The bulb

In your house, there are lots of cables connecting the light switches to the light sockets. These were laid under the floor, behind the walls and above the ceiling when the house was built. In houses where extra lights or sockets have been added, you may be able to see thin tubes called conduit or trunking covering the extra cables.

When you switch on a room light, electricity in the cables and the light bulb begins to flow. As it flows through the bulb, electricity passes through the wire filament. The filament is very thin and made of a metal called tungsten. It glows brightly when electricity passes through it.

The gas inside the bulb helps to stop the filament from burning away. However, one day when you switch on the light, you will hear a "ting" which is the sound of the filament snapping, which means it is time to change the bulb.

The bulbs used for making the circuits described in the rest of this unit are much smaller than the light bulbs used in houses but they work in the same way.