Abstract: An instrument for measuring the dimensions of a rigid object (10) comprises a fixed member (1), a mobile member (2) slidably mounted on the fixed member, means for indicating the resulting measurement, and a locking device (3) allowing the mobile member (2) to be moved on the fixed member (1) to hold two bearing points (X, X') on the members in contact with the object (10) being measured with a substantially constant bearing force. The locking device comprises a first sleeve (20) slidably mounted on the mobile member (2) under the action of springs (24), and a second sleeve (30) slidably mounted on the fixed member (1) and inside which is a tipping ring (36) connected by a rod (40) to the first sleeve. The second sleeve (30), by acting on the first sleeve (20), enables loading the springs (24) to produce the bearing force when the instrument is in contact with the object (10) being measured.