VI. C A L O I? I M K T a 1 C M E A S U R E M E N T S 189 



4. Differential Caloriiiietry 



Instead of measuring the heat flow from a calorimeter chamber as 

 described above, one may match this flow by an equal flow in a quasi- 

 equal second chamber, the "compensating chamber." The rate of 

 heat production in the calorimeter chamber is then measured by the 

 rate of heat production in the compensating chamber necessary to 

 produce an equal "thermal head," Ti — Ts- The arrangement of a 

 differential calorimeter, also called a compensating calorimeter, is il- 

 lustrated in Figure 2. 



Since, by definition of "quasi-equal conditions" for both chambers, 

 Ts is the same for both, it is only necessary to match the two inside 

 temperatures. A galvanometer is connected to thermoelectric junc- 

 tions distributed on the inside walls of both chambers. It serves as a 

 null instrument, whose sensitivity may be very high. The heat flow 

 in the compensating chamber is usually produced by an electric cur- 

 rent whose intensity and potential drops may be measured very ac- 

 curately. The rate of heat production in the calorimeter chamber is 

 then calculated as: 



AQ/M = 0.239iE (13) 



where AQ/ At is the rate of heat production in calories per second, 

 i is the electric current in amperes, and E is the potential difference 

 in volts. 



Detailed discussions of differential calorimetry are found in the 

 literature {20,21). For experiments on a large scale a major source of 

 error appears to be the difficulty in making the thermal conditions in 

 the two chambers equal. If this equality for two chambers, used 

 simultaneously, is not more accurate than the equality of the thermal 

 conditions of the same chamber used at different periods, ordinary 

 calorimetry may be as accurate as differential calorimetry. For an 

 appraisal of the differential method in microcalorimetry the reader 

 should consult the recent book by Swietoslawski {17). 



5. Prevention of Heat Leaks 



Instead of measuring the heat flow through the walls of the calorim- 

 eter, one may prevent this flow or decrease it to a small fraction of 

 the measured heat, which is then called a "heat leakage." 



For experiments of long duration and relatively steady rate of 

 heat production, heat leakage may be reduced to an insignificant rate 



