VI. CALORIMETRIC MEASUREMENTS 195 



E V = nV. + ;-^ (^V^ +Y.T- nTA (17) 



1 z — I „ \ Z 1 / 



n 



where ^V = total tempemture loss in main period = temperature 







correction, n = number of intervals in main period (as a rule, number 

 of minutes), Va = mean rate of temperature loss in preliminary pe- 

 riod (degrees Centigrade per minute), Fj = mean rate of temperature 

 loss in after period, Ta = mean temperature of calorimeter water dur- 

 ing preliminary period, T^ = mean temperature of calorimeter water 

 during after period. To = temperature of calorimeter water at ignition 

 time, Tn = last temperature reading of main period, and T = tem- 

 perature of calorimeter during main period. This is the calorimeter 

 correction formula of Regnault-Pfaundler. For its derivation one 

 has to assume that the environmental temperature Tg, which in a 

 well constructed calorimeter is equal to the temperature of the jacket, 

 remains constant. This constancy may be approximated sufficiently 

 by using jackets of sufficient heat capacity. The requirement may be 

 estimated as follows: 



One may assume for an ordinary bomb calorimeter a cooling coef- 

 ficient of 0.002° per minute per degree Centigrade (unpublished result 

 by author). For a duration of five minutes and an increase in tem- 

 perature of the calorimeter of 5° the temperature loss would amount 

 to 0.05°C. Assuming a heat capacity of the calorimeter of 2.5 kcal. 

 per degree Centigrade, the temperature loss represents a heat loss of 

 0.05 X 2500 = 125 cal. If this amount of heat were to raise the 

 temperature of the jacket only 0.001°, the jacket should have a heat 

 capacity of 125 kcal. (125/0.001) or an equivalent of 125 liters of 

 water. If a change of 0.005°, w^hich amounts to 0.1% of the result, 

 is allowable, a jacket of 25 liters capacity will do. 



Errors from inaccurate assumptions, such as the constancy of en- 

 vironmental temperature, may be eliminated to a considerable ex- 

 tent if the calorimeter is calibrated by combusting a reference sub- 

 stance under conditions similar to those prevailing during subsequent 

 measurements. Benzoic acid is supplied for this purpose by the 

 National Bureau of Standards. Salicylic acid is used in the Swiss 

 Institute for Fuel Investigations. 



A more elaborate discussion of the Regnault-Pfaundler formula 

 and its improvements is found in White's monograph (33). For most 

 determinations of heats of combustion by bomb calorimetry the old 

 formula is sufficiently accurate. 



