a precision electromechanical measuring device utilizes a strain gauge or pressure transducer to sense changes of position in a workpiece contacting finger . the finger is pivotable about a relatively stiff pivot to different positions , enabling the instrument &# 39 ; s use as a center finder for circular bores and pins or for verifying the flatness and perpendicularity of a disc or plate to the axis of rotation of a spindle . a housing of the device supports a rotatable shank extending from one end for engagement in a chuck , mandrel or collet associated with a spindle of a machine , and also supports pivotal apparatus including the finger extending out the other end of the housing . brushes conduct a variable signal from the strain gauge in the housing to the exterior , where the signal is fed to an ammeter . in use as a center finder , the device is fixed in a chuck or mandrel , with a workpiece approximately alligned with the center of rotation of the mandrel , the finger is engaged against the interior of a bore or the exterior of a pin of the workpiece , and the machine spindle is power - rotated to sweep the finger around the bore or pin . variations in the reading of the ammeter indicate eccentricity and direction and degree of eccentricity .