|Themes > Science > Physics > Electromagnetism > Electrostatics > Electric Current > Magnetic effect of an electric current|
From at least the eighteenth century, people were trying to determine the connection between electricity and magnetism. Benjamin Franklin tried to magnetize a needle by electrical discharge. Sir Edmund Whittaker in the classical treatise History of the Theories of Aether and Electricity writes: "In 1774 the Electoral Academy of Bavaria proposed the question, `Is there a real and physical analogy between electric and magnetic forces?' as the subject of a prize." In 1805, two French investigators attempted to determine whether a freely suspended voltaic pile orients itself in any fixed direction relative to the earth. In 1807, Hans Christian Oersted (1777 - 1851), professor of natural philosophy at the University of Copenhagen, announced his intention to investigate the effects of electricity on the magnetic compass needle. Oersted's intention did not bear fruit for some time, but in July 1820 he published a pamphlet describing the results of experiments that "were set on foot in the classes for electricity, galvanism, and magnetism, which were held by me in the winter just past."
In these experiments, Oersted showed that a magnetic compass needle is subjected to a systematic pattern of forces in the presence of a wire closing a voltaic circuit and carrying an electric current. Note, we use the convention in which electric current flows from the positive terminal to the negative terminal through the wire. [demo Oersted's experiment: undisturbed needle; wire above; wire below; vertical wire current coming and going]
Following Oersted's discovery, it was immediately surmised that the magnetic effect of the current should induce magnetism in pieces of iron just as is done by an ordinary magnet, and this was quickly verified. Since the magnetism thus induced in small iron filings causes the filings to line up in the direction of external magnetic force like an array of tiny compass needles, a sprinkling of iron filings is frequently used to map out the direction of magnetic force throughout a given region [demo]. The same, done around a current-carrying conductor, shows that the resulting magnetic force at any point is directed along the tangent to a circle centered at the wire. Oersted deduced the tangential or "circular" pattern directly from the behavior of the compass needle, before the effect was visually revealed with iron filings.
A mnemonic (memory-aiding) device for recalling the relation between direction of current and direction of force on a north magnetic pole is referred to as "right-hand rule I:" With the thumb of your right hand pointing in the direction of conventional (positive) current, the curl of the fingers around the wire shows the direction of the circular pattern of force on a north magnetic pole. We choose the north pole for the sake of simplicity. There is simultaneously an oppositely directed force acting on the south pole. The direction of force on a north magnetic pole at any given point is called the "direction of the magnetic field" at that point.
By observing the magnitude of deflection of the compass needle from its normal north-south orientation, Oersted had access to a rough measure of the intensity of the effect. The influence of the earth's magnetism on the compass needle provides the restoring force with which the effect of the conductor is being compared. To obtain appreciable deflections, Oersted found it necessary to use currents "able to make a metallic wire red hot." The compass needle deflections were smaller with voltaic piles having a smaller number of plates (lower current) and decreased with increasing distance between wire and needle. When the circuit is broken, no needle deflection from normal north-south orientation occurs and no iron-filing pattern tends to form.
One of the many questions prompted by Oersted's discovery was: What would happen if an electrically charged object moved at different velocities? That is, is a moving charge analogous to an electric current in a wire and would therefore produce similar magnetic effects? Most physicists were fully convinced that the answer would be affirmative. Faraday asserted in 1838: "If a ball be electrified positively in the middle of a room and be then moved in any direction, effects will be produced as if a current in the same direction had existed." Maxwell also agreed that a moving electrified body must be equivalent to an electric current. Such effects, however, are very weak because electrostatic charges are extremely small. A direct experimental test was not achieved until 1875 when H. A. Rowland (an American physicist who, at the time was early in his career and spending a year in Europe prior to assuming professorship of physics at the newly founded Johns Hopkins University) showed the presence of the expected magnetic effects in the neighborhood of a rapidly rotating, electrostatically charged disc.
Oersted's pamphlet of 1820 made an immediate sensation throughout Europe, promoted a host of additional questions, and stimulated a flurry of investigation, particularly in France where there was at the time great interest in electric and magnetic phenomena. Such episodes recur from time to time in the history of science. Usually they are characterized by an experimental or theoretical discovery for which the scientific community is well-prepared conceptually. No revolution is involved, no sharp break in which old models and deeply entrenched modes of thought must be discarded in favor of new and unfamiliar conceptual patterns. Under these circumstances, the discovery, even if somewhat surprising and unexpected, is quickly assimilated and appreciated. Further consequences are readily pursued, and there is no prolonged period of gradual acceptance, residual doubt, and conservative opposition such as characterized the Copernican and Newtonian revolutions or the acceptance of heat as energy and subsequent conservation of energy law propounded by Mayer and Joule. The announcement of the discovery of the fission of the uranium atom in January 1939, for example, created a sensation and a flurry of investigation not unlike that which occurred after the publication of Oersted's pamphlet.