Build a Detector

You don’t just have to check AW UK to find out when the Northern Lights might be visible from where you are, you can make your own magnetometer to act as a detector. You needn’t be a rocket scientist to do it!

Pop bottle magnetometer

This really is as simple as it sounds! It is just a magnet with a mirror on it hanging inside a pop (soda) bottle.




You will need:

  • one clear plastic two litre pop-bottle, with cap
  • fine cotton thread
  • one small bar magnet
  • one small mirror, piece of mirror card, or reflective sequin
  • one piece of card
  • stabilizing material (e.g., sand)
  • Blu-tac™
  • one drinking straw (or some thick copper wire)
  • sticky tape, scissors, and glue
  • non-frosted light source, paper, and ruler


Remove any labels from the pop-bottle, and carefully cut around the bottle to remove the top third. Watch out for sharp edges where you have cut. Keep the top part of the bottle as it will be stuck back on later. Fill the bottom part of the bottle about half full with sand, this will keep the bottle upright and stop it moving about.

To suspend the magnet you can either:

  • Cut length of drinking straw slightly shorter than the magnet and stick it to the top of the magnet. Feed one end of the thread through the straw and then tie it to itself to make a triangular loop with sides of a couple centimetres in length, with the long end of the thread still attached.


  • Make a cradle from the copper wire by taking a length of around ten centimetres and bending each end with three right angles such that they will hold the magnet. Then bend the wire in the middle so that, when you sit the magnet in the u-bends that you made, it makes a triangle with the magnet as the base. Tie the thread to the point of the triangle.

It is important that the magnet hangs level, so spend a little time making sure that it does.

Next cut the piece of card to into a rectangular shape slightly longer than the magnet and several centimetres wide. This is to act as an air-brake to stop the magnet swinging too much. Remember that the magnet should still be able to swing freely when it is inside the bottle, so the card should have a bit of space between it and the edge of the bottle. The card should also not scrape on the sand in the bottom of the bottle, but you will probably have to adjust for this later.

The card should be stuck to one side of the magnet, so that the magnet is in the centre of the top edge of the card.

Next the mirror/sequin should be stuck to the other side of the magnet. The whole lot should hang vertical, and the magnet should be horizontal so you may need to use a bit of Blu-tac™ to balance them.

Puncture a hole in the centre of the bottle cap (be careful!) and feed the thread through the top of the bottle, and then through the cap. Next screw the cap on.

Now carefully reconnect the top part of the bottle so that the magnet assembly hangs inside. Adjust the length of the thread so that the mirror is a couple of centimetres below the cut in the bottle, and not hitting any of the sand in the bottom. When you are happy with the height stick the thread to the top of the bottle to stop it slipping, and use some sticky tape to stick the bottle back together.

The detector is now finished, and should look something like this:

Pop-bottle magnetometer

Taking Readings

You will need to set up the pop-bottle detector so that if a light is shone on the mirror the reflection hits a wall head on. The wall should be about one metre away. The light source can be a normal desk lamp, as long as it has a clear bulb in it, as a frosted one will not have a very clear reflection. Using the ruler draw a line on the paper and mark off every centimetre, and number them. Stick the paper to the wall so that the line marked is vertical, and the reflection from the mirror is in the centre of the line. We used a more advanced method of taking readings, involving a light following plotter, that allowed it to be done automatically for long periods of time.

Readings should be taken every hour to start with, and if it starts to move a lot then more frequent readings should be taken.

When taking readings it is important that the detector is as far from moving metal objects as possible as they will disturb it. This includes busy roads, railways, and lifts (elevators). The results page will give some idea of what might be expected from your detector.

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