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Connecting Electricity and Magnetism

It is remarkable how many discoveries came about by accident (although it should be borne in mind that these ‘accidents’ were normally preceded by much hard work on allied research). One of these happened to the Danish physicist, Hans Christian Oersted (1777–1851). He was applying a potential across a metal wire to measure the current, when he noticed to his amazement that a compass that happened to be near the wire deflected when the voltage was switched on. Since he was aware, of course, of the interaction between two magnetic poles (laid down in the 1785 Law of Coulomb),he concluded that an unknown magnetic force was active in his experiments. An important conclusion, because until then nobody had connected electricity and magnetism. Until the middle of this century, the electromagnetic unit of magnetic fieldstrength was the oersted (2 Pi oersted is the field at the centre of a circular coil 2 cm in diameter carrying a current of 1 ampere). It was replaced by the SI unit A /m. Oersted’s discovery was the spur to further research by other scientists. Around 1820, the French physicist Dominique Francois Jean Arago (1786–1853) discovered that an iron rod can be made magnetic by winding a current-carrying (insulated) conductor around it. The magnetism disappeared when the current was switched off. Today, Arago is better known by the experiments he conducted before the discovery of electromagnetic induction by Michael Faraday in which a rotating copper disk was made to cause rotation of a pivoted magnet (Arago’s rotation). Another scientist fascinated by electromagnetism was the French physicist and mathematician André Marie Ampere (1775–1836), who discovered that two wires carrying direct currents flowing in the same direction attract one another and repel each other when the direction of one current is reversed. The force with which this happens is directly proportional to the level of current and inversely proportional to the distance between the wires. From these findings, he formulated the law named after him: nHdl=i (where H is the field around the conductor, l is the length of the conductor, and i is the current through the conductor) He also worked out the principle of a moving-coil ammeter, which depends for its action upon the force on a current-carrying coil in the field of a permanent magnet. In the early part of the 19th century there were not yet standardized units for current, voltage and resistance. Although the British chemist Sir Humphrey Davy (1778–1829) had already discovered that the electrical resistance of metals increases when the temperature of the metal is raised, it was not until some years later that the German physicist Georg Simon Ohm (1787–1854) found that the current through a conducting wire is directly proportional to the potential across the wire and inversely proportional to the resistance of the wire. This was formulated in the well-known Law of Ohm: I=E/R.

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Birth of the Battery

Count Alessandro Giuseppe Anastasio Volta (1745–1827) belonged to an aristocratic family and was Professor of Physics at the universities of Como and Pavia. He became interested in a phenomenon described by Galvani in 1786 to the effect that “an electric spark, or contact with copper and iron, causes a frog’s leg to twitch”. This gave rise to the, fortunately short-termed, belief that animal tissue was necessary for the generation of electricity. Experiments showed Volta that an electric current could be generated by bringing different metals into contact with each other. There are different versions of which metals he used: some writers claim silver and zinc, others, copper and zinc. In 1799, he succeeded in making a construction of metal discs, alternately silver (or copper) and zinc, with brine-soaked card between them. This ‘voltaic pile’ as it became known was the first man-made source of electricity. Its invention was made known by Volta to Sir Joseph Banks, President of the Royal Society, in a letter in early 1800. In this letter, Volta says that he used 25 mm dia. copper and zinc discs. After his invention was made known, Volta did little further work on the device. His name survives, however, in the SI unit of electric potential difference, the volt. It is interesting to note that in 1848 Scyffer in his Geschichtliche Darstellung der Galvanisms (Historical Notation of Electric Phenomena) states that others besides Volta carried out experiments with dry cells between 1800 and 1812, namely Ludicke, Einhof, Ritter, Hachette, Desornes, Biot, and others. Several physicists of that era, particularly Zamboni, expressed as their opinion that the best performance was not that of Volta but that of De Luc. Be that as it may, Volta’s invention transformed the study of electricity and was, therefore, invaluable to men such as Nicholson, Davy and Faraday. It also put paid to the belief that animal tissue was needed for the generation of electricity. It may be said that all this work in the early part of the 19th century was experimental. The first reliable, practical source of electric current, based on the interactions of carbon and zinc in an electrolyte consisting of, among others, ammonium chloride, manganese dioxide, zinc chloride and water, was described by the French physicist Georges Leclanché in 1868. The Leclanché cell, improved many times since its inception, remains the best known dry or primary cell in common use today. The secondary battery, invented in 1803 by Johann Wolfgang Ritter (1776–1808) consists of discs of one metal separated by circular pieces of cardboard that are moistened in a liquid that cannot chemically affect the metal. When the extremities of this pile are linked to the poles of a voltaic pile, it becomes electrified and can be substituted for the latter and it will retain the charge. However, the first practical secondary battery, the lead-acid battery, was produced in 1859 by another French physicist, Gaston Planté (1834–1889). In spite of all sorts of other type of secondary battery, the lead-acid battery remains the most widely used secondary battery in the world today.

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