The Loop That Must Close

A torch has a bulb, a battery and a switch. Press the switch and it lights. That much everyone knows.

Now here's the thing worth noticing. The bulb has two contacts, not one. The battery has two ends, not one. And the wire doesn't just run from the battery to the bulb — it runs from the battery to the bulb and back again.

Electricity doesn't get used up at the bulb and stop there. It goes round. Out of one end of the cell, through the wire, through the bulb, onward through the switch, and back into the other end of the cell — a complete ring, with no beginning and no end.

And that gives you the single most useful fact in this whole topic:

Break the ring anywhere at all, and everything stops.

Not "it gets dimmer nearer the break". Not "the part before the break still works". The whole thing stops, instantly, everywhere. A gap the width of a hair is as good as a gap a metre wide.

Once you believe that, a switch stops being mysterious. A switch is nothing but a break you are allowed to open and close on purpose.

By the end of this lesson you'll be able to:

• name the parts of a simple circuit and say what each one does
• explain why a circuit has to be a complete loop
• work out why a bulb is not lighting, by tracing the loop and finding the break
• tell conductors from insulators, and stay safe around electricity