Electrical phenomena are omnipresent in nature, so mankind started from simple observations and slowly built his understanding of electricity.
AMBER AND MAGNETITE
The ancient Greeks started mankind on the long path toward a comprehensive and uni ffed description of all electric and magnetic phenomena by making two simple observations. They noted the observable magnetic forces between pieces of certain black stones (magnetite), and they noted the attractive electric forces between a piece of amber, when rubbed with wool, and miscellaneous light objects such as fragments of leaves or straw. The awareness of the ancients is brought to our attcntion by the word electron, which is the Greek word for amber, and by the word magnet, which is the Greek name for the black stones from Magnesia in Asia Minor.
The first known technical description of the use and testing of magnets and their properties was written in 1269 by Petrus Peregrinus, a French physicist. For the next 300 years or so, rather little was added to the understanding of magnetism, though many fanciful explanations of magnetic behavior were suggested.
In 1600, William Gilbert, physician to Queen Elizabeth, published De Magnete, thereby inaugurating an era in which greater emphasis was placed on the careful observation of experimental facts and on the development of more functional theories. To explain both electric and magnetic attraction, Gilbert rejected such animistic concepts as sympathy or antipathy and instead introduced the idea of “physical force.” Although Gilbert confused magnetic and gravitational force, his work resulted in greater emphasis on the importance of measuring forces in order to understand magnetic and electric behavior.
Gilbert made a major contribution to the measurement of forces by designing the versorium, the first electroscope. It made possible a simple measurement of the presence of electric charge. The versorium consisted of a light metallic needle, balanced on a delicate pivot like a compass needle. The attractive force of an electrified body brought close to the needle produced charge on the needle and caused it to move on its pivot toward the electrified body. This instrument, and improvements on it, provided the first means of systematic measurement of charge.
Interest in magnetism and electricity continued throughout the 17th century, with contributions by Descartes, Huygens, and many others. About 1660, Otto von Guericke built the first electric friction machine. It was a globe of sulfur that sparked and crackled when rotated and rubbed by hand. Von Guericke also performed experiments on electric repulsion, surface charges, and conductivity. By the end of the 17th century the stage was set for future advances in understanding that would rapidly dwarf all past advances. The importance of the work up to this point was that it aided in developing a state of mind that increasingly turned to experiments. This was the path toward precise measurements of natural phenomena, rather than to magical or animistic explanations.
About 1720, a new era in electrical investigations began with the experiments of Stephen Gray in England. His work in the 1720’s and 1730’s led to a clear understanding that some materials were conductors of electric charge and some were insulators. About 1730, Gray and Granville Wheler conveyed electricity from rubbed glass a distance of 886 feet (270 meters) along a string supported by silk threads.
Another discovery in this period was that two kinds of electricity could be produced by friction, depending on the materials used. A related discovery was that bodies charged with similar kinds of electricity repel each other, and those charged with opposite kinds of electricity attract each other. These fundamental discoveries were made in 1733 by the French scientist Charles Du Fay. Du Fay and others had made these advances possible by developing an improved electroscope, which had pith balls suspended on conducting threads. By charging up the pith balls with a known kind of electricity, it was possible to distinguish the kind of charge on another charged body by observing whether the pith balls were attracted or repelled by the charged body.
Thus by 1740 there were crude machines for producing charges and electroscopes for measuring the existence of charges. Also, the concept that there were conductors and insulators was becoming well known.
Charge Storage. The next big step came in 1745 when the Leyden jar, a device in which charge could be collected and stored, was developed. The first Leyden jar, devised independently by E. Georg von Kleist, a Pomeranian parson, and Pieter Van Musschenbroek, a professor of physics at Leiden, was a glass bottle filled with water with an electrical contact made to the water by a nail put through a cork stopper. When the nail was put in contact with a charging machine, the charge placed on the water was insulated by the glass. Thus the charge could be used later to produce a strong spark when the nail was touched to another body.
Although the Leyden jar was poorly understood at the time, its chargestoring property made possible many experiments involving electric discharge and the first experiments involving electric currents.
The experiments of Gray and Du Fay resulted in great popular interest in electrical experiments. Perhaps the most spectacular demonstration was performed by Abbe Jean-Antoine Nollet before members of the court of Louis XV at a monastery in Paris. Seven hundred monks joined hands, and one end of this long line of men was connected to the outer contact of a charged Leyden jar. When the man at the other end of the line was connected to the other terminal of the Leyden jar, ali 700 monks simultaneously leaped into the air, thus convincingly demonstrating the effects of an electric shock.
Electric Charge THeories and Experiments. In the mid-18th century, the idea was gradually developing that charge is a quantity that is conserved. That is, charge is a quantity that may be moved from one place to another, like a fluid, and the amount of charge on a body is the simple sum of separate amounts put there. However, the idea of positive and negative charges» which in effect cancel each other when placed on the same body, was not yet investigated, and there was much confusion. Further development had to await the ability to make more quantitative measurements than were then possible.
By about 1744, Benjamin Franklin had become intensely interested in electrical phenomena, and he devoted most of his energies to electrical experiments for the next 10 years or so. Franklin s very thoughtful experiments greatly advanced the idea of conservation of charge. He proposed a one-fluid theory—that is, that there were excesses and differences of a single electrical fluid—rather than a two-fluid theory as proposed by the immediate successors of Gray and Du Fay. His proposal was not entirely correct in that there are in fact two kinds of charge, but his model is easily modifîed to fit this fact. His “plus” and “minus” description of charge is now used to characterize the two kinds of charge.
Many other clarifying ideas were generated by Franklin from his experiments. His famous kite experiment, made in 1752, is perhaps best known. In the experiment he flew a kite in a thunderstorm and collected charge in a Leyden jar, thus convincingly showing the electrical nature of lightning.
In the latter half of the 18th century, beginning with the work of Franklin, there was a greatly accelerated development of ideas about static electricity. Better experimental equipment, the beginning of accurate quantitative measurements of forces, and the accompanying development of theoretical ideas resulted in a more rapid advancement in understanding than in any previous half century. On the theoretical side, Newton’s ideas about gravitation, especially that gravitational force is proportional to the inverse square of the distance between two bodies, stimulated investigations of the law of forces between charged bodies. In 1767, Joseph Priestley showed that if the electrical law of forces between charges varies as the inverse square of the distance, as in the case of gravitational forces, then there is no net force on a charge placed anywhere inside a hollow charged conductor. In 1772, Henry Cavendish provided the experimental proof of this important conclusion.
MEASUREMENT OF ELECTRICAL AND MAGNETIC FORCES
For further progress in understanding electric and magnetic forces, it was necessary to put investigations on a quantitative basis. The quantitative work done by Cavendish largely remained unknown until 1879, when James Maxwell published Cavendish’s works. In the meantime, the work of Charles Augustin de Coulomb marked the beginning of a new emphasis on the study of force in a quantitative way. His work provided the experimental faets necessary for developing a mathematical theory of electric and magnetic forces.
Coulomb’s most important work was concerned with the establishment of an inverse square law for both electric and magnetic forces. His work with the torsion balance, which began in 1784, made possible very accurate measurement of electric forces between charged bodies. By using it in 1785, Coulomb not only demonstrated the law of forces between electrically charged bodies but also demonstrated a similar law that applies to magnetic forces.
DEVELOPMENT OF THE VOLTAIC PILE
During the 18th century, experimenters had a serious handicap because their basic source of electric charge was the Leyden jar. It could store charge only at relatively high voltage, and it could provide charge only for a short time before the stored charge was dissipated. Before the effects of continuous current, or charge flow, could be investigated in detail, a continuous source of charge had to be developed. This came about in a surprising way as the result of a seemingly unrelated chance discovery by Luigi Galvani in 1786.
Galvani first noticed that the severed leg of a frog contracted under the influence of a nearby electrostatic machine. Further investigations led him to discover that the same muscular twitching occurred when a nerve and a muscle in the frog’s leg were connected together by two different metals in contact with each other. Understandably, Galvani thought that the source of this effect was in the frog’s leg. In fact, the two metals in contact with the şaline liquids in the frog constituted the first chemical electric celi. The chemical action produced a current that caused the muscles to react, as was shown by Alessandro Volta.
In following up the discoveries of Galvani, Volta developed the “voltaic pile” in 1800. It was the first chemical battery and the first source for producing a constant current. In one of its original forms, this device had a silver disk, a zinc disk, a brine-soaked cardboard spacer, another silver disk, and so on, fornıed in a stack.
The publication of Volta’s work in 1800 aroused immense interest, and experimenters soon were using the voltaic pile in many forms for numerous investigations involving currents. One important development was the discovery that water is decomposed by passing a current through it. Humphry Davy was among the first to develop this new field of electrochemistry. Among other things, he identified the elements sodium and potassium in 1807.
ELECTRODYNAMICS: CURRENTS AND MAGNETIC FIELDS
In the first two decades of the 19th century batteries were available, and metals were recognized as excellent conductors of electric current. Quantitative measurement of current was not yet possible, but that was not necessary to the establishment of a relationship between electric currents and magnetic effects.
Oersted’s Work. In 1820, Hans Ghristian Oersted, a professor of physics at the University of Copenhagen, first demonstrated the effect of an electric current on a compass needle placed near a wire carrying a current, thus discovering that there was a connection between tire flow of charge and magnetic phenomena. It seems certain that he had been searching for a connection between electricity and magnetism, and he may have discovered it during a lecture demonstrating electrical effects. In any case, he circulated a document describing his experiments to scientists and scientific societies in 1820. In these experiments Oersted demonstrated all of the qualitative features of the force on a magnetized compass needle caused by nearby currents. He showed that a wire carrying current in a direction parallel to a magnetized needle causes deflection of the needle, and that the direction of the deflection depends on the direction of the current and on whether the needle is above or below the wire.
Oersted’s fundamental discovery was quicky followed by experiments that probed further into relationships between electric currents and magnetism. These experiments were made chiefly by Europeans.
Ampere’s Work. In the 1820’s, Andre Marie Ampere followed Oersted’s discovery by showing stili another basic fact: two adjacent parallel wires carrying currents in the sarne direction attract each other, and two parallel wires carrying currents in opposite directions repel each other. This effect, which is clearly distinguishable from the electrostatic forces between stationary charges, probably is the simplest manifestation of magnetic force.
Besides making this fundamental discovery, Ampere performed quantitative measurements using movable coils, and thus he was able to investigate the laws of the forces involved. He showed mathematically that all of the experimental facts could be understood on the basis that each element of length of a current-carrying wire contributed a magnetic force in the region around it, according to a simple law. Ampere’s formulation was a major contribution to the development of the concept of the field.
Ampere also showed that the force between magnetized rods follows the same rules as the force between appropriate coils carrying currents. This work led him to hypothesize that a magnetized rod of iron is equivalent to a coil carrying a continuous current. This insight is the key to the modern understanding of magnetic matter.
A further result of Ampere’s work was the development of the galvanometer, the basic instrument for measuring the magnitude of electric currents. By 1821, Ampere and others had designed and built galvanometers consisting of a coil of wire surrounding a magnetic needle. The deflection of the needle out of the plane of the coil is a sensitive measure of the current passing through the coil.
Ohm’s Work. The development of the galvanometer for the quantitative measure of currents led to the work of Georg Simon Ohm. Following the work of Jean Fourier on heat conduction in solids, Ohm applied similar reasoning in 1826 to describe electrical conduction in solids. Fourier had investigated the rate of heat flow in a body whose ends are at different temperatures, and he had shown that the rate of heat flow depends on how rapidly the temperature changes with position—the temperature gradient—along the conducting body. Ohm suggested that a similar result should hold for electrical conduction, with “electric potential” replacing “temperature,” and “electric current” replacing “heat flow.” Ohm’s law, which is discussed in a later part of this article, follows from these ideas.
Ohm backed up his reasoning by electrical experiments that verified the correctness of his conclusions. His results show that electric current depends on processes in which the carriers of charge drift from one end of a solid conductor to the other end at a rate depending on the electric potential gradient—that is, on how rapidly the electric potential changes with position along the conducting body.
Ohm’s work had great practical use. Also, it clarified the idea of potential and the relationship between potential diff erence and electric field. These concepts, which were just beginning to be developed, were crucial in the further development of electrical theory.
Faraday’s Work. Michael Faraday, who became involved in electrical experiments soon after the work of Oersted, provided a needed giant step toward a complete electrical theory. His experimental work led him to important new theoretical ideas and also to the development of electric motors and generators.
Faraday pursued the basic idea that a magnetic “field of force” around a current in a wire should cause a magnet to movc, or alternatively, a fixed magnet should cause a current-carrying wire to move. He built devices that demonstrated these effects in 1821.
Even more important was his discovery of “induced” currents in 1831, an effect that came to be called Faraday’s law of electromagnetic induction. Faraday discovered that a changing magnetic field sets up an electric force and that this force causes current to flow in a closed conducting circuit. For instance, if a coil of wire is connected to a galvanometer, any change in the magnetic field in the coil will result in current through the galvanometer while the field is changing. This changing field can be produced by moving a magnetized bar in and out of the coil. Alternatively, the changing field can be produced by changing the current in a nearby coil or by moving the second coil while holding its current constant.
Maxwell’s Synthesis. The phenomena of electricity were discovered by Faraday and many others, each making a limited contribution to the gradually forming knowledge of electricity. In contrast, one person alone—James Clerk Maxwell —was responsible for the uniflcation of ali the known facts into one concise theoretical structure. Maxwell’s remarkable theory, formulated in 1864, rests on the earlier results of Coulomb, Oersted, Ampere, and Ohm, as well as on Faraday’s law of induction. Maxwell showed how these separate discoveries could be brought together in a rigorous mathematical theory that provides the framework for describing ali electric and magnetic phenomena in nature. In particular, his theory predicted that electromagnetic waves can be propagated in space. This immediately led to the identification of light as one kind of electromagnetic wave. It was then possible to show that the transmission of light, refraction of light, interference effects, and so on, could be understood in terms of Maxwell’s electromagnetic theory. Indeed, this theory concisely describes a broad body of physical phenomena.
Four equations summarize Maxwell’s theory. His fîrst equation relates the electric field—defîned as the force that would act on an electric charge at any point in space—to the time rate of change of the magnetic field in the region. This is essentially Faraday induction. Maxwell’s second equation relates the magnetic field in a region to the time rate of change of the electric field in the region. This equation, which is complementary to his first equation, was crucial to the understanding of electromagnetic waves. Two other equations concisely express the spatial properties of electric and magnetic fields—in particular, that the electric field of a concentrated charge depends on the reciprocal of the square of the distance from that charge, a relationship known as the “inverse square” law. Much has been learned about the electric and magnetic properties of matter since Maxwell’s theoretical work, first published in 1873, but basic electrical theory is unchanged. I
Einstein’s Uniflcation. A further unification of electromagnetic theory became possible when Albert Einstein developed his special theory of relativity in 1905. This theory shows that magnetism—that is, the magnetic field—can be understood as the direct result of the application of his theory to electricity. Thus in principle one could explain ali magnetic effects by applying this theory of relativity to the electric fields of charges in motion.