LONDON (AP) _ It takes just a penny to keep Big Ben on atomic time.

Two stacks of pennies stand on top of the pendulum, and by removing one, the timekeepers are slowing the pendulum enough to give it the leap second created by the Earth's erratic rotation.

For the start of the New Year, London's favorite old timepiece is in step to the half-second with the atomic clocks that keep the world's time.

The accuracy of this giant clock, which for 129 years has survived pollution, foul weather and war, is a tribute to Victorian engineering genius. It is London's best-known landmark and its chimes are heard throughout the world through the British Broadcasting Corp., which uses them to introduce its newscasts.

When something goes wrong and the bells are silenced, London's heart seems to miss a beat. Cold weather sometimes freezes a hinge, a workman's ladder once stopped the clock, and another time a nest of starlings caused a disruption.

Stoppages are rare, however. ''It's a beautiful machine, and a really very good clock,'' says Bryan Sewell of the Property Services Agency, the government company that services Big Ben and other state buildings and landmarks.

The 20-story clocktower stands over the Houses of Parliament and is reached by climbing 334 winding stairs. The name Big Ben actually applies only to the big bell which sounds the hour, but it has become the nickname for the whole clock.

Big Ben and its four smaller bells hang in a belfry above a room in which the machinery stands. Painted black, its mechanism of fly wheels, weights and levers looks like a Rube Goldberg machine, but is a model of simplicity.

Sewell says it could go on working forever with just occasional changes of parts. There is no need to modernize it, he says, ''and anyway, it would cause an international outcry.''

Indeed, so precious is Big Ben that it cannot be stopped without written permission from the secretary of the environment.

Big Ben is stopped twice a year when Britain goes on and off daylight saving time, and its accuracy is checked twice a week. A timekeeper stands in the machine room, dials the telephone company's speaking clock and makes adjustments by adding or subtracting pennies from the pendulum.

According to the PSA experts, each penny removed slows the pendulum by two- fifths of a second every 24 hours.

The pennies themselves are special. Heavier than modern pennies, they went of circulation after Britain decimalized its currency in 1971.

''We still get little old ladies sending in their old pennies for Big Ben,'' says Rick Braddock, a PSA official. The unwanted coins are kept in a locked box in the machine room.

The 314-foot clocktower was built as part of the reconstruction of the Palace of Westminster after a devastating fire, and was dogged by mishaps.

Work repeatedly fell behind schedule. In 1857, disaster struck when Big Ben cracked during testing and had to be remade. It then proved too big to go up the shaft and had to be maneuvered in sideways.

The Times of London fumed: ''The series of blunders and misconceptions, and misunderstandings and squabbles respecting this clock are a disgrace to all concerned in it, and to the government which permits them to go on.''

When it finally began ringing July 11, 1859, members of Parliament below complained it was too loud.

But once in action, nothing could silence Big Ben. Its chimes signaled the end of World War I, and when its lantern was lit again after 2,062 days of blackout during Hitler's blitz, huge crowds watched and cheered.

The origin of its name is obscure. Some believe Ben was a prizefighter of the time, Ben Caunt, others that it referred to a bulky Welshman, Sir Benjamin Hall, a commissioner of works whose name is inscribed on the bell.

Its first face lifted was in 1934, and underwent a $2 million cleanup from 1983 to 1985.