What is the principle of electromagnetic relay

A relay is an electro-mechanical switch, which is made of lacquered copper wire wound hundreds to thousands of times on a round iron core. When the current flows through the coil, the round iron core generates a magnetic field, which attracts the iron plate with a contact piece on the top of the round iron core, causing it to disconnect the first contact and connect the second switch contact. When the coil is powered off, the iron core loses its magnetism, and due to the elastic action of the contact copper, the iron plate leaves the iron core and recovers the connection with the first contact. Therefore, a small current can be used to control the switch of other circuits. The whole relay is protected by plastic or plexiglass dust cover, and some are fully sealed to prevent electric oxidation.
When the switch K in the control circuit is closed, the electromagnet is magnetic, the armature is sucked down, the relay contact is contacted, and the power circuit connected with the contact is switched on; When the control switch K is disconnected, the magnetism of the electromagnet is withdrawn, the relay contact pops open, and the power circuit is also disconnected.