Relay, Polarized.
A relay whose armature is of steel, and polarized or permanently
magnetized, or in which a permanent magnet is used as the basis for the
electro-magnets. In the relay shown in the cut the coils shown are
mounted on cores carried on the end of a powerful bent permanent magnet.
Thus when no current passes their upper poles are both of the same sign,
and the horizontally vibrating tongue is held by the magnetic attraction
against one or the other pole piece. If a current is sent through the
electro-magnet it gives opposite polarity to the two polar extensions.
As the end of the vibrating tongue is of polarity determined by the
permanent magnet it is attracted to one pole and repelled from the
other. On cessation of current it remains attached by the permanent
magnetism. If now a current is sent in the opposite direction the two
poles again acquire opposite polarity, the reverse of the former, and
the tongue flies across to the opposite side. On cessation of current it
remains attached as before by the permanent magnetism.
In its movements to and fro the relay tongue opens and closes a contact,
so as to work a sounder or other apparatus. The polarized relay is of
high sensibility, and requires little or no change of adjustment.