A simple technique has been proposed and tested in a dc plasma machine to determine the second derivative of a Langmuir probe characteristic. The principle of current continuity was used to arrange the current sampling network of a group of three probes so that the second‐order difference was achieved without any active arithmetic circuits. In this way, the second derivative signal of the probe characteristic is obtained automatically and accurately. The error remaining in this technique can be monitored easily and then minimized to a tolerable extent.