LITMIR.BIZ

      VI .Translate at sight

      Noble metals I

      Gold. A yellow metal of very high plasticity, with a tensile strength of 150 MPa and elongation at rupture of 40 %. Its uses in electrical engineering include corrosion-resistant contact coats, photocell electrodes, vacuum-deposited thin-film microcircuits, etc.

      Silver, A white, lustrous metal, resistant to oxidation at normal temperature. Of all the metals, silver has the lowest resistivity at normal temperature. The tensile strength of silver wire is about 200MPa, and elongation approximately 50 %. This wire is used for contacts intended to operate at low currents.

      Silver is also employed for direct deposition of coats by fusion or vacuum evaporation on dielectrics, which serves as silver plates in ceramic and mica capacitors.

      The metal presents a drawback in that migrates from the surface of a dielectric into its body at high humidity and also at elevated temperatures of the surrounding medium. Silver is inferior to other noble metals in chemical resistance.

      Iron

      Iron (steel), the cheapest and most common commercial metal of high mechanical strength, is sometimes used as a conducting material. However, even pure iron has a considerably higher resistivity (about 0.1 Mflm) than copper and aluminium; steel, i.e. iron with carbon and other elements chemically dissolved in it, has a still greater resistivity.

      Being a ferromagnetic material, steel is liable to a pronounced skin effect in an a.c. field; therefore steel conductors offer a higher opposition to alternating current than to direct current. Besides, the hysteresis loss appears in steel conductors when they carry alternating current.

      Ordinary steel is vulnerable to corrosive attack; even at normal temperature, especially at high humidity, it rusts rather rapidly, while at elevated temperature the rate of corrosion is even higher. This calls for the protection of steel conductors with a surface layer of a more corrosion-resistant material. Commonly, steel conductors are zinc-plated.

      Since iron has a high temperature coefficient of resistivity, a thin iron wire, if protected against corrosion by placing it, for example, into a vessel filled with hydrogen, can be used in devices, where the wire resistance rises rapidly with increases in current tends to maintain a constant current despite the variations in the line voltage.

      Exercises:

      I. Find the Kazakh (Russian) equivalents to the following words and word combinations

      А) Iron; steel; mechanical strength; carbon; alternating current (a. c.); hysteresis loss; skin effect; vulnerable to corrosive attack; humidity; rust; surface layer; placing into a vessel filled with hydrogen; variations in the line voltage.

      В) колебание напряжения; обладает малой коррозионной стой-костью; сталь; механическая прочность; потеря мощности на гис-терезис; железо; поверхностный эффект; поверхность; ржаветь; помещение в баллон, заполненный водородом; углерод; пере-менный ток; влажность.

      С) кернеудің тербелуі; төменгі тот басу қасиетіне ие; болат; механикалық беріктілік; қуатты гистеризиске кетіру; темір; беттік құбылыс; тоттану; балонға орналастыру; сутегімен толтырылған; көміртегі; айнымалы тоқ; ылғалдылық.

      II. Translate into English using the active vocabulary of the lesson:

      III. Translate at sight

      Noble metals II

      Platinum. A metal that does not react with oxygen and shows rather high resistance to chemical agents. It displays superb ductility and malleability, and readily draws into very thin wire and foil. The tensile strength of annealed platinum is about 150 MPa, with elongation of 30 to 35 %. Platinum is used for thermocouples designed to measure temperature up to 1,60 °C, for the production of pastes intended for fusion of electrodes on to the body of ceramic capacitors, etc.

      Platinum filaments drawn to about 1 Mm across are used for suspension of movable systems in elecrometers and other sensitive devices. They are obtained through multiple drawing of the composite platinum-silver wire with the subsequent dissolution of the silver coat in nitric acid (platinum is resistant to this acid).

      Platinum is too soft for use alone as a contact material, and is generally alloyed with harder metals. Platinum-iridium alloys are wear- and corrosion-resistant, hard and tough, allow for a high repetition rate of switching, but are costly, and so their uses are limited to critical applications.

      Palladium. It stands close to platinum in many of its properties, and often serves as a substitute for the latter. It is valued in the electron- tube industry for hydrogen absorption. Palladium and its alloys with silver and copper go into the production of contacts. Palladium paste, like platinum paste, is used for the deposition of electrodes on to

Скачать книгу