The magnetic tachometer consists of rotary bell-shaped permanent
magnet 1 and core 2, between which there is a narrow gap.
Arranged in this gap is hollow a lu m in iu m cylinder 3, held
against rotation by coil spring 4. The rotating magnetic field
induces currents in cylinder 3 which tend to rotate the cylinder
w ith a force whose torque is proportional to the speed of rotation
of magnet 1. A scale w ith uniform divisions is engraved on
the outside circumference of cylinder 3. It indicates the speed
(rpm) of magnet 1, and of the shaft whose speed is to be measured,
in opening 5.
$4366$SmE,M$