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 1700’s.
The Ancient Greeks observed, that when amber (amber is a yellowish translucent mineral) is rubbed, it attracts small and light objects. About 600 BC Greek philosopher Thales of Miletus held, that amber had a soul, since it could make other objects move. Three centuries after Thales, another Greek philosopher Theopastrus has written, that other substances also have this power. For almost 2,000 years after Theopastrus, little progress was made in the study of electricity. In 1600 English physician William Gilbert published a book in which he noted, that many other substances beside amber could be charged by rubbing. He gave these substances the Latin name electrica, which is derived from the Greek word electron (which means “amber”). English writer and physician Sir Thomas Brawne first used the word electricity in 1646.
Gilbert’s experiments led to a number of investigations by many pioneers in the development of electricity technology over the next 350 years:
Franklin was an American writer, publisher, scientist and diplomat. He had theorised that electricity is a kind of fluid. According to his theory, when two objects are rubbed together, electric fluid flows from one object to the other. The object that gains electric fluid makes what he called positive change and the one that looses electric fluid goes to another change, which he called negative. He also demonstrated that lightning is a form of electricity. In 1752 he constructed a kite and flew id 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. The experiment proved Franklin’s theory, but it was extremely dangerous – He could easily have been killed.
Luigi Galvani and Alessandro Volta
In 1786, Luigi Galvani, an Italian biologist, found that when the leg of a dead frog was touched by a metal knife, the leg twitched violently. Galvani thought that the frogs muscles must contain electricity and he also found out, that the muscles in a frog’s leg would contract if he applied an electric current to them. By 1792 another Italian scientist, Alessandro Volta, disagreed: he realised that the main factors in Galvani’s discovery were the two different metals – the steel knife and the tin plate – upon which the frog was lying. Volta showed that when moisture comes between two different metals, electricity is crated. This led him to invent the first electric battery, the voltaic pile, which he made from thin sheets of copper and zinc separated by moist pasteboard.
In this way, a new kind of electricity was discovered, electricity that flowed steadily like a current of water instead of discharging itself in a single spark or shock. Volta showed that electricity could be made to travel from one place to another by wire, thereby making an important contribution to the science of electricity. The unit of electrical potential, the Volt, is named after Volta.
The credit for generating electric current on a practical scale goes to the famous English scientist, Michael Faraday. Faraday was greatly interested in the invention of the electromagnet, but his brilliant mind took earlier experiments still further. If electricity could produce magnetism, why couldn't magnetism produce electricity.
In 1831, Faraday found the solution. Electricity could be produced through magnetism by motion. He discovered that when a magnet was moved inside a coil of copper wire, a tiny electric current flows through the wire. Of course, by today's standards, Faraday's electric dynamo or electric generator was crude, and provided only a small electric current be he discovered the first method of generating electricity by means of motion in a magnetic field.
Thomas Edison and Joseph Swan
Nearly 40 years went by before a really practical DC (Direct Current) generator was built by Thomas Edison in America. Edison's many inventions included the phonograph and an improved printing telegraph. In 1878 Joseph Swan, a British scientist, invented the incandescent filament lamp and within twelve months Edison made a similar discovery in America.Swan and Edison later set up a joint company to produce the first practical filament lamp. Edison used his DC generator to provide electricity to light his laboratory and later to illuminate the first New York Street to be lit by electric lamps, in September 1882. Edison's successes were not without controversy, however - although he was convinced of the merits of DC for generating electricity, other scientists in Europe and America recognized that DC brought major disadvantages.
When Edison's generator was coupled with Watt's steam engine, large-scale electricity generation became a practical proposition. James Watt, the Scottish inventor of the steam condensing engine, was born in 1736. His improvements to steam engines were patented over a period of 15 years, starting in 1769 and his name was given to the electric unit of power, the Watt.
Watt's engines used the reciprocating piston, however, today's thermal power stations use steam turbines, following the Rankine cycle, worked out by another famous Scottish engineer, William J.M Rankine, in 1859.
Andre Ampere and George Ohm
Andre Marie Ampere, a French mathematician who devoted himself to the study of electricity and magnetism, was the first to explain the Electro-dynamic theory. A permanent memorial to Ampere is the use of his name for the unit of electric current.
George Simon Ohm, a German mathematician and physicist, was a college teacher in Cologne when in 1827 he published, "The galvanic Circuit Investigated Mathematically". His theories were coldly received by German scientists but his research was recognised in Britain and he was awarded the Copley Medal in 1841. His name has been given to the unit of electrical resistance.
11. duben 2008