Poles apart

摘要

They seem magical: magnets, every child's favourite science toy. Two otherwise ordinary lumps of metal draw inexorably closer, finally locking together with a satisfying snap. Yet turn one of them round and they show an entirely different, repulsive face: try as you might to make them, never the twain shall meet. If magnets seem rather bipolar, that's because they are. Every magnet has two poles, a north and a south. Like poles repel, unlike poles attract. No magnet breaks the two-pole rule - not the humblest bar magnet, not the huge dynamo at the heart of our planet. Split a magnet in two, and each half sprouts the pole it lost. It seems that poles without their twins -magnetic "monopoles" - simply do not exist.