Abstract: An apparatus for measuring or monitoring changes in the periphery of a vessel, especially a pressure vessel for containment of a nuclear-reactor core, comprises a wire which extends around the periphery of the vessel and is held along this periphery by a plurality of mechanical elements establishing a predetermined spaced relationship between the wire and the periphery. The wire extends polygonally around these elements and is provided with a displacement-measuring device responsive to any tendency of the wire to shift from its original position. According to the invention, the connecting elements between the wire and the periphery of the vessel are constituted as sheet-metal plates or vanes lying in planes substantially perpendicular to the axis of the wire and extending radially with respect to the axis of the vessel. The vanes are elastically deformable laterally or shiftable in the direction of the axis of the wire and are retained under tension by the tension applied to the wire.