style="font-size:15px;"> 2. , the metric system has been legal for trade since 1866 but other measurements such as the gallon, inch, and the pound are still widely used.
3. The original base unit of mass in the metric system was chosen to match the mass of one cubic centimeter of water.
4. is the only base unit not to have been redefined in terms of an unchanging natural phenomenon.
5. The metric unit of time is It was originally defined as
LESSON 3
Active vocabulary
Superconductivity- Сверхпроводимость
discover- обнаружить
phenomenon- явление
occur- произойти
electrical resistance- электрическое сопротивление
exclusion- исключение
interior – интерьер
property- собственность
digital circuits- цифровые схемы
microwave filter- микроволновый фильтр
storage devices- устройства хранения данных
abruptly- резко
impose- наложить
electrical current- электрический ток
cryogenic temperatures- криогенные температуры
refrigerant- хладагент
Superconductivity, discovered in 1911 by , is a phenomenon occurring in certain materials at extremely low temperatures (on the order of −200 degrees Celsius), characterized by exactly zero and exclusion of the interior (the Meissner effect). Materials with such properties are called superconductors.
Superconductors are used to make some of the most powerful electromagnets known to man, including those used in MRI machines. They have also been used to make digital circuits, highly sensitive magnetometers, and filters for mobile phone base stations. They can also be used for the separation of weakly magnetic particles from less magnetic or nonmagnetic particles, as in the industries. Promising future applications include high-performance , power storage devices, electric power transmission, (such as for ), and magnetic levitation devices.
The (the measure of how much a material resists an electric current) of a metallic decreases gradually as the temperature is lowered. However, in ordinary conductors such as and , impurities and other defects impose a lower limit. Even near , a sample of copper shows non-zero resistance. The resistance of a superconductor, on the other hand, drops abruptly to zero when the material is cooled below a temperature called its "critical temperature" – typically 20 (K) or less. An electrical current flowing in a loop of superconducting wire will persist indefinitely with no power source (provided that no energy is drawn from it).
Superconductivity occurs in a wide variety of materials, including simple elements like and , various metallic , and certain kinds of ceramic materials known as high-temperature superconductors (HTS). Superconductivity does not occur in noble metals like and , nor in most metals that can be spontaneously magnetized.
In 1986, the discovery of HTS, with critical temperatures in excess of 90 K, spurred renewed interest and research in superconductivity for several reasons. As a topic of pure research, these materials represented a new phenomenon not explained by the current theory. Also, because the superconducting state persists up to more manageable temperatures, more commercial applications become feasible, especially if materials with even higher critical temperatures could be discovered.
History of superconductivity
Superconductivity was discovered in 1911 by , who was studying the resistance of solid at cryogenic temperatures using the recently discovered liquid as a refrigerant. At the temperature of 4.2 K, he observed that the resistance abruptly disappeared. For this discovery, he was awarded the in 1913.
In subsequent decades, superconductivity was found in several other materials. In 1913, was found to be superconductive at 7 K, and in 1941 niobium nitride was found to be superconductive at 16 K.
The next important step in understanding superconductivity occurred in 1933, when Walter Meissner and Robert Ochsenfeld discovered that superconductors expelled applied magnetic fields, a phenomenon that has come to be known as the "Meissner effect." In 1935 F. and H. London showed that the Meissner effect was a consequence of the minimization of the electromagnetic free energy carried by superconducting current.
In 1950 Lev Landau and Vitaly Ginzburg formulated what came to be called the phenomenological Ginzburg-Landau theory of superconductivity. This theory had great success in explaining the macroscopic properties of superconductors. In particular, Alexei Abrikosov showed that the theory predicts the division of superconductors into the two categories, now referred to as Type I and Type II. Abrikosov and Ginzburg were awarded the 2003 for their work (Landau died in 1968).
Also in 1950, James Maxwell and Reynolds et al. found that the critical temperature of a superconductor depends on the of the constituent . This discovery revealed that the internal mechanism responsible for superconductivity was related to the attractive force between electrons and the ion lattice beneath – known as electron-phonon interactions.
The complete, microscopic theory of superconductivity
