An electric bell works on the principle of electromagnetism and includes a coil, iron armature, spring, and hammer. When the switch is pressed, electric current flows through the coil, turning it into an electromagnet. This magnet attracts the iron armature attached to the hammer, causing the hammer to strike the bell and produce sound.
However, when the armature moves towards the electromagnet, it breaks the circuit at the contact point. As soon as the circuit is broken, current stops flowing, and the electromagnet loses its magnetism. The spring then pulls the armature back to its original position, reestablishing the circuit. This makes the bell ring repeatedly as long as the switch is pressed and released automatically in rapid succession.
If the switch is kept pressed continuously, the circuit remains open after the first contact break, and no current flows through the coil. As a result, the electromagnet stops working, the hammer stops moving, and the bell stops ringing.
In short, the bell rings due to rapid making and breaking of the circuit. Keeping the switch pressed continuously breaks this process, stopping the ringing.