All electromechanical relays have a coil of wire with a rod passing through the middle (to form an electromagnet), an armature, and one or more sets of contacts. When the coil is energized, the electromagnet attracts one end of the armature mechanism, which in turn moves the contacts.
The figure below shows a typical electromechanical relay with the main operating elements labeled and a scheme for actuating the armature.
A typical coil must generate a strong magnetic field to completely actuate the relay. Several parameters relate to the force necessary to actuate the relay, among them:
Spring tension of the armature
Spacing of the contacts
The mass of the armature mechanism.
Electromechanical Relay