Архитектура Аудит Военная наука Иностранные языки Медицина Металлургия Метрология
Образование Политология Производство Психология Стандартизация Технологии
EARLY HISTORY OF ELECTRICITY
History shows us that at least 2,500 years ago the Greeks were already familiar with the strange force (as it seemed to them) which is known today as electricity. Generally speaking, three phenomena made up all of man's knowledge of electrical effects. The first phenomenon was the familiar lightning flash – a dangerous power which could both kill people and burn or destroy their houses. The second manifestation of electricity was more or less familiar to people: a strange yellow stone which looked like glass was sometimes found in the earth. On being rubbed, that strange yellow stone – amber – obtained the ability of attracting light objects of a small size. The third phenomenon was connected with the so-called electric fish which possessed the property of giving more or less strong electric shocks which could be obtained by a person coming into contact with it.
Nobody knew that the above phenomena were due to electricity. People could neither understand their observations nor find any practical applications for them. All of man's knowledge in the field of electricity has been obtained during the last 370 years. It took a long time before scientists learned how to make use of electricity. Most of the electrically operated devices, such as the electric lamp, the refrigerator, the tram, the lift, the radio are less than one hundred years old. In spite of their having been employed for such a short period of time, they play a most important part in man's everyday life all over the world.
Famous names are connected with the scientific research on electricity, its history. As early as about 600 B. C. the Greek philosopher Phales discovered that when amber was rubbed, it attracted and held minute light objects. However, he could not know that amber was charged with electricity owing to the process of rubbing. Then Gilbert, the English physicist, began the first systematic scientific research on electrical phenomena. He discovered that various substances possessed the property similar to that of amber: they generated electricity when they were rubbed. He gave the name "electricity" to the phenomenon he was studying. He got this word from the Greek "electrum" meaning "amber".
Many learned men of Europe began to use the new word "electricity" in their conversation as they were engaged in research of their own. Scientists of Russia, France and Italy made their contribution as well as the Englishmen and the Germans.
FROM THE HISTORY OF ELECTRICITY
There are two types of electricity, namely, electricity at rest or in a static condition and electricity in motion, that is, the electric current. Both of them are made up of electric charges, static charges being at rest, while electric current flows and does work. Thus, they differ in their ability to serve mankind as well as in their behaviour.
Static electricity was the only electrical phenomenon to be observed by man for a long time. At least 2,500 years ago the Greeks knew how to get electricity by rubbing substances. However, the electricity to be obtained by rubbing objects cannot be used to light lamps, to boil water, to run electric trains, and so on. It is usually very high in voltage and difficult to control, besides it discharges in no time.
As early as 1753, Franklin made an important contribution to the science of electricity. He was the first to prove that unlike charges are produced due to rubbing dissimilar objects. To show that the charges are unlike and opposite, he decided to call the charge on the rubber-negative and that on the glass-positive.
In this connection one might remember the Russian academician V. V. Petrov. He was the first to carry on experiments and observations on the electrification of metals by rubbing them one against another. As a result he was the first scientist in the world who solved that problem.
Volta’s discovery of electric current developed out of Galvani's experiments with the frog. Galvani observed that the legs of a dead frog jumped as a result of an electric charge. He tried his experiment several times and every time he obtained the same result. He thought that electricity was generated within the leg itself.
Volta began to carry on similar experiments and soon found that the electric source was not within the frog's leg but was the result of the contact of both dissimilar metals used during his observations. However, to carry on such-experiments was not an easy thing to do. He spent the next few years trying to invent a source of continuous current. To increase the effect obtained with one pair of metals, Volta increased the number of these pairs. Thus the voltaic pile consisted of a copper layer and a layer of zinc placed one above another with a layer of flannel moistened in salt water between them. A wire was connected to the first disc of copper and to the last disc of zinc.
The year 1800 is a date to be remembered: for the first time in the world's history a continuous current was generated.
Volta was born in Como, Italy, on February 18, 1745. For some years he was a teacher of physics in his home town. Later on he became professor of natural sciences at the University of Pavia. After his famous discovery he traveled in many countries, among them France, Germany and England. He was invited to Paris to deliver lectures on the newly discovered chemical source of continuous current. In 1819 he returned to Como where he spent the rest of his life. Volta died at the age of 82.
Nature of Electricity
The first recorded observation on electricity was made by the ancient Greek philosopher Phales. He stated that a piece of amber rubbed with fur attracted light objects. But more than 22 centuries passed before the study of magnetism and of electrical phenomena began by Galileo and other scientists.
It was well known that not only amber, but many other substances having been rubbed behave like amber i.e. can be electrified. It was discovered that any 2 dissimilar substances forced into contact and then separated became electrified, or acquired electrical charges.
During the 19th century the idea of the nature of electricity was completely revolutionized. The atom was regarded as the ultimate subdivision of matter. Today the atom is regarded as an electrical system. In this electrical system there is a nucleus containing positively charged particles called protons. The nucleus is surrounded by lighter negatively charged units – electrons. So the most essential constituent of matter is made up of electrically charged particles. Matter is neutral and produces no electrical effects when it has equal amounts of both charges.
But when the number of negative charge is unlike the number of positive ones, matter will produce electrical effects. Having lost some of its electrons, the atom has a positive charge: having an excess of electrons – it has a negative charge.
Electricity plays such an important part in modern life that in order to get it, men have been burning millions of tons of coal. Coal is burned instead of its being mainly used as a source of valuable chemical substances which it contains. Therefore, finding new sources of electric energy is a most important problem that scientists and engineers try to solve.
Hundreds of millions of volts are required for a lightning spark about one and a half kilometre long. However, this does not represent very much energy because of the intervals between single thunderstorms. As for the power spent in producing lightning flashes all over the world, it is only about 1/10,000 of the power got by mankind from the sun, both in the form of light and that of heat. Thus, the source in question may interest only the scientists of the future.
Atmospheric electricity is the earliest manifestation of electricity known to man. However, nobody understood that phenomenon and its properties until Benjamin Franklin made his kite experiment. On studying the Leyden jar (for long years the only known condenser), Franklin began thinking that lightning was a strong spark of electricity. He began experimenting in order to draw electricity from the clouds to the earth. The story about his famous kite is known all over the world.
On a stormy day Franklin and his son went into the country taking with them some necessary things such as: a kite with a long string, a, key and so on. The key was connected to the lower end of the string. "If lightning is the same as electricity," Franklin thought, "then some of its sparks must come down the kite string to the key." Soon the kite was flying high among the clouds where lightning flashed. However, the kite having been raised, some time passed before there was any proof of its being electrified. Then the rain fell and wetted the string. The wet string conducted the electricity from the clouds down the string to the key. Franklin and his son both saw electric sparks which grew bigger and stronger. Thus, it was proved that lightning is a discharge of electricity like that got from the batteries of Leyden jars.
Trying to develop a method of protecting buildings during thunderstorms, Franklin continued studying that problem and invented the lightning conductor. He wrote necessary instructions for the installation of his invention, the principle of his lightning conductor being in use until now. Thus, protecting buildings from strokes of lightning was the first discovery in the field of electricity employed for the good of mankind.
In studying the electric current, the following relation between magnetism and the electric current can be observed; on the one hand magnetism is produced by the current and on the other hand the current is produced from magnetism.
Magnetism is mentioned in the oldest writings of man. Romans, for example, knew that an object looking like a small dark stone had the property of attracting iron. However, nobody knew who discovered magnetism or where and when the discovery was made. Of course, people could not help repeating the stories that they had heard from their fathers who, in their turn, heard them from their own fathers and so on.
One story tells us of a man called Magnus whose iron staff was pulled to a stone and held there. He had great difficulty in pulling his staff away. Magnus carried the stone away with him in order to demonstrate its attracting ability among his friends. This unfamiliar substance was called Magnus after its discoverer, this name having come down to us as "Magnet".
According to another story, a great mountain by the sea possessed so much magnetism that all passing ships were destroyed because all their iron parts fell out. They were pulled out because of the magnetic force of that mountain.
The earliest practical application of magnetism was connected with the use of a simple compass consisting of one small magnet pointing north and south.
A great step forward in the scientific study of magnetism was made by Gilbert, the well-known English physicist (1540–1603). He carried out various important experiments on electricity and magnetism and wrote a book where he put together all that was known about magnetism. He proved that the earth itself was a great magnet.
Reference must be made here to Galileo, the famous Italian astronomer, physicist and mathematician. He took great interest in Gilbert's achievements and also studied the properties of magnetic materials. He experimented with them trying to increase their attracting power.
At present, even a schoolboy is quite familiar with the fact that in magnetic materials, such as iron and steel, the molecules themselves are minute magnets, each of them having a north pole and a south pole.
Последнее изменение этой страницы: 2016-03-25; Просмотров: 868; Нарушение авторского права страницы