Substrate
Fe₃O₄ magnetite
How the plate reads you
Real field lines, computed under your cursor.
No footage, no loop. Every filing you see is turned in the moment by a magnetic field summed from the poles on the plate — and the strongest pole is the one riding your cursor.
Each pole is treated as a point of magnetic charge, the way William Gilbert modelled a lodestone in 1600: a north face and a south face, each pushing or pulling along the line between it and every point around it. Add the contributions and you get the field H at any spot:
H(p) = Σ qi (p − ri) / |p − ri|³
// q = ±1 charge · ri = pole position · inverse-square, radial
// q = ±1 charge · ri = pole position · inverse-square, radial
A filing is a tiny needle with no head of its own, so it aligns to the axis of that field, not its arrow — brightening and lengthening as the field grows stronger, fading back to slack grey where it dies to nothing. Pair a north with a south and the filings draw the loops of a dipole between them; that is the pattern every schoolroom knows. Everything here is that sum, recomputed sixty times a second, for a few thousand needles at once.
Working the plate
Three ways to move the iron.
Your cursor is a north pole
It rides with you at full strength. Wherever it goes, the nearest filings swing to point along its field and darken; the loops sweep after your hand like filings under a moving magnet.
Tap to pin a fixed pole
Choose north or south, then tap the plate to leave a pole standing there. Set a north against a south and walk away — the filings hold the dipole between them. Tap a pinned pole again to lift it.
Compose a pattern
Pin four poles in a square and the field knots into a quadrupole; scatter a handful and read the saddle points where the loops cancel. Clear the plate to start again. Up to fourteen poles hold at once.