The accuracy of cientific observations and calculations is always at the mercy of the scientist's timekeeping methods. For this reason, scientists are interested in devices that give promise of more precise timekeeping.

　　In their search for precision, scientists have turned to atomic clocks that depend on various vibrating atoms or molecules to supply their "ticking". This is possible because each kind of atom or molecule has its own characteristic rate of vibration. The nitrogen atom in ammonia, for example, vibrates or"ticks" 24 billion times a second.

　　One such atomic clock is so accurate that it will probaly lose no more than a second in 3000 years. It will be of great improtance in fields such as astronomical is a sesium atom that vibrates 9.2 billion times a second when heated to the temperature of boiling water.

　　An atomic clock that operates with an ammonia molecue may be used to check the accuracy of predictions based on Einstein's relativity theories, according to which a clock in motion and a clock at rest should keep time differently. Placed in an orbiting satellite moving at a speed of 18000 miles an hour, the clock could broadcast its time readings to a ground station, where they would be compared with the readings on a similar model. Whatever differences develop would be checked against the differences predicted.

　　1. Scientists expect that the atomic clocks will be ________.

　　a. more precise

　　b. absolutely accurate

　　c. more durable

　　d. indestructible

　　2. The heart of the atomichron is ________.

　　a. acesium atom

　　b. an ammonia molecule

　　c. a nitrogen atom

　　d. a hydrogen

　　3. From theselection, we may assume that temperature changes ________.

　　a. affect only ammonia molecules

　　b. may affect the vibration rate of atoms

　　c. affect the speed at which atoms travel

　　d. do not affect atoms in any way

　　4. Identical atomic clocks may be used to check ________.

　　a. the effect of outer space on an atomic clock

　　b. the actual speed of an orbiting satelite

　　c. the accuacy of predictions based on theories of relativity

　　d. all of Einstein's theories

　　5. Implied but not stated: ________.

　　a. Precise timekeeping is essential in science

　　b. Scientists expect to disprove Einstein's relativity theories

　　c. Atomic clocks will be important in space flight

　　d. The rate of vibration of an atom never varies

　　參考答案: aabcc