Starting up an improved atomic clock.

It doesn't look at all like the trusty digital have at your bedside, but it certainly keeps better time. Placed into operation on April 22 by the National Institute of Standards and Technology (NIST) in Boulder, Colo., this new atomic clock will neither gain nor lose a second in the next I million years. Designated NIST-7, it replaces NBS-6, the atomic clock started up in 1975 to serve as the U.S. contribution to setting and maintaining the international standard for time and frequency

The clock's glistening, cylindrical facade, about 2.2 meters long, hides several layers of magnetic shielding Within this cocoon, a small oven at one end gently heats up cesium metal to release cesium atoms, which are collimated into a narrow beam only I millimeter wide. As the cesium beam passes down the center of a long, evacuated tube, laser light excites the individual atoms to ensure that they all end up in the same electronic state.

These atoms then enter a 1. 55-meter-long chamber in which microwaves reflect back and forth. The frequency of these microwaves- 9,192,631,770 hertz- corresponds precisely to the energy needed to excite a cesium atom from its initial electronic state to a state of slightly higher energy Bathed by another laser, the microwave-excited atoms then fluoresce, giving off electromagnetic radiation. Electronic circuitry locks the microwave signal to this atomic signal, so the system maintains a constant frequency One second is represented by 9, 192,631,770 of these vibrations.

"It's actually a fairly simple procedure,"says NIST's John P. Lowe. But it took years of effort to refine the technique and build a better clock