A sinusoi¨dal temperature modulation method has been developed to provide direct access to the derivative of the resistance with respect to temperaturedR/dT. This quantity is of a high physical interest since it gives information on the critical behavior of the specific heat of magnetic materials near the critical points. The original aspect of such an experimental device is the high sensitivity of its detection. Indeed it provides a resolution &Dgr;R/Rof 10−9withR=3 m&OHgr;. Finally, we demonstrate the high accuracy of this technique with experimental measurements ofdR/dTof a rare‐earth single crystal.