When the coil of electromagnet 1 is energized by alternating
current, armature 2, carrying flexible tip a, vibrates. When
workpiece b, to be inspected, is advanced to measuring tip a of
armature 2, the ratio of the period of the closed state of the
contacts they constitute to the period of the open state depends
upon the size of workpiece b. The armature tip is connected to
the grid of an electron tube, and a negative po tential is applied
to the workpiece being inspected. The average value of the
plate current in the electron tube can serve as a measure of the
clearance between the measuring tip and the workpiece. An am meter,
connected into the plate circuit of the tube, can be
graduated in units of size of the workpiece.
$4359$SmE,M$