I recommend rethinking the magnet metaphor, on the grounds that it is physically wrong. If you have two magnets on either end of a ruler, and one is twice as strong as the other, then an iron ball at the center of the ruler is going to roll all the way to the larger magnet (accelerating as it goes, because inverse square law), unless I'm missing something. Perhaps a better physical metaphor would be something like rubber bands, with each bit of evidence adding another rubber band from the belief level to pins at the ends of the ruler?