Ok, I dug this thread out because I had a reply ready for you while I was at work when WHAM! someone thought it was a good idea to ban my work IP from GLP and I couldn't post it (and I got my unban request refused so, no more helpful posts like this from me here anymore, just this last one)

How about building a magnetometer to measure changes in the earth's magnetic field (to tell you when the earth's getting battered by the solar wind)?

I built one a lot of years ago (or tried to with limited time, resources, and patience) but I just found this one which looks pretty easy to make. There'll be plenty of other designs out there but this is one which I think you'll be able to demonstrate in a classroom/

How to make a very sensitive jam jar magnetometer:

[link to www.eaas.co.uk]

Bye.