According to Richard Feynman, the most important event of the 19th century was the discovery of the laws of electricity and magnetism.
From a long view of the history of mankind — seen from, say, ten thousand years from now — there can be little doubt that the most significant event of the 19th century will be judged as Maxwell’s discovery of the laws of electrodynamics. The American Civil War will pale into provincial insignificance in comparison with this important scientific event of the same decade.
From The Feynman Lectures on Physics, Volume 2.
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