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Сообщение ВАЛЕНОК » 23.09.2015, 16:06

There is electricity everywhere in the world. It is present in the atom, whose particles are held together by its forces; it reaches us from the most distant parts of the universe in the form of electro- magnetic waves. Yet we have no organs that could recognize it as we see light, hear sound. We have to make it visible, tangible or audible; we have to make it perform work to become aware of its presence. There is only one natural phenomenon which demonstrates it unmis- takably to our senses of seeing and hearing – thunder and lightning; but we recognize only the effects – not the force which causes them. Small wonder, then, that Man lived for ages on this earth without knowing anything about electricity. He tried to explain the phenome- non of the thunderstorm to himself by imagining that some gods or other supernatural creatures were giving vent to their heavenly, anger, or were fighting battles in the sky. Thunderstorms frightened our primitive ancestors; they should have been grateful to them instead
because lightning gave them their first fires, and thus opened to them the road to civilization. It is a fascinating question how differently life on earth would have developed if we had an organ for electricity. We cannot blame the ancient Greeks for failing to recognize that the force which causes a thunderstorm is the same which they ob- served when rubbing a piece; of amber: it attracted straw, feathers, and other light materials. Thales of Miletos, the Greek philosopher who lived about 600 BC, was the first who noticed this. The Greek word for amber is “electron”, and the therefore Thales called that mysteri- ous force electric. For a long time it was thought to be of the same na- ture as the magnetic power of the lodestone since the effect of attrac- tion seems similar, and in fact there are many links between electricity and magnetism. There is just a chance, although a somewhat remote one, that the ancient Jews knew something of the secret of electricity. Perhaps the Israelites did know something about electricity; this theory is supported by the fact that the Temple at Jerusalem had metal rods on the roof which must have acted as lightning-conductors. In fact, during the thousand years of its existence it was never struck by lightening although thunderstorms abound in Palestine. There is no other evidence that electricity was put to any use at all in antiquity, except that the Greek women decorated their spinning- wheels with pieces of amber: as the woolen threads rubbed against the amber it first attracted and then repelled them – a pretty little spectacle which relieved the boredom of spinning. More than two thousand years passed after Thales’s discovery without any research work being done in this field. It was Dr. William Gilbert, Queen Elizabeth the First’s physician-in-ordinary, who set the ball rolling. He experimented with amber and lodestone and found the essential difference between electric and magnetic attraction. For sub- stances which behaved like amber – such as glass, sulphur, and seal- ing-wax – he coined the term “electrica”, and for the phenomenon as such the word “electricity”. In his famous work “De magnete”, pub- lished in 1666, he gave an account of his studies. Although some sources credit him with the invention of the first electric machine, this was a later achievement by Otto von Guericke, inventor of the air pump.Von Guericke’s electric machine consisted of large, disc spin- ning between brushes; this made sparks leap across a gap between two
metal balls. It became a favourite toy in polite society but nothing more than that. In 1700, an Englishman by the name of Francis Hawksbee produced the first electric light: he exhausted a glass bulb by means of a vacuum pump and rotated it at high speed while rub- bing it with his hand until it emitted faint glow of light. A major advance was the invention of the first electrical con- denser, now called the Leyden jar, by a Dutch scientist, a water-filled glass bottle coated inside and out with metallic surfaces, separated by the non-conducting glass; a metal rod with a knob at the top reached down into the water. When charged by an electric machine it stored enough electricity to give anyone who touched the knob a powerful shock. More and more scientists took up electric research. A Russian scientist Professor Richmann from St. Petersburg, was killed when he worked on the same problem. Benjamin Franklin, born in Boston, was the fifteenth child of poor soap-boiler from England. He was well over 30 when he looked up the study of natural phenomena. “We had for some time been of opinion, that the electrical fire was not created by friction, but col- lected, being really an element diffused among, and attracted by other matter, particularly by water and metals”, – wrote Franklin in 1747. Here was at last a plausible theory of the nature of electricity, namely, that it was some kind of “fluid”. It dawned on him, that thunderstorms were merely a discharge of electricity between two objects with dif- ferent. He saw that the discharging spark, the lightning, tended to strike high buildings and trees, which gave him an idea of trying to attract the electrical “fluid” deliberately to the earth in a way that the dis- charge would do no harm. In order to work this idea out he undertook his famous kite-and- key experiment in the summer of 1755. It was much more dangerous than lie realized. During the approach of thunderstorm he sent up a silken kite with an iron tip; he rubbed the end of the kite string, which he had soaked in water to make it a good conductor of electricity, with a large iron key until sparks sprang from the string – which proved his theory. Had the lightning struck his kite he, and his small son whom he had taken along, might have lost their lives. On the next experiment he fixed an iron bar to the outer wall of his house, and through it charged a Leyden jar with atmospheric elec-
tricity. Soon alter this he was appointed Postmaster General of Brit- ain’s American colonies, and had to interrupt his research work. Tak- ing it up again in 1700, he put up the first effective lightning- conductur on the house of a Philadelphia business man. His theory was that during a thunderstorm a continual radiation of electricity from the earth through the metal of the lightning- conductor would take place, thus equalizing the different potentials of the air and the earth so that the violent discharge of the lightning would be avoided. The modern theory, however, is that the lightning- conductor simply offers to the electric tension a path of low resistance for quiet neutralization. At any rate – even if Franklin’s theory was wrong – his invention worked. Yet its general introduction in America and Europe was delayed by all kinds of superstitions and objections: if God warned to punish someone by making the lightning strike his house, how could Man dare to interfere? By 1782, however, all the public buildings in Phila- delphia, first capital of the USA, had been equipped with Franklin lightning-conductors, except the French Embassy. In that year this house was struck by lightning and an official killed. Franklin had won the day. It was he who introduced the idea of “positive” and “negative” electricity, based on the attraction and repulsion of electrified objects. A French physicist, Charles Auguslin de Coulomb, studied these forces between charged objects, which are proportional to the charge and the distance between the objects; he invented the torsion balance for measuring the force of electric and magnetic at1raction. In his honor, the practical unit of quantity of electricity was named after him. To scientists and laymen alike, however, this phenomenon of ac- tion at a distance caused by electric and magnetic forces was still rather mysterious. What was it really? In 1780, one of the greatest sci- entific fallacies of all times seemed to provide the answer. Aloisio Galvani, professor of medicine at Bologna, was lecturing to his stu- dents at his home while his wife was skinning frogs, the professor’s favorite dish, for dinner with his scalpel in the adjoining kitchen. As she listened to the lecture the scalpel fell from her hand on to the frog’s thigh, touching the zinc plate at the same time. The dead frog jerked violently as though trying to jump off the plate. The signora screamed. The professor, very indignant about this interruption of his
lecture, strode into the kitchen. His wife told him what had happened, and again let the scalpel drop on the frog. Again it twitched. No doubt the professor was as much perplexed by this occur- rence as his wife. But there were his students, anxious to know what it was all about. Galvani could not admit that he was unable to explain the jerking frog. So, probably on the spur of the moment he explained: “I have made a great discovery – animal electricity, the primary source of life”. “An intelligent woman had made an interesting observation, but the not-so-intelligent husband drew the wrong conclusions, was the judgment of a scientific author a few years later. Galvani made nu- merous and unsystematic experiments with frogs’ thighs, most of which failed to prove anything at all; in fact, the professor did not know what to look for, except his animal electricity. These ex- periments became all the rage in Italian society, and everybody talked about galvanic electricity currents – terms which are still in use al- though Professor Galvani certainly did not deserve the honor. A greater scientist than he, Alessandro Volta of Pavia, solved the mystery and found the right explanation for the jerking frogs. Far from being the “primary source of life”, they played the very modest part of electric conductors while the steel of the scalpel and the zinc of the plate were, in fact, the important things. Volta showed that an electric current begins to flow when two different metals are separated by moisture (the frog had been soaked in salt water), and the frog’s mus- cles had merely demonstrated the presence of the current by contract- ing under its influence. Professor Volta went one step further – a most important step, because he invented the first electrical battery, the “Voltaic pile”. He built it by using discs of different metals separated by layers of felt which he soaked in acid. A “pile” of these elements produced usable electric current, and for many decades this remained the only practical source of electricity. From 1800, when Volta announced his invention, electrical research became widespread among the world’s scientists in innumerable laboratories.


Сообщение English Guru » 23.09.2015, 16:12

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Описание: Поможем с английским языком. В первую очередь помогаем тем, кто просит проверить задание, которое было выполнено самостоятельно. Отвечаем на вопросы, консультируем. Помощь с английским оказывается бесплатно.

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