| Themes > Science > Physics > About Physics, Generalities > A Brief History and Philosophy of Physics > The Development of Classical Physics: Mechanics, Heat, Optics, Electromagnetism, Atoms > Electromagnetism |
The study of electromagnetism began in experimental studies of such effects as static electricity and magnetism. People had known from ancient times that rubbing certain materials on dry hair would make the two attract each other, and the naturally occurring, magnetic lodestone was used as a navigating compass by the Chinese from about 100 B.C. Systematic studies of electricity began in earnest once apparatus had been invented for generating and storing electrical charge. The first electrostatic generator, a machine which rubbed a cloth against a rotating ball of sulphur, was invented by Otto von Guerike (1602-86), while Pieter van Muschenbroek (1692-1761) made the first Leiden jar to store electrical charge. In contrast to the spark discharges of an electrostatic generator, the voltaic cell (battery), invented by Volta in Italy in 1799, could provide a continuous flow of current. In a famous (and dangerous!) experiment in 1752, Benjamin Franklin used a kite to collect charge from a thunder cloud and store it in a Leiden jar. He then showed that this charge had identical properties to that produced by an electrostatic generator, proving that lightning was just one manifestation of electricity. However, Franklin's main contribution to the theory of electricity was his suggestion that charge came in two types, which he called positive and negative, with like charges repelling each other and unlike charges attracting. By these simple assumptions he could explain all known experimental facts about electricity, whereas previous theories had required about 20 different assumptions, including different shapes for particles of electricity in different media. This is one example of the use of Ockham's Razor in deciding between rival theories. Franklin also showed that there was a connection between electricity and magnetism, because iron needles could be magnetized by placing them near a wire carrying an electrical current. In 1750 John Mitchell, at Cambridge, had discovered the inverse-square repulsion of magnetic poles, by using a "torsion balance" to measure the twisting of a thread supporting one magnet when another was brought close. In a period beginning in 1785, the Frenchman Charles Augustin Coulomb reinvented the torsion balance and showed that both magnetic and electric forces experienced an inverse-square dependence on distance, now called "Coulomb's law" in the case of electrostatics. In Germany there developed a separate school of thought, that of the "nature philosophers". They believed that matter was not inert, as claimed by the mechanist school, but alive, with a universal world spirit that interconnected all forces. One member of this movement was the philosopher Immanuel Kant (1724-1804), who asserted that it was the interplay of innate repulsive and attractive forces that governed matter. If only repulsive forces existed, all matter would disperse; if only attractive forces were present, all matter would coalesce into a point. This balance between attractive and repulsive forces is today the starting point for the theoretical analysis of the structure of solids and liquids, although the forces are no longer believed to reflect a life force. The study of both electricity and magnetism was popular with German scientists, because the presence of opposite polarities in these phenomena fitted with their philosophy. These ideas also led to the conviction that every effect in nature had its inverse effect, since the vital forces were all connected. This idea that every effect has its inverse is fundamental to modern physics. For example, if you connect two wires made of different materials, and heat the junction, a voltage develops between the free ends of the wires. This effect, discovered by Thomas Seebeck, another German Nature-Philosopher, is the principle behind the use of a "thermocouple" for measuring temperatures. Conversely, a voltage applied with the correct polarity across the free ends of the two wires causes the junction to decrease in temperature. This is the principle behind the "thermoelectric cooler", often used to cool devices in electronic circuits. The belief in the interconnectedness of all forces in nature led Hans Christian Oersted, in Copenhagen, to announce in 1807 that he was looking for a connection between magnetism and electricity. He found that a magnet would move in a circle around a wire carrying a current, and that a wire carrying a current would move around a magnet. This is the principle required for the construction of an electric motor. The magnetic forces near current-carrying wires were the first forces which had been discovered which did not operate radially from the two interacting bodies. The next major contributions in electricity and magnetism came from the theoretician André Marie Ampère in France, and the experimentalist Michael Faraday in England. Ampère (1775-1836) developed a theory for the calculation of magnetic forces caused by a given electrical current, and suggested that the magnetic effects of some solids were caused by small circulating currents in the particles making up these materials. Faraday (1791-1867), on the other hand, had very little mathematics but was a superb experimentalist. His most important experimental observation in electromagnetism was that of induced currents, made in 1831: a wire loop would have an electric current developed in it, if either the loop was moved near a magnet, or the magnet was moved. This is the principle behind the generation of electricity by mechanical means, as occurs in every hydro- or thermo-electric power generating station, or in every car alternator. Even though mathematically unlearned, Faraday made a very important contribution to the development of the theory of electromagnetism by constructing a qualitative model of how electrical and magnetic forces acted. He supposed that each "particle" of electricity or magnetism produced a "line of force" which emanated from a positive pole of a particle and returned to a negative pole. These lines tended to contract along their length, and to expand perpendicular to their length. The lines could not cross. The number of such lines passing through a given area (i.e. the areal density) was a measure of the strength of the force provided by them. These assumptions explained the repulsion and attraction of magnetic and charged bodies: the tendency to contract lengthwise would pull bodies of opposite polarity together, whereas the tendency for them to expand laterally would push bodies of opposite polarity apart. Since the area of a sphere increases with the square of the radius, the inverse-square decrease in intensity of the forces was a natural consequence of the decrease in the areal density of the lines of force with distance from the charge or magnetic poles. The visual appeal of these lines of force still plays an important role in our understanding of electromagnetic phenomena. Moreover, Faraday believed that the lines of force would be present even if only a single charged or magnetic object existed; that is, even if there were no other body on which the first one could exert a force. Thus he invented the concept of the "field", as a physical presence which had the ability to produce a force -- magnetic, electric or gravitational -- if a second body happened to come into its vicinity. The concept of the field has served as one of the most powerful of all theoretical tools of modern physics. James Clerk Maxwell (1831-79) set out to make Faraday's ideas quantitative. He described the lines of force using Newtonian mechanics, envisioning them as rotating tubes of fluid (the ether) which had the properties required by Faraday: the rotation would cause the tubes to expand laterally and contract longitudinally. The resulting set of only four equations ("Maxwell's equations") described all known electric and magnetic phenomena exactly. Maxwell, however, realized that the enormous machinery with which he had filled all space was not an essential part of his theory, and eventually just used his equations as though the machinery did not exist. This is how we use his equations today. The relationship between the original machinery and the final equations was not without its detractors, however. One French reader stated that when he started to read Maxwell's work he expected to find himself in the midst of the quiet groves of electromagnetic theory, and instead found himself inside a factory! [Williams, p.122]. One of the unexpected results of Maxwell's work was that it predicted that electromagnetic waves could be produced which would propagate at the speed of light. This showed that light was an electromagnetic phenomenon, and not a separate subject. Discoveries in electromagnetism were applied quite rapidly to the development of useful devices. For example, the telegraph was invented in 1837 by Charles Wheatstone only one year after the development of the first reliable battery, and the first practical electrical generator was invented by Werner Siemens in Germany in 1866, 35 years after Faraday's discovery of induced currents. |
|
|