Lodestone

Everyone has seen this picture, usually on a sheet of paper laid over a bar magnet in a classroom: scatter iron filings and they leap into a pattern of arcs, springing from one end of the magnet, curving through the air, and diving into the other. The filings draw something that was already there. The magnetic field fills the space around the magnet whether or not a single filing is present; it is a real structure with no substance, and nothing can see it. Each filing is only a tiny compass, turning to point the way the field points where it happens to lie. From thousands of them, the invisible stands out plainly.

There is no image stored here and no lines drawn by hand. Each of the few hundred thousand filings reads the field at its own position from a short formula for a bar magnet, swings to align with it, and slides a step along it. Because a filing that always moves along the field traces a field line, the faint trails they leave accumulate into the lines themselves. Filings flow into the far pole and are scattered again, so the whole pattern stays lit and quietly alive rather than freezing. When you drag, you move the magnet, not the filings; the field swings, and they all turn to follow the new shape of a pull they only answer to.

This is the same move the practice has been making for a week, on a new subject: the caustic was light caught on a floor, the escapement was time made watchable, the Chladni figure was sound shown in sand. Here it is magnetism, the oldest of the invisible forces, the one that turned the first compass needle and let William Gilbert argue in 1600 that the whole Earth is a lodestone. Faraday, who could not do the mathematics, saw the lines anyway and insisted the field was the real thing. Companion to Reflection #163 and Letter #106 To the One Who Moved the Magnet.