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Drop the needle!

By David Shaw, 11 July 2019

When you heat the needle you rearrange the atoms.

Think you understand magnets? In this hands-on activity, use a magnet to make a needle float and then fall.

Safety: This activity uses a lighter and gets very hot. Get an adult to help.

First aid: If you burn yourself, run the burn under cold water for 20 minutes.

You will need

• Sewing needle (metal)
• Spool of thread
• Magnetic clip
• Bowl
• Stick lighter
• Baking tray
• Jug of water (in case of fire)

What to do

1. Put the bowl on the tray.
2. Clip the magnet to the edge of the bowl.
3. Unwind a length of thread from the spool, then use it to thread the needle. (You don’t need to cut the thread.)
4. Tie a knot in the thread so it makes a loop and the needle can’t escape.
5. Unwind the thread until it’s about 20 centimetres long. (Or wind the thread back up until there’s about 20 centimetres unwound!)
6. Attach the thread to the spool so it won’t unwind any further. Many spools have a groove near the top that you can use to hold the thread in place. If your spool doesn’t have this feature, you can wrap the thread around the spool and knot it to itself.
7. Put the needle on the magnet, and make sure the spool is in the tray.
8. Slowly pull the spool of thread away until the thread is tight. Then keep pulling very slowly. You should be able to get the needle to stay attracted to the magnet without the two touching. Then let go. Everything should stay in place!
9. Check your that setup is stable and that it’s all on the tray. Have a jug of water nearby. (If you accidentally set fire to the thread, you can pour water over the fire and into the tray.)
10. Use the stick lighter to hold a flame under the pointy end of the needle (not the thread!) for about 15 seconds. What happens to the needle? Careful, the needle will heat up to about 800 °C! Wait for a few minutes before you touch it.

What’s happening?

There are lots of different types of magnets. Permanent magnets always have a magnetic field. Electromagnets are only magnets when there’s electricity flowing through them. And some objects turn into magnets when they are in a magnetic field.

In an electromagnet, the magnetism is caused by electricity. But in permanent magnets, and in substances that are attracted to magnets (such as iron and some types of steel), the magnetism comes from within. These substances are ferromagnetic.

Sewing needles, such as the one in this activity, are typically made from a type of steel. In a ferromagnetic substance like this, every atom acts as a tiny magnet. Usually, these magnets are pointed in different directions. But a powerful magnet nearby can make all the atoms line up in the same direction, turning all those mini-magnets into one big one.

There are other ways to reorder atoms in iron or steel. When you heat the needle above about 770 °C, the atoms in the metal change into a different pattern. In this new pattern, the iron atoms can’t line up to form a magnet, and it’s no longer attracted to magnetic fields. When it cools down below 770 °C, the pattern changes back and it becomes ferromagnetic again.

Colourful metal?

If you look at the needle afterwards, it’s probably black, covered in soot. If you carefully wipe the soot off, you might see colours on the needle!

As you heat the steel, it starts to react with the air. A thin layer of oxides will start forming on the outside of the needle. If the layer is only a few atoms thick, it might appear yellowish. As the layer gets thicker it will become more golden, then purple, and then bluish–black.

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