Third grade physics! The picture shows the schematic diagram of a DC bell. Can you tell me its principle? ! Thanks

When energized, current passes through the electromagnet, which generates magnetism and attracts the elastic piece B under the hammer, causing the hammer to strike the bell and make a sound.

But because point B has come down, it cannot be disconnected from the "metal needle contact" circuit above. The electromagnet loses its magnetism, and the hammer is bounced back by the spring leaf at A. After returning, the circuit is closed. , point B was sucked down again, and the small hammer struck the bell again to make a sound.

Keep repeating, and the electric bell will make a continuous sound.