Tuesday, 28 January 2014

A Cake Stand Magnetometer

A few weeks ago I was asked to build a device to illustrate the science behind the machines used to detect eruptions of solar wind on Earth caused by coronal mass ejections from the Sun. It's a deceptively simple thing called a magnetometer and it gave me an excuse to buy a cake stand.

All the bits of the magnetometer
There's a clue in the name, so it should come as no surprise that at the heart of the magnetometer is a small magnet. This is dangled on a length of fishing line inside a draft proof, glass bell jar. Or if you haven't got one of them, a £12 cake stand with cover from Ikea works a treat. Stuck to the front of the magnet is a tiny mirror and pointing at the mirror is a laser beam, that reflects and finally rests on a graduated scale. Now, once the system has stopped flailing around it is an exquisitely sensitive detector of magnetic field. Eruptions of magnetic field from the Sun should cause a sufficiently large change in the Earth's field such that the dangling magnet will twist ever so slightly. This tiny movement makes the laser beam sweep along the scale and can be measure. Assuming of course you are watching it when this happens. 

Close up of the cake stand cover
The trick with this magnetometer was to make it less sensitive. Clearly, the stronger the magnet on the fishing line, the more magnetic field it has itself and thus the more sensitive it will be to changing fields. I started by trying a neodymium magnet, but this was so sensitive it was impossible to get it to settle. A small square ferrite magnet did the job. It's the sort of thing you would find on a fridge magnet. 

There is also the issue of damping the system. The original Victorian designs, upon which this machine is based used another magnet to damp the motion of the main magnet. I found a couple of small neodymium magnets helped enourmously. They need to be aligned so their field is horizontal, in the same direction as the field in the dangling magnet. By using two magnets you effectively create a magnetic well for the main magnet to sit in. 

Butchered laser pointer in a box
Other than this, I used a cheapo laser pointer, hacked open and built into a small box with a tiny on/off switch. In theory you can increase the sensitivity of the device by moving the screen further away from the mirror. However, after about a metre, the laser spot is so big in this system it is unusable. The problem is imperfections in the glass of the dome and the mirror. These both cause the beam to lose coherence. 

Magnet (bottom) causing laser spot to deflect
So, how sensitive is it? Well, detecting a medium sized neodymium magnet (15mm x 5mm) is trivial at distances under half a metre. However, with patience and a bit of magnet wiggling, you can detect the same magnet from nearly nearly three metres away. Which is astonishingly sensitive for a cake stand. It is also easily sensitive enough to detect magnetic storms caused by coronal mass ejections from the sun.

Now I just need to work out how to record movement of the laser spot, so I don't need to sit staring at the magnetometer waiting for a Carrington Event to happen. 

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Science TV Presenter, live show performer, writer, strange prop builder and all round Science Bloke. All opinions expressed are mine alone. Not the BBC's, just mine.