Magnetic poles and electric charges at one time seemed rather similar: in each case, two kinds existed, opposites attracted and likes repelled. As Coulomb found around 1777, the forces between charges or poles always diminished with distance r like 1/r2, which was also the way gravity decreased.

    Yet there were differences. Magnetic poles always came in matched pairs, and steel needles could be magnetized by stroking them repeatedly in one direction by the pole of another magnet, or a natural "lodestone" magnet. Soft iron (like the kind used in paperclips and baling wire) also became magnetized when touched by a magnet, but this magnetization was temporary and ended when the magnet was removed.

    The Earth itself was a giant magnet. This "geomagnetism" was a boon to sailors in mid-ocean, but magnetic north differed by a few degrees from astronomical north, and its direction slowly shifted over the decades. Edmond Halley--remembered for his comet--came up with an ingenious explanation of that slow variation: Earth consisted of layers, spheres within spheres, each magnetized differently and rotating slightly differently (it was more complicated than that).