Field Notes: Magnetism — Plate III, iron filings

Drag a magnet. Turn it by its handle. Double-click to flip its poles.

Fig. 3 — Iron filings around a bar magnet

The lines leave the north pole, curve round, and re-enter at the south pole. None of them ever cross.

Arrangements

Instruments

Streamlines S Compass needle C

Marginal note

Iron is not so much attracted by a magnet as turned by it. Each filing becomes, for the moment, a small magnet itself, and swings until it lies along the direction of force. What we see is therefore not the field, but the field’s direction — written in six thousand short strokes.

Where the filings crowd and darken, the force is strong: at the poles. Where they thin and fade, it is weak.

At the bench

R turn F flip + add Del remove T tap
Two fingers turn a magnet on touch screens.

Field strengths and scale are illustrative; the pattern is computed from summed point poles.