Movable cy lin d ric al coil /, wound with a large number of turns of copper
wire, moves in the radial magnetic field set up in the air gap of permanent
magnet 2 in accordance with the axial motion of stylus 3, rig id ly
secured to coil 1. Stylus 3 is moved at constant speed along surface a
being inspected. This induces an emf in coil /, which is proportional to
the velocity of its displacement. This emf in the coil is transmitted through
amplifier 4 to electric measuring and recording instrument 5.
$441$SmE,M$