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This simple circuit can detect a magnet!

By Mike McRae and David Shaw

Attracted to doing some hands-on science? Why not use a magnet to create a ‘reed switch’?

Safety: If the wires or batteries get warm, unclip the wires immediately.

You will need

• Magnet
• 3 wires with crocodile clips
• Battery clip for 2 x AA batteries
• 2 AA batteries
• LED
• Straw*
• Scissors
• 2 shiny metal paperclips

*Note: Paper straws are better for the environment. But if you have a clean, see-through, plastic straw to repurpose, you’ll be able to see what’s happening inside your reed switch!

What to do

1. Before you start, check that your paperclips are attracted to your magnet. If they aren’t, the activity won’t work!
2. Unbend the first curve on the outside of each paperclip. You’ll end up with two pieces of wire that each have a loop at one end.
3. Lie the two paperclips end to end next to the straw.
4. Cut the straw so it’s about one centimetre shorter than the two paperclips.
5. Put the long wire end of one paperclip into one end of the straw. The loop will grab onto the straw and help keep it in place.
6. Put the long wire end of the other paperclip into the other end of the straw.
7. Put the batteries into the battery clip.
8. Clip one end of a crocodile clip wire to the positive (+) terminal of your battery and the other to the longest leg of your LED.
9. Clip another crocodile clip wire to the short leg of the LED and one of the paperclips.
10. With the third crocodile clip wire, clip one end to the second paperclip, the other end to the negative (-) terminal of the battery.
11. If the LED lights up, adjust the paperclips slightly until it goes dark.
12. Bring a magnet close to the tubing. The LED should light up. You might hear a clicking noise too as the switch turns on!

What’s happening?

You’ve made a reed switch that can turn an LED on and off. When brought close to the straw, the magnet pulls on both paperclips until they come into contact with one another. When they touch, they complete the circuit – a loop of metal conductors from one end of the battery to the other. Electricity can flow and the LED lights up.

When the magnet is removed, the paperclips return to their original positions. The circuit is broken, and the LED turns off.

When a professionally-made reed switch is properly sealed inside a tube, it can be used in hazardous, dirty or wet environments that would otherwise ruin other switches. All it takes is a moving magnet to turn it on and off.

Today, reed switches are still used in devices such as burglar alarms and some electronic instruments. Speed sensors on some bicycles rely on a small magnet in the wheel to record its rotation. Laptops and tablets often have reed switches that put them into sleep mode when the lid is closed.

Real-life science

In this activity, we use magnets to control electricity. But electricity can make a magnet too.

In some reed switches, there is a coil of wire around the tube that contains the wires. When electricity flows through the coil, it creates a magnetic field that also causes the wires to move together and join the circuit. Different combinations of switches and coils can store information or do logical operations a bit like a computer.

Reed switches were being used like this to connect telephone calls back in the 1930s, before the invention of electronic computers.

If you’re after more science activities for kids, subscribe to Double Helix magazine!