When switch 1 is closed, electromagnet coil 2 is energized and
a magnetic field is set up around it. The direction of the lines
of force in the field inside coil 2 is opposite to th at of the lines
of force in the field of permanent magnet 3. As a result of the
algebraic a d d itio n of the lines of force of the two fields, magnet
3 is turned clockwise about fixed axis A. H and a of the in strument,
rig id ly mounted on magnet 3, also turns. When the
direction of the current in coil 2 is reversed, the direction of the
lines of force in the field inside coil 2 is also reversed, b u t the
direction of the lines of force in the field of permanent magnet 3
remains as before. In this case, magnet 3 and hand a turn counterclockwise.
The deviation of hand a from the zero position is
proportional to the force of interaction of the fields of coil 2
and magnet 3, i.e. to the current in coil 2.
$4370$SmE,M$