The nineteenth century opened with the discovery by Alessandro Volta of the electric battery, the first generator capable of supplying a long-lasting electric current. Volta's invention made possible a series of decisive advances. They include the observations by Hans Christian Oersted on the reciprocal action of electrical currents and magnets, and the discoveries by Michael Faraday and Joseph Henry on induced currents. François Arago, meanwhile, proposed an elaborate mathematical theory of electromagnetic phenomena. These breakthroughs gave a powerful impetus to the development and diversification of electrical instruments for research and teaching purposes. The first half of the century saw the first practical applications of electrical currents—most notably in the fields of electrochemistry (galvanoplasty, for example) and telegraphy. However, the technology for electric lighting and for the conversion of electricity into mechanical energy and vice versa on an industrial scale was not developed until the last decades of the nineteenth century. In 1867, James Maxwell, in a brilliant synthesis, defined the equations that provided an explanation of classical electromagnetic phenomena in all their complexity. Some twenty years later, Heinrich Hertz demonstrated, through experiment, the existence of electromagnetic waves. Predicted by Maxwell's equations, the waves were to be used, starting in the late nineteenth century, for the development of wireless telegraphy. At the century's close, the study of gas discharges led to the discovery of X-rays and the electron, opening new horizons for twentieth-century physics.