Electricity

XI

History

Humans have known about the existence of static electricity for thousands of years, but scientists did not make great progress in understanding electricity until the 1700s.

A

Early Theories

The ancient Greeks observed that amber, when rubbed, attracted small, light objects. About 600 bc Greek philosopher Thales of Miletus held that amber had a soul, since it could make other objects move. In a treatise written about three centuries later, another Greek philosopher, Theophrastus, stated that other substances also have this power.

For almost 2,000 years after Theophrastus, little progress was made in the study of electricity. In 1600 English physician William Gilbert published a book in which he noted that many substances besides amber could be charged by rubbing. He gave these substances the Latin name electrica, which is derived from the Greek word elektron (which means “amber”). The word electricity was first used by English writer and physician Sir Thomas Browne in 1646.

The fact that electricity can flow through a substance was discovered by 17th-century German physicist Otto von Guericke, who observed conduction in a linen thread. Von Guericke also described the first machine for producing an electric charge in 1672. The machine consisted of a sulfur sphere turned by a crank. When a hand was held against the sphere, a charge was induced on the sphere. Conduction was rediscovered independently by Englishman Stephen Gray during the early 1700s. Gray also noted that some substances are good conductors while others are insulators.

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Also during the early 1700s, Frenchman Charles Dufay observed that electric charges are of two kinds. He found that opposite kinds attract each other while similar kinds repel. Dufay called one kind vitreous and the other kind resinous.

American scientist Benjamin Franklin theorized that electricity is a kind of fluid. According to Franklin’s theory, when two objects are rubbed together, electric fluid flows from one object to the other. The object that gains electric fluid acquires a vitreous charge, which Franklin called positive charge. The object that loses electric fluid acquires a resinous charge, which Franklin called negative charge.

Franklin demonstrated that lightning is a form of electricity. In 1752 he constructed a kite and flew it during a storm. When the string became wet enough to conduct, Franklin, who stood under a shed and held the string by a dry silk cord, put his hand near a metal key attached to the string. A spark jumped. Electric charge gathered by the kite had flowed down the wet string to the key and then jumped across an air gap to flow to the ground through Franklin’s body. Franklin also showed that a Leyden jar, a device able to store electric charge, could be charged by touching it to the key when electric current was flowing down the string.

Around 1766 British chemist Joseph Priestley proved experimentally that the force between electric charges varies inversely with the square of the distance between the charges. Priestley also demonstrated that an electric charge distributes itself uniformly over the surface of a hollow metal sphere and that no charge and no electric field of force exists within such a sphere. French physicist Charles Augustin de Coulomb reinvented a torsion balance to measure accurately the force exerted by electric charges. With this apparatus he confirmed Priestley’s observations and also showed that the force between two charges is proportional to the product of the individual charges.

In 1791 Italian biologist Luigi Galvani published the results of experiments that he had performed on the muscles of dead frogs. Galvani had found earlier that the muscles in a frog’s leg would contract if he applied an electric current to them.

B

19th and 20th Centuries

In 1800 another Italian scientist, Alessandro Volta, announced that he had created the voltaic pile, a form of electric battery. The voltaic pile made the study of electric current much easier by providing a reliable, steady source of current. Danish physicist Hans Christian Oersted demonstrated that electric currents are surrounded by magnetic fields in 1819. Shortly afterward, André Marie Ampère discovered the relationship known as Ampere’s law, which gives the direction of the magnetic field. Ampère also demonstrated the magnetic properties of solenoids. Georg Simon Ohm, a German high school teacher, investigated the conducting abilities of various metals. In 1827 Ohm published his results, including the relationship now known as Ohm’s law.

In 1830 American physicist Joseph Henry discovered that a moving magnetic field induces an electric current. The same effect was discovered a year later by English scientist Michael Faraday. Faraday introduced the concept of lines of force, a concept that proved extremely useful in the study of electricity.

About 1840 British physicist James Prescott Joule and German scientist Hermann Ludwig Ferdinand von Helmholtz demonstrated that electricity is a form of energy and that electric circuits obey the law of the conservation of energy.

Also during the 19th century, British physicist James Clerk Maxwell investigated the properties of electromagnetic waves and light and developed the theory that the two are identical. Maxwell summed up almost all the laws of electricity and magnetism in four mathematical equations. His work paved the way for German physicist Heinrich Rudolf Hertz, who produced and detected electric waves in the atmosphere in 1886, and for Italian engineer Guglielmo Marconi, who harnessed these waves in 1895 to produce the first practical radio signaling system.

The electron theory, which is the basis of modern electrical theory, was first advanced by Dutch physicist Hendrik Antoon Lorentz in 1892. American physicist Robert Andrews Millikan accurately measured the charge on the electron in 1909. The widespread use of electricity as a source of power is largely due to the work of pioneering American engineers and inventors such as Thomas Alva Edison, Nikola Tesla, and Charles Proteus Steinmetz during the late 19th and early 20th centuries.

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