An effective way to help children learn the basic properties of an electric bell is through a practical science project that teaches them how the bell works and why. An electric bell is made to ring by passing a current through an electromagnet, which become magnetised and attracts a small metal hammer within the bell unit. When this hammer moves it strikes the bell and makes it ring, however it also breaks the circuit, demagnetising the electromagnet. The hammer returns to its position in the circuit, reconnecting it and repeating the process. The full process happens very quickly and produces the intermittent ringing associated with electric bells.
Place the battery in the battery holder. Point out to pupils the negative and positive terminals of the battery and explain that direct current flows from the positive terminal to a negative terminal in a complete circuit. Explain to the children that this flow of current is what powers things like the electric bell.
Connect one of the coated wires to the connector at the positive end of the battery and then connect the other end of the wire to the one of the connectors on the switch. Explain the properties of the switch -- that it serves to break and complete the circuit depending on the position of the lever -- and reinforce the idea that breaking the circuit causes a cessation in the flow of current. Put the switch in the "Off" position.
Connect one of the coated wires to the other connector of the switch and connect the other end of the wire to one of the connectors on the electric bell. Explain to the children that the current flows from the positive terminal of the battery, through the switch and through the bell, making it ring but that the circuit is not yet complete.
Complete the circuit by connecting the final wire to the other connector on the bell and to the negative terminal of the battery. Explain to the children that the circuit is now complete but that the switch is breaking the circuit. Ask one of them to flick the switch and make the bell ring. Explain what the current does within the bell to make it ring.
Have the children experiment with removing wires from the circuit and observing when a circuit is complete and incomplete. Continue the project by having the children build their own circuits with electric bells in series:connected together in a chain on a singly branched circuit, or in parallel:connected on individual branches of the main circuit. Have the children to observe the difference between the two types of connections.
Things you need
- Battery holder with connectors
- 3 coated wires with crocodile clips
- Basic switch with connectors
- Electric bell with connectors