When rotor 1 of generator A is driven by the shaft of the motor
whose speed is to be measured, the emf induced in the w ind in g
of rotor 1 is proportional to its speed of rotation. The voltage of
generator A is applied to an in d ica tin g device— a voltmeter.
Armature 2 of the voltmeter is turned due to interaction between
the electromagnetic field set up by the current in its w ind in g
and the magnetic field of permanent magnet 3. The angle through
which armature 2 and hand a, rig id ly mounted on the armature,
turn is proportional to the voltage over the armature w ind in g
and, consequently, to the speed being measured of rotor 1.
The point of hand a slides along scale d.
$4397$SmE,M$