It doesn't tick, there's no snooze alarm and at 1,500 pounds it is too heavy to throw against the wall.

For a $400,000 clock, about the only thing the world's most accurate chronometer does inarguably is measure time down to the trillionth of a second.

"Not much happens in a trillionth of a second," said Lauren J. Rueger, the man who prefected the latest in a line of atomic clocks that splinter time into infinitesimally small segments.

For the first time since atomic clocks were developed in the 1950s, Rueger and other scientists at the John Hopkins University Applied Physics Laboratory in Laurel have built a device that can instantly measure the "pico second" -- about the time it takes light to travel the thickness of a credit card.

Although Reuger's clock will not cause much of a ripple in the ordinary world where zillions of "pico seconds" fly by in the time it takes to read this sentence, the accomplishment holds special importance for scientists studying outer space with probes such as Voyager.

"The clock is so precise that error doesn't accumulate over time," said Dr. Richard Kershner, 61, a space consultant for the lab. "The more ambitious the mission, the more accurate a clock you need to track it."

A timepiece as accurate as Rueger's hydrogen maser clock, a veritable paragon of precision that varies less than one-millionth of a second over a three-year period, is also useful in measuring minute changes in the earth's crust.

It was by the atomic clock's ability to time radio waves that "we found out Australia was whipping along at 20 centimeters a year," Kershner said with a laugh.

Funded by the National Aeronautic Space Administration, the 5-foot-high device of aluminum cylinders and mor than 50,000 parts took 14 people more than five years to perfect.

Because technicians needed a way to clock the clock, they built two deices simultaneosly and turned them on each other. They were assembled in a laboratory that looks like a hygienic body shop and come complete with a spotless winch. Tools as prosaic as screwdrivers, wrenches and mallets lie among trays of diodes and high technology hardware.

Greasing a few bolts, mechanical designer Lee Stillwell, who helped assemble the atomic timepiece, joked, "I'd be lucky if I could fix an alarm clock."

Oddly enough, telling time for a clock as prodigious as the hydrogen maser begins with an ordinary wristwatch.

"I used to set the master with my wristwatch all the time," said engineer Ed Mengel, displaying his digital timepiece. Even Rueger, who for 15 years sported a stem-wind Omega, recently bought a new electronic Sanyo watch because his old one lost five second a month. "I kept track of it," he said. "I used to be teased so much I finally had to get an electronic watch."

While it lacks the elegant simplicity of a traditional clock such as an egg timer, the principle of the hydrogen maser chronometer is fairly simple, at least to engineers with doctoral degrees.

The maser was first developed in 1960 by Harvard Unversity researchers who discovered that electrically excited hydrogen atoms beamed into a chamber will produce a steady vibrating tone, like a tuning fork. "The tone," Rueger explained, "can be used to run the atomic clock."

Rueger and his colleagues perfected the maser for use as a clock by finding ways to shield the vibrating element from outside disturbances. When they had completed construction, final testing included wheeling the maser up and down the halls of the lab. While it's not fit for use in a demolition derby, the clock is sturdy enough for remote tracking stations.

There, in esoteric service to mankind, it may help forecast earthquakes by making measurements of the earth's movements even more precise. And surely, Rueger says, his invention's potential uses are manifold. "If we can shrink the maser down to one-tenth the size," he said, "we could use it in airplanes to help keep them from colliding with each other."

Although the two original clocks have been shipped off to labs in California and Texas, two more are in the works. The applied physics lab also has prepared special charts for curious members of the public with time on their hands to contemplate an awesome monument to punctuality.

"Hell," said public information officer Cyril O'Brien, "this is something everybody's going to be interested in, largely because you can't imagine what a trillionth of a second is."