Abstract: The displacement sensor employing a magnetoresistive effect laminate structure consisting of a bottom ferromagnetic layer with a field oriented in one direction, a middle non-magnetic layer, and a top softer magnetic layer. The top softer magnetic layer has two regions of opposing magnetic fields separated by a domain wall, where the magnetic field in one region is aligned with the bottom layer magnetic field. The resistance of the laminate structure changes as the location of the domain wall changes. In one improvement, the domain wall is induced by a pair of opposing semicylindrical magnets magnetized along their semicylindrical surface, forming parallel field lines and leading to a stronger domain wall. In another improvement, there are four laminate structures in an electrical bridge configuration. The bridge is excited by an alternating current source and the output is synchronously measured.

Abstract: A bore gauge centralizer comprising a frame having a pair of parallel legs connected by a cross piece. One end of the frame is pivotally mounted to the gauge head so that the legs are generally parallel to the longitudinal axis of the gauge and the ends of the legs pivot in unison away from the head. A spring biases the leg ends away from the head. A roller is rotatably attached to the each leg end. The roller is a truncated solid oval so that a single contact point is made with the bore wall regardless of the position of the frame.

Abstract: A dial indicator comprising a body having an dial face with marked graduations; a longitudinally reciprocating rod extending from the body, the rod having a standard rack internal to the body and an end of the rod external to the body having a contact surface; an interacting series of gear assemblies inside the body, one of the gear assemblies having a crowned pinion, the crowned pinion being driven by the rack; and a dial hand attached to the axle of another of the gears; whereby longitudinal pressure on the contact surface causes the rod to retract longitudinally into the body, the distance of retraction being indicated by the dial hand on the dial face.