I'm not sure the FM receiver will be sensitive enough to detect the magnetic field of the earth. But you can test it. Tune the FM receiver to a frequency where there is no radio station (static). Start recording from the FM receiver. Now, SPIN the N900 at a frequency that corresponds to the FM frequency you are tuned to. The FM receiver should see the magnetic field of the earth oscillating (because of relative motion) at the FM frequency it is expecting and you should hear a tone. Assuming you are tuned to a frequency of 87.7 mhz, you would have to spin the N900 at an rpm of 87,700,000. For reference, that is 551,034,887 rad/s. At those speeds, the N900 would experience G-forces on the order of 789-million Gs (assuming radius of about 10cm). Be sure to let us know how it goes