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2.2.2 Read the text about the metric system and anwer which sentanses below it are true and which are false.

The metric system. The metric system is the international decimal system of weights and measures based on the metre and kilogram.

The metre, the unit of length, is defined in terms of the bar of platinum-indium known as the international prototype metre at the International Bureau.

This is a line standard of length made with a cross section known as the Tresca section, selected because of its great rigidity for a giv­en weight, and having microscopic lines engraved on the plane of its neutral axis.

The composition of the alloy is 90% platinum, 10% iridium. The distance between the central and one of the group of lines at each end when the bar, being subjected to normal atmospheric pres­sure, is supported on two rollers at least 1 centimetre in diameter placed symmet­rically 572 mm. apart and the bar is at temperature of 0 °C. is defined as one metre.

It was shown by Albert Abraham Michelson that a standard of lengths could be replaced by reference to the measurement of wave length of light. In 1927 the Seventh General (International) Con­ference on Weights and Measures adopt­ed provisionally a supplementary defi­nition of the metre in terms of the wave length of light. According to this def­inition the relation for red cadmium light waves under specified conditions of tem­perature, pressure and humidity is 1 metre = 1,553,164.13 wave lengths. The kilogram is the mass of a definite platiiuim-iridium standard, the international prototype kilogram, kept at the International Bu­reau of Weights and Measures. The composition of this cylinder, which has a height approximately equal to its diameter, is the same as that of the prototype metre, namely, 90% platinum and 10% iridium.

The litre is defined as the volume of a kilogram of pure water at the temperature of its maximum density and under standard pressure.

False or true?

  1. The weights and measures of the metric system are based on the metre and kilometer.

  2. The platinum-indium alloy consists of 90% platinum and 10% iridium.

  3. In the 30s of the 20th centure the explanation of the metre with the help of the wave length of light was adopt­ed.

  4. One litre may be defined as the volume of a kilogram of any liquid at the temperature of its maximum density and under standard pressure.

2.2.3 Read the text, translate it and choose the right form from brackets.

Effects of Temperature and Pressure and Gravity. Because the dimensions of any standard change with temperature, it is … (necessary, necessarily) to state the temperature at which standards of length, area, volume, or capacity are to be used, or, if used at any other than standard temperature, then the coefficient of expansion must be stated. The standard temperatures most often used are: 0 °C. (32 °F.), the standard temperature for the prototypes of the metric system, also used for some secondary metric standards and for some measurements; 20 °C. (68 °F.), the internationally adopted standard temperature which is being … (increasing, increasingly) used for weights and measures work; 16.67 °C. (62 °F.), the official temperature used in connection with the British imperial system; 4 °C. (39.20 °F.), the temperature of maximum density of water, used in density and volumetric work in which water is … (direct, directly) or … (indirect, indirectly) involved; 25 °C. (41 °F.), a standard temperature … (used, using) in some work in physical chemistry; and 15.56 °С. (60 °F.), the standard temperature … (used, using) in the petroleum industry for density and volume work.

Changes in … (atmospheric, atmospherically) pressure have very little effect on length standards, a change in pressure from 710 to 790 mm. causing a change in length of about 0.00005 mm. in the case of the prototype metre. Changes in … (atmospheric, atmospherically) pressure are usually disregarded in measurements of length.

Any change in air pressure, as well as changes in the temperature and humidity of the air is, however, of importance in any comparison of masses because these are factors … (affected, affecting) the buoyant effect of the air.

Any definitions of weights which involve comparisions of the weights of bodies having different densities are incomplete unless the atmospheric conditions are specified together with the densities of mate­rial employed. In the United States most commercial weights are verified on the basis of apparent mass in air against brass standards of density 8.4 g./cm.3 at 0 °C., no correction being made for the buoyant effect of the air, the values for the brass standards being their true mass or weight in vacuo.

When weighing on an equal arm balance, if the body being weighed does not have the same density as the weights, a correction must be made.

The standard air pressure usually … (used, using) in weights and measures work is 760 mm. mercury, at 20 °C., mercury having a specified density of 13.5951 g. per cm.3, with gravity 980.665 cm./sec.2

Since a spring balance indicates weight and not mass, a constant mass suspended from a spring balance will produce different readings when measurements are made in a series of places having sufficient changes in the force of gravity, the indicated weights varying … (direct, directly) with the force of gravity, other conditions being equal. An equal arm balance likewise really compares weights rather than masses, but on the assumption that the two pans of the balance are acted upon by the same force of gravity, the result is an … (indirect, indirectly) comparison of masses. If other conditions remained constant, a balance would every­where give the same balance between the same two bodies, no matter how the force of gravity varied from place to place — provided always as above, the assumption is justified that the force of gravity is the same on the two pans of the balance.