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The construction of an adiabatic calorimeter and its use in measuring specific heats Swanson, Max Lynn


A fluidless adiabatic calorimeter was constructed and was used to measure the specific heats of manganese-aluminum-carbon and manganese-zinc-carbon alloys from -150° to 150°C. In an adiabatic calorimeter, the temperature of a shield surrounding the calorimeter vessel is kept at approximately the same temperature as that of the vessel, so that the thermal leakage between the two is reduced to a negligible quantity. Thus the ordinary rating period, in which the thermal leakage modulus is calculated, can be eliminated. Since leakage modulus variations are reduced by the adiabatic method, it can be used for large temperature rises, resulting in fast and accurate measurements. The aneroid (fluidless) adiabatic calorimeter eliminates stirring and evaporation errors, and makes possible measurements at extreme temperatures. The calorimeter consisted of a cylindrical silver-plated copper vessel surrounded by an electrically heated adiabatic shield and an evacuated outer case. A platinum resistance thermometer-heater was used to supply heat to the calorimeter vessel and to measure the vessel temperature. The heat input and the thermometer resistance were measured by using a potentiometer in conjunction with standard resistances. The thermometer was calibrated by measuring its resistance at -183, -40, 0, and 100°C. The calorimeter was calibrated from -150 to 150°C. The accuracy of the calorimeter was approximately 0.5%, the main error arising from the method of measuring the temperature of the calorimeter vessel. The specific heat curves of the single phase magnetic alloys Mn₃AIC and Mn₃ZnC were measured. A second order specific heat anomaly was found, as expected, for the ferromagnetic alloy Mn₃AlC at its Curie point, -10°C. Although the anomaly was close to the theoretical shape, dropping to zero over only a 10°C range at the Curie point, its maximum height was less than saturation magnetization measurements would indicate. The alloy Mn₃ZnC showed second order specific heat anomalies at -35°C, and at 65°C. This double specific heat anomaly indicates, in agreement with neutron diffraction results, a complex magnetic behaviour for the alloy. Although the high temperature Curie point anomaly did not have a sharp peak, the low temperature anomaly's shape approached that of the theoretical Weiss curve.

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