(1) A relay is really an electromagnet when current flows through its winding. The magnetic field around the
core causes the armature to be attracted to the core. As it moves on its pivot, the armature causes the armature spring to
be pushed up, moving the contact springs to open or close the various circuits controlled by the relay. When the circuit of
the relay winding is broken, the magnetic field collapses, permitting the armature to spring back to its original position
and restoring the relay contacts and springs to their normal position.
(2) The number and arrangement of contact springs which a relay may have depend on its application in a
particular circuit, and on the power available to move the armature and the springs. The force with which the armature is
attracted is proportional to the strength of the magnetic field, which, in turn, depends on the number of ampere turns of the
electromagnet. This can be made large by using either a winding of many turns and a small current, or a winding of fewer
turns and a large current.
(3) When a relay is connected in series with other circuit components, the current available to operate the relay
depends on the total resistance of all the components in series, and tends to be relatively small. The winding (or
windings) of the relay therefore should have a relatively low resistance but a sufficient number of turns to operate the
relay properly. When relays are connected across the battery, however, it is necessary to "increase" the resistance of the
windings in order to limit the current to a safe value. This usually is done by adding some German silver wire, or some
high-resistance wire, to the regular relay windings.
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