330 



Journal of Agricultural Research 



Vol. V, No. 8 



sorber is shown in Table I, which summarizes data obtained in an alcohol 

 check test (see p. 342) of the calorimeter made in January, 191 5, which 

 continued for two consecutive periods of three hours each. 



Table I. — Comparison of data for heat measurement obtained by use of temperature 

 difference recorder and of thermoelement with potentiometer 



In order that the recording device may continue to measure tempera- 

 ture differences with the accuracy required, not only must the bridge be 

 sensitive to a change as small as 0.002 per cent in the resistance of the 

 thermometer coils, but also the resistances of the various parts of the 

 bridge circuit other than the thermometers must not change as much as 

 0.003 per cent. Provision is made for testing the component parts of 

 the bridge by the substitution of duplicate parts, which are mounted 

 with the ratio coils of the bridge in a check box, and tests of this character 

 are made at frequent intervals. After the apparatus had been in use for 

 a short time a very slight change in one of the ratio coils was detected 

 and corrected. Since that time the bridge has remained remarkably 

 constant. It is possible also to test with the check box and recorder 

 whether the thermometer coils remain alike in resistance at the same 

 temperature. Provision is made in the check box for correcting sHght 

 inequalities in them by a variable shunt across a coil of small resistance 

 in series with one of the thermometers. 



PREVENTING TRANSFERENCE OF HEAT THROUGH THE WALLS OF THE 



CHAMBER 



In order that the quantity of heat produced in the chamber may be 

 accurately measured, either there must be no increase or decrease in it 

 due to the passage of heat through the walls of the chamber, or if heat 

 is thus added or subtracted, the quantity must be determined and allow- 

 ance made for it. This calorimeter is constructed and operated in 

 accordance with the method employed in the original calorimeter of 



