Physics in the Nineteenth Century

At the very beginning of the nineteenth century Volta was perfecting his battery and Count Rumford making his experiments on heat as a mode of motion. Volta’s battery at once led to the great advances in electricity made during the first third of the century, but Rumford’s experiments were unappreciated for almost half a century.

In light, Young solved the problem of the interference of light waves in accordance with the wave theory of light in 180l. In the previous year Herschel had discovered that the hottest rays of the spectrum lie beyond the red rays, being invisible to the eye, and in 1801 Ritter of Jena found that, at the other end of the spectrum, invisible and light lavender rays beyond the violet ones have the curious chemical power of turning nitrate of silver black. In 1802 Humphrey Davy managed to take a few sun pictures on this principle, using chloride of silver, but he could not keep them from fading, and it remained for Daguerre, in 1839, to make photography a success. Fraunhofer (1787–1826), the optician and manager of a physical laboratory near Munich, noted the black lines in the sun’s spectrum in 1814, and in 1822 Sir John Herschel showed that burning gases and vapors show bright lines at different portions of the spectrum. He even suggested that this be used for chemical analysis, but the matter was not taken up fully until later.

An important advance made in electricity the same year as the invention of Volta’s battery in 1800 was its application to chemistry in the electrolysis, or decomposition by an electric current, of water by Nicholson and Carlisle. This was soon extended by Davy to the discovery of new substances. The next great step was not until 1819. Hans Christian Oersted of Denmark in this year discovered that a magnetic needle was affected by a current passing along a wire near it. By later experiments he proved that an electric current passing near a magnetic needle makes it turn so as to lie across the path of the current. This was the first step in connecting electricity and magnetism.

Andre Ampere (1775–1864) taking up the subject, showed that if a man will imagine himself standing so that the positive current comes out of his mouth and returns by his feet the north pole of the needle or magnet turns to his left side. He concluded that if an electric current causes a magnetic current across itself, then two electric wires ought to have magnetic currents running across them and ought to attract or repel each other. He experimented on this and proved that if the current is sent in the same direction through the wires they move toward each other, but if in opposite directions they repel each other. It now occurred to him that if electric currents cause magnetic ones he should be able to make a magnet by passing a current around a steel bar. This he found he could do. This was the last great step made in the new science by Ampere. This work was all done in 1820 while he was professor at the Polytechnique in Paris.

Henry in America and Faraday in London followed up Ampere’s experiments. Their results were much the same. On the theory that electricity is a force and gives rise to magnetic lines of force, Faraday saw that if a current going around a wire makes a magnet, a magnet ought to make an electric wire revolve around it and ought itself to revolve around a live wire. To prove this he took two cups of mercury with the wire for the current inserted in the bottom of each. The only connection between the two cups was by means of a small bar magnet floating on end in one cup, then along a copper wire, the end of which was free to move around a fixed bar magnet in the other cup. Sending a current through, he found that in the first cup the magnet would move around the wire and that in the second the wire would move around the magnet. This is the principle and first beginning of the electric motor. Soon after this he wound about three hundred yards of wire around a hollow cylinder and showed that when a magnet was passed into and out of the cylinder it developed an electric current in the wire. This was the beginning of the dynamo.