To do a simple experiment to understand the working of a ding dong bell.
- A few feet of insulated wire.
- A 6 volt battery.
- A cardboard of plastic tube.
- A big nail.
- Wrap the insulated wire around the tube making as many loops as possible. Leave the ends free.
- Strip off the insulation from the ends of the wire and keep them ready to be connected to the terminals of the battery.
- Insert the nail partly into the coil and connect the wire to the battery terminals briefly. You will observe that the nail is sucked into the coil.
- Reverse the connections to the battery and repeat the experiment. Again you will see that the nail is sucked into the coil.
When electricity flows throw a conductor it generates a magnetic field around it. A loop of wire generates a magnetic field through the loop. When the wire is made into a coil, the strength of the magnetic field increases as the number of loops increases and the assembly acts as a strong electromagnet. Hence when the battery is connected to the coil and the electromagnet turns on, the nail is quickly sucked in. This is exactly what happens in a ding dong bell. In a real bell a plate is kept for the nail to strike as it is sucked in, thereby creating a ringing sound. Even when you reverse the direction of current, the iron nail is still sucked in as the iron atoms reorient so that they line up with opposite poles to the electromagnets. Therefore the nail is always attracted and pulled in by the coil electromagnet.