2011英语六级模拟试卷及解析之四(2)
Mechanical ClocksIn 1656, "Christian Huygens' (Dutch scientist)， made the first "Pendulum(钟摆) clock", with a mechanism using a "natural" period of oscillation(振幅). "Galileo Galilei" is credited, in most historical books, for inventing the pendulum as early as 1582, but his design was not built before his death. Huygens' clock, when built, had an error of "less than only one minute a day". This was a massive leap in the development of maintaining accuracy, as this had previously never been achieved. Later refinements to the pendulum clock reduced this margin of error to "less than 10 seconds a day"。
The mechanical clock continued to develop until they achieved an accuracy of "a hundredth-of- a-second a day", when the pendulum clock became the accepted standard in most astronomical observatories。
Quartz Clocks
The running of a "Quartz clock" is based on the piezoelectric property of the quartz crystal. When an electric field is applied to a quartz crystal, it actually changes the shape of the crystal itself, If you then squeeze it or bend it, an electric field is generated. When placed in an appropriate electronic circuit, this interaction between the mechanical stress and the electrical field causes the crystal to vibrate, generating a constant electric signal which can then be used for example on an electronic clock display. The first wrist-watches that appeared in mass production used "LED", "Light Emitting Diode" displays. By the 1970's these were to be replaced by a "LCD", "Liquid Crystal Display"。
Quartz clocks continue to dominate the market because of the accuracy and reliability of the performance, also being inexpensive to produce on mass scale. The time keeping performance of the quartz clock has now been surpassed by the "Atomic clock"。
Atomic Clocks
Scientists discovered some time ago that atoms and molecules have "resonances" and that each chemical element and compound absorbs and emits "electromagnetic radiation" within its own characteristic "frequencies". This we are told is highly accurate even over "Time and Space"。
The development of radar and the subsequent experimentation with high frequency radio communications during the 1930s and 1940s created a vast amount of knowledge regarding "electromagnetic waves", also known as "microwaves". which interact with the atoms. The development of atomic clocks focused firstly on microwave resonances in the chemical Ammonia and its molecules. In 1957. "NIST". the "National Institute of Standards and Technology", completed a series of tests using a "Cesium Atomic Beam" device, followed by a second program of experiments by NIST in order to have something for comparison when working at the atomic level. By 1960, as the outcome of the programs, "Cesium Time Standards" were incorporated as the official time keeping system at NIST。
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