Probe for attaching at certain times to the tool spindle of a machine tool, which probe emits by means of a stylus (5) a measuring signal which is transmitted via a transmission channel (8) to a receiving and evaluating device (9) of the machine tool. For power-saving purposes, the transmission channel (8) is kept in the functioning state, by means of an activation system contained in the probe, only during the actual measuring operation. The activation system comprises a switching device (15) and an electronic assembly (13), which is characterized in that, from when it reaches its switching position, a control check is carried out with respect to continuance of the switching position and the activation or deactivation of the transmission channel (8) is only triggered when the switching position continues uninterruptedly for the duration of a certain interrogation time.