Three quarters of a century elapsed between Ampere's definition of electrodynamics and Einstein's reform of the concepts of space and time. The two events occurred in utterly different worlds : the French Academy of Sciences of the 1820s seems very remote from the Bern Patent Office of the early 1900s, and the forces between two electric currents quite foreign to the optical synchronization of clocks. Yet Amperes electrodynamics and Einstein's relativity are firmly connected through a historical chain involving Gerona extensions of Amperes'work, competition with British field conceptions, Dutch synthesis, and " fin de siècle " criticism of the aether-matter connection. Olivier Darrigol retraces this intriguing evolution, with a physicist's attention to conceptual and instrumental developments, and with a historians awareness of their cultural and material embeddings. This book exploits a wide range of sources, and incorporates the many important insights of other scholars. Thorough accounts are given of crucial episodes such as Faradays redefinition of charge and current, the genesis of Maxwell's field equations, and Hertz's experiments on fist electric oscillations. Thus there emerges a vivid picture of the intellectual and instrumental variety of Nineteenth-century physics. The most influential investigators worked at the crossroads between different disciplines and traditions : they did not separate theory from experiment, they frequently drew on competing traditions, and their scientific interests extended beyond physics into chemistry, mathematics, physiology, and other areas. By bringing out these important features, this book offers a tightly connected and yet sharply contrasted view of early electrodynamics.