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A circuit is described for the measurement of the temperature difference between input and output heat exchangers by means of resistance thermometers. Two thermometer resistors form part of a resistance-to-frequency converter, the frequency being proportional to their difference in resistance value. The frequency is also dependent on a few additional resistors and a capacitor. Drifts and offsets of amplifiers are eliminated to a first order. No stabilized voltages are used. The circuit was developed as part of a simple and reliable calorimeter for domestic uses.