relays are explained in this paragraph before proceeding to a detailed discussion of the switchboard circuits which they
b. Definition of relay. A relay, as used in telephone circuits, is an electrically operated switch by means of
which one switchboard circuit can be made to control the operation of one or more other switchboard circuits; in some
cases the use of a relay enables a circuit to regulate or control its own operation. The basic operation of a relay is similar,
therefore, to that of a manually operated switch, except that a relay is automatic in its operation.
c. Structure of typical relay.
(1) Although there are many different types of relays, differing in the details of their construction, operation, and
application, the principle of operation is the same for all, and structural features are similar. Consider therefore what
might be called a typical relay, illustrated in figure 49.
(2) The essential components of any relay include an armature, the motion of which opens and closes the circuit
or circuits to be controlled, and one or more windings, through which the control current flows. The core on which the
windings are placed is composed of a magnetic material such as silicon steel or permalloy. The number and types of the
windings depend on the particular function of the relay.
(3) Associated with the armature are an armature spring and one or more contact springs. Each spring has one or
more contacts made of pure silver, some silver alloy, or alloys or platinum or other metals. The contacts are arranged to
be either open normally, as in figure 49, or closed normally; or, in relays with several contacts, some contacts may be
open normally and others may be closed normally.