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ammonia clock[ə′mōn·yə ′klak]
A time-measuring device dependent on the pyramidal ammonia molecule's property of turning inside out readily and oscillating between the two extreme positions at the precise frequency of 2.387013 × 1010 hertz. Also known as ammonia maser clock.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.