Abstract: A caliper logging sonde is equipped with a plurality of fingers (54) mounted swivelably with respect to its body member (51). The movement of each finger (54) is transmitted to a respective bar (176) in magnetic material. This bar moves within a longitudinal tube (136) which is mounted in a sealed manner with respect to the sonde body member so that its interior is subjected to the pressure outside the sonde and so that the space around this tube is not subjected to this pressure. Around the tube is mounted a transducer winding (150) whose self-inductance depends on the degree of penetration of the bar (176) and hence on the displacement to be measured. A measurement of this self-inductance is obtained by the study of the variation of a current going through the coil when a voltage transient is applied to the coil terminals.