2 5 MEASUREMENT OF TIME To measure any time interval we need a clock We now use an atomic standard of time which is based on the

Question

2 5 MEASUREMENT OF TIME To measure any time interval we need a clock We now use an atomic standard of time which is based on the periodic vibrations produced in a cesium atom This is the basis of the cesium clock sometimes called atomic clock used in the national standards Such standards are available in many laboratories In the cesium atomic clock the second is taken as the time needed for 9 192 631 770 vibrations of the radiation corresponding to the transition between the two hyperfine levels of the ground state of cesium 133 atom The vibrations of the cesium atom regulate the rate of this cesium atomic clock just as the vibrations of a balance wheel regulate an ordinary wristwatch or the vibrations of a small quartz crystal regulate a quartz wristwatch The cesium atomic clocks are very accurate In principle they provide portable standard The national standard of time interval second as well as the frequency is maintained through four cesium atomic clocks A cesium atomic clock is used at the National Physical Laboratory NPL New Delhi to maintain the Indian standard of time In our country the NPL has the responsibility of maintenance and improvement of physical standards including that of time frequency etc e Indian Standard Time IST is set of atomic clocks The efficient pie plecl Note 17

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