An electric bell uses an electromagnet, an interrupter mechanism (screw contact), a hammer, and a gong. What causes the hammer to repeatedly strike the gong when the switch is pressed?

A. The permanent magnet attracts and repels the hammer.
B. The electromagnet attracts the hammer, which breaks the electromagnet's circuit, releasing the hammer. The hammer then re-closes the circuit, repeating the cycle.
C. The AC current directly vibrates the hammer at the supply frequency.
D. The heat generated by the coil causes the hammer to move.

The correct answer to this question is B. The electromechanical process of an electric bell involves the use of an electromagnet, which plays a key role in causing the hammer to repeatedly strike the gong.

Here's how it works step-by-step:

Initial Activation: When the switch is pressed, an electric current flows through the circuit.
Electromagnet Activation: The current flows through a coil of wire wrapped around a core, creating an electromagnet. This electromagnet attracts a metal arm connected to the hammer.
Hammer Movement: As the electromagnet attracts the metal arm, the hammer strikes the gong, producing a sound.
Circuit Interruption: The same movement that allows the hammer to hit the gong also causes a break in the electrical contact, usually through an interrupter mechanism or screw contact. This action interrupts the current, causing the electromagnet to lose its magnetic pull.
Re-closure: With the electromagnet inactive, the spring mechanism associated with the metal arm pulls it back to its original position, which re-closes the circuit contact.
Cycle Repeats: When the circuit re-closes, the flow of electricity resumes. This process repeats rapidly as long as the switch is pressed, causing the hammer to strike the gong repeatedly.

This cycle of attraction and release of the hammer is what results in the bell ringing continuously. This principle of operation leverages the on-and-off cycle of the electromagnet to produce regular, repeated mechanical movements.