the usual length of a 

 period, the water-current is deflected from the meter, the meter is weighed, 

 and the average temperature-difference of the water obtained by averaging 

 the results of all the temperature differences noted during the hour. Usu- 

 ally during an experiment of this nature, records of the water-temperatures 

 are made every 4 minutes ; occasionally, when the fluctuations are somewhat 

 greater than usual, records are made every 2 minutes. 



The calculation of the heat developed in the apparatus is made by means 

 of the formula CxExf X0.2385 = calories, in which C equals the current 

 in amperes, E the electromotive force, and t the time in seconds. This 

 gives the heat expressed in calories at 15° C. This procedure we have fol- 

 lowed as a result of the recommendation of Dr. E. B. Eosa, of the National 

 Bureau of Standards. In order to convert the values to 20°, the unit com- 

 monly employed in calorimetric work, it has been necessary to multiply by 

 the ratio of the specific heat of water at 15° to that of water at 20°. As- 

 suming the specific heat of water at 20° to be 1, the specific heat at 15° 

 is 1.001.* 



Of the many electrical check-tests made with this type of apparatus, but 

 one need be given here, pending a special treatment of the method of con- 

 trol of the calorimeter in a forthcoming publication. An electrical check- 

 experiment with the chair calorimeter was made on January 4, 1909, and 

 continued 6 hours. The voltmeter and mil-ammeter were read every few 

 minutes, the water collected in the water-meter, carefully weighed, and the 

 temperature differences as measured on the two mercury thermometers 

 were recorded every 4 minutes. 



The heat developed during the experiment may be calculated from the 

 data as follows: Average currents 1.293 amperes; average E. M. F. 

 = 109.15 volts; time = 21,600 seconds; factor used to convert watt-seconds 

 to calories =0.2385. (1.293x109.15x21600x0.2385) x 1.001 = 727.8 calo- 

 ries produced. 



During the 6 hours 237.63 kilograms of water passed through the absorb- 

 ing system. 



The average temperature rise was 3.04° C, the total heat brought away 

 was therefore (237.63x3.04) x 1.0024 f =724.1 calories. 



Thus in 6 hours there were about 3.7 calories more heat developed inside 

 the apparatus than were measured by the water-current, a discrepancy of 

 about 0.5 per cent. 



* W. O. Atwater and E. B. Rosa : Description of a new respiration calorimeter 

 and experiments on the conservation of energy in the human body. U. S. Dept. 

 of Agr., Office of Experiment Stations Bui. 63. (1899.) 



f Specific heat of water at average temperature of the water in the heat-absorb- 

 ing system referred to the specific heat of water at 20° C. 



