There is another configuration of relay which was commonly used for remote switching at one time. Instead of the control circuit connecting power to a coil to energize the relay when needed, power is kept applied to the coil all the time, but the flux is designed to be too weak by itself to operate the contacts.
A second coil is then connected completely independently to a control circuit. When the leads from that coil are shorted together, current flows due to the primary winding and core acting like a transformer. The increased flux then provides enough pull to operate the contacts.
Obviously that type of relay can be used only with A.C. on the coil.
