Magnets and magnetic fields

The magnets, compass, filings, objects, rods and clips are simulated. The field is calculated as if each pole were a single point a little inside each end of the magnet, and the compass and filings follow it exactly, so the plotted line, the filings and the neutral point come out where that field puts them. The filings are a few thousand short marks, many fewer than a real sprinkle.

A real compass also feels the Earth’s field. Beside a bar magnet it is weak, but at a neutral point it is all there is, so a real needle there turns to point north; the experiment leaves the Earth’s field out, so the needle there has no preferred direction and stays pointing whichever way it is left. Real plotted lines wobble a little, because each dot is placed by eye.

The clip counts come from the model, not from a measurement: soft iron holds 6 clips while touching and none after, steel 4 and then 3, to show the two behaviours clearly. Real counts depend on the magnets, the rods and the clips. The eight objects are pure samples of their materials.

With reduced motion each step happens at once: the needle, the dots, the lines and the filings appear straight away as they are at the end of the step, and the view cuts from the paper to the tray and the clamps.