美國英語聽力80篇3 Lesson58
Clock
The United States has a new official clock.
The clock began operating late last year in the American city of Boulder,Colorado.
Experts say the new clock is so exact that it will neither gain nor lose so much as one second in the next 20,000,000 years.
Scientists with the National Institute of Standards and Technology developed the new atomic clock.
The agency is part of the United States Department of Commerce.
The new clock was built and tested in less than four years.
It is now being used to measure the official world time, known as Coordinated Universal Time.
The clock measures time by counting the movements of atoms of the element cesium.
The natural rate of movement of cesium atoms has been used to measure time since 1967.
Researchers have found that cesium atoms move back and forth at a rate of almost 9,200,000,000 times in one second.
The new clock is less than two meters tall.
This is how it works.
First, a gas of cesium atoms is released in an empty container.
All other matter has been removed from this vacuum chamber.
Six beams of laser light are directed at the center of the chamber.
The lasers slow the movement of the cesium atoms.
The lasers force the atoms into a cloud of gas shaped like a ball.
Two lasers push the ball of gas upward through a passage filled with microwave energy.
Then,all of the lasers are turned off.
Gravity pulls the ball of cesium atoms back down through the passage.
The microwaves change some of the atoms.
The microwaves cause some of the atoms to move quickly back and forth.
Another laser causes the cesium atoms to produce light.
The light is measured by special observation equipment.
This process is repeated many times while different levels of mierowave energy are used.
The researchers find a level of ehergy that makes the cesium atoms produce the most light.
This is the rate of movement used to define one second.
The new device replaces another atomic clock that has served as the official clock of the United States since 1993.
The new clock is said to be three times more exact than the atomic clock it replaced.
