Why can automotive relays control large currents with small currents?

Update:30-08-2019
Summary:

To make it easier to understand, we divide the circuit […]

To make it easier to understand, we divide the circuit into two parts, a low voltage (small current) circuit and a high voltage (high current) circuit. In the figure, the power supply, the switch S1, and the coil are low voltage circuit parts; the contacts, the load power supply, and the consumer are high voltage circuit parts.

When the switch S1 is closed, the low voltage circuit is turned on, current flows from the positive pole of the power source through the switch S1, and then through the coil to the negative pole of the power supply. The current is generated by the current through the coil. This is the process of electromagnetism. The magnetic force attracts the iron piece. The movable contact and the fixed contact in the high-voltage circuit are closed, the high-voltage circuit forms a loop, and the load power supply supplies power to the electrical appliance. When the switch S1 is opened, the coil no longer generates a magnetic force and is not attractive to the iron piece. The return spring breaks the contact in the high-voltage circuit due to the elastic force, and the load power supply stops supplying power to the electric appliance, and the electric appliance stops working.

More information: single-phase relays