How to: make a magnetic compass

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Mark Miodownik, presenter from Dara O Briain's Science Club on BBC Two, reveals how you can perform simple science experiments at home. By magnetising a needle you can create your own working compass.

Our planet acts like a giant bar magnet creating a magnetic field that protects Earth from space radiation. Magnetised metals naturally align themselves with this field and you can harness this invisible effect with Mark's simple step-by-step instructions.

Mark Miodownik holds a magnet and a needle You can harness Earth's magnetic field to create a compass that will point towards north.
cork Cut a circle around 1/4 inch (5-10 mm) thick from the end of a cork.
Mark strokes a needle with a magnet Stroke a needle with the north end of a bar magnet around 50 times, lifting between strokes. Stroking from tail to tip pulls the needle's iron atoms into alignment, turning the needle into a weak temporary magnet.
Mark places cork and needle in a dish of water Attach the magnetised needle to your circle of cork with tack and carefully place it into a dish of water.
Homemade compass showing magnetic north alongside a field compass The needle will rotate until its north pole (the tail) points towards the magnetic north pole (shown on the compass). Stroking the needle from tip to tail with the north pole of a bar magnet makes the tip turn towards north.

Things you'll need:

  • Sewing needle
  • Cork or plastic bottle top
  • Bar magnet
  • Sticky tack
  • Shallow dish of water
  • Sharp knife or scissors
  • Towel (optional)
  • Compass (optional)

How to do it:

magnetic field imagined Earth has magnetic fields like that of a bar magnet with a pole at each end

Be careful when handling sharp needles and using knives or scissors.

1.

Cut a circle around 1/4 inch (5-10 mm) thick from the end of a cork with scissors or a knife and place to one side. You could also use an upturned plastic bottle cap.

2.

Magnetise the needle by stroking it with one end of the bar magnet 50 times from tail to tip. If the magnet has its north pole labelled, then stroke the needle with this end. Otherwise, pick one of the ends and use this for every stroke to magnetise the needle.

How do you magnetise a needle?

Iron, nickel and cobalt contain tiny regions called magnetic domains, in which electrons align in the same direction. These domains point in different directions so tend to cancel each other out.

When one of these metals is exposed to a strong magnetic field the domains are encouraged to align, which turns it into a temporary magnet.

This is how a steel needle (which contains iron) becomes magnetised when stroked by a permanent magnet.

Why does a compass point north?

Once the needle is magnetised it naturally wants to align with the Earth's stronger magnetic field.

Scientists believe this field, called the magnetosphere, is created by electrical currents that are generated by a churning molten iron core deep inside the planet.

It means the Earth acts as if it has a bar magnet running through it with the magnet's south pole located near the planet's geographic north. Since opposites attract the north pole of a magnetised needle is attracted towards it.

Lift the magnet away from the needle after each stroke to reduce the chance of de-magnetising the needle as you return it back to the tail.

3.

Stick the magnetised needle to the circle of cork with some tack, this will keep the needle floating on the water.

4.

Float the cork in a dish of water. Water creates an almost frictionless surface which allows the cork to rotate until the north pole of the needle points towards the magnetic north pole - i.e. the same as shown on the manufactured compass.

5.

Keep the dish away from computers and other devices that contain magnets as they can disrupt the field lines.

You can then check that the result is correct using a field compass or a smart phone with a compass tool. The needle will slowly lose its magnetic charge over time.

Mark Miodownik demonstrates how magnetism could be used to clean up offshore oil slicks

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