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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