Magnet Filings

Magnet Filings is the school physics demonstration with the paper and the bar magnet, except you can pick the magnets up and move them. A few thousand iron filings cover the screen, each one free to pivot but not to travel. Drag a magnetic pole across the field and every filing within reach swings around to line up with it, closest ones snapping over almost instantly while distant ones drift round a moment later. The result is the familiar pattern of looping field lines, drawn not by an illustrator but by the filings themselves.

The field is calculated properly rather than faked. Each pole contributes a force falling off with the square of distance, and every filing takes the direction of the total. That means two like poles genuinely repel — the filings between them turn to form a hard boundary neither side crosses — while opposite poles produce the classic arcs running from one to the other. Add a third or fourth pole and the pattern becomes something no textbook diagram shows, with null points where competing fields cancel and the filings sit unaligned.

Filings turn rather than snap, easing toward their new angle over a few frames, so moving a magnet sends a visible wave of rotation across the screen. Brightness tracks field strength, which makes the dense regions near a pole glow and the weak far corners fade almost to nothing. The Density slider sets how fine the filing grid is, and you can have up to eight poles at once. There is no objective — free in the browser, nothing to install.