Why do some bulbs glow dim?

Here is a puzzle. You have one battery and two bulbs. You can connect them in two different ways, and the bulbs will glow differently each time. Why?

Try it first

Below, press the two wiring buttons. Look at how bright each bulb gets, and read the percentages. Then press Unscrew the second bulb in each layout.

What did you notice?

Layout 1: in a chain (series)

In the first layout, the bulbs are in one chain. The electricity goes through bulb 1, then bulb 2, then back to the battery. There is only one path.

• Both bulbs glow, but dimly, at about a quarter of normal brightness.
• When you unscrew one bulb, both go dark. The chain is broken, so the loop is broken.

Why dim? Think of the battery's push as a fixed amount. In a chain, the two bulbs share it. Each bulb only gets half the push. And a half-push gives much less than half the glow. It gives about a quarter.

This is called a series circuit.

Layout 2: side by side (parallel)

In the second layout, each bulb has its own path to the battery. The electricity can choose either road.

• Both bulbs glow brightly, almost as much as a bulb on its own.
• When you unscrew one bulb, the other keeps shining. It still has its own loop.

Each bulb gets the full push of the battery. They don't have to share.

This is called a parallel circuit.

Which one do we use at home?

Your house is wired in parallel. Imagine if all the lights in your house were in a chain: every time one bulb broke, the whole house would go dark. And every extra light would make all the others dimmer.

Old strings of fairy lights used to be wired in series. When one little bulb failed, the entire string went out, and you had to test them one by one to find the culprit. Many modern strings are built differently, so they keep going.

So why not always use parallel?

There is a catch. Each path takes some electricity from the battery. Two bulbs side by side use twice as much as one bulb. So the battery runs out faster. A chain makes the battery last longer, but gives dim lights.

It is a trade-off, and engineers choose the right one for the job. Sometimes they even use both layouts in one circuit.

Switches and batteries

The same idea works for switches. A switch in series with a bulb switches that bulb off. A switch in the main wire, before the bulbs split apart, switches off all of them with one click. That is the trick in the last Circuit Rescue mission.

And batteries? Batteries end to end are like a chain: their pushes add up. That makes a bulb brighter, much brighter. Two batteries give about four times the power.

Your turn

• In Circuit Rescue, One chain is series, and Side by side is parallel.
• Can you build a circuit where one switch turns off three bulbs?