Temperature

Date: 2015-10-07; view: 449.

Time

Mass

Length

FREE – READING PASSAGE

It is advisable that you read the following passage about some basic units in SI system of measurements. You can pick up some new vocabulary items. Try to do some practice on translation.

The meter and the kilogram had their origin in the metric system. By international agreement, the standard meter had been defined as the distance between two fine lines on a bar of platinum-iridium alloy. The 1960 conference redefined the meter as 1,650,763.73 wavelengths of the reddish-orange light emitted by the isotope krypton-86. The meter was again redefined in 1983 as the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 of a second.

When the metric system was created, the kilogram was defined as the mass of 1 cubic decimeter of pure water at the temperature of its maximum density (4.0° C/39.2° F). A solid cylinder of platinum was carefully made to match this quantity of water under the specified conditions. Later it was discovered that a quantity of water as pure or as stable as required could not be provided. Therefore the primary standard of mass became the platinum cylinder, which was replaced in 1889 by a platinum-iridium cylinder of similar mass. Today this cylinder still serves as the international kilogram, and the kilogram in SI I defined as a quantity of mass of the international prototype of the kilogram.

For centuries, time has been universally measured in terms of the rotation of the earth. The second, the basic unit of time, was defined as 1/86,400 of a mean solar day or one complete rotation of the earth on its axis. Scientists discovered, however, that the rotation of the earth was not constant enough to serve as the basis of the time standard. As a result, the second was redefined in 1967 in terms of the resonant frequency of the cesium atom-that is, the frequency at which this atom absorbs energy, or 9,192,631,770 hertz (cycles per second).

The temperature scale adopted by the 1960 conference was based on a fixed temperature point, the triple point of water, at which the solid, liquid, and gas are in equilibrium. The temperature of 273.16 K was assigned to this point. The freezing point of water was designated as 273.15 K, equaling exactly 0° on the Celsius temperature scale. The Celsius scale, which is identical to the centigrade scale, is named for the 18th-century Swedish astronomer Anders Celsius, who first proposed the use of a scale in which the interval between the freezing and boiling points of water is divided into 100 degrees. By international agreement, the term Celsius has officially replaced centigrade.