The measurement feelers (15) are operated by a movable member consisting of a rod (17) which is movable axially and upon which can slide a cylinder (11a) subjected to the action of a main spring (13) which forces it towards its position of bearing against a stop (19) integral with the rod (7a). This rod is furthermore under the influence of a secondary spring (16) which is weaker and acts in the opposite sense to the main spring. The trigger (17) which enables the cylinder (11a) to be acted upon may be actuated as far as the end of its travel without the rod (6) of the comparator attached to the body (1,2) of the instrument ever undergoing a thrust stronger that the force of the secondary spring (16). Thus the disadvantage is avoided of direct action of the trigger upon the portion of the movable member which enters into action against the feeler (10) of the comparator and consequently can cause the slipping of this rod in the tightener bush (4) and resultant errors in reading.