The terminations of the cores of field or other electro-magnets, or of
permanent magnets. These terminations are variously shaped, sometimes
being quite large compared to the core proper of the magnet.

They are calculated so as to produce a proper distribution of and
direction of the lines of force from pole to pole. As a general rule the
active field should be of uniform strength and the pole pieces may be of
contour calculated to attain this end.