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Electromagnetism is the force involving the interaction of electricity and magnetism. It is the science of electrical charge, and its rules govern the way charged particles of atoms interact. Electromagnetism is one of the four fundamental forces of the universe (gravity and the "strong" and "weak" forces that hold an atomic nucleus together are the other three). Because its effects can be observed so easily, electromagnetism is the best understood of these four forces.

Some of the rules of electrostatics, or the study of electric charges at rest, were first noted by the ancient Romans, who observed the way a brushed comb would attract particles. Until the nineteenth century, however, electricity and magnetism were thought to be totally different and separate forces. In 1820, a direct connection between the two forces was confirmed for the first time when Danish physicist...