• theft prevention (as shown)and so on. OBSERVATION: I am seeking to push this design to its physical limits. At first I was just below the limits -- at the moment I am just above them. Incidentally if anyone has two (matching) heavy copper transformers or motors to throw out, do let me know. I could use these as sensors. Some magnetometers use sensors of 100kg or more.
• deep searches under the earth
• an earthquake warning device
• to prospect for minerals
• to open a cat flap
• to detect a car rotating in the Earth's magnetic field
• to detect trains or trucks at great distances
POSCSRIPT: Ten days later, my magnetometer picks up a (nominally) unmagnetised hairpin at 0.8 metres. But I am back below the limits now -- it can do more.