WASHINGTON, April 29 -- The U.S. Department of Commerce's National Institute of Standards and Technology issued the following news: * * * New Atomic Fountain Clock Joins Elite Group That Keeps the World on Time * NIST scientists have published results establishing a new atomic clock, NIST-F4, as one of the world's most accurate timekeepers, priming the clock to be recognized as a primary frequency standard -- the ultimate source of time. * The NIST-F4 clock was carefully and painstakingly as  more