The Working Principle of an Electric bell and a Telephone Earpiece
This consist of a u-shaped magnet with coil of wire wound in opposite direction
When the bell is pressed, current flows through the coil of electromagnets and soft iron armature is attracted by the electromagnet and the hammer attached to the armature moves such that the hammer strikes the gong thereby producing sound.
The movement of the armature breaks the circuit and current flow is cut off, the electromagnet loses its magnetism and the armature is pulled back to the screw by the spring, the circuit is completed and current flows through, the process is continuously repeated and sound is produced.
When a person speaks into a microphone at the other end of the line, a varying electric current is set up having the same frequency as the sound waves. A similar electric current passes through the solenoid in the earpiece. This alters the strength of the magnetic flux in the u-shaped magnet producing a corresponding variation in the pull of the diaphragm. This latter vibrates and reproduces a copy of the sound waves which enter the microphone initially.