Science: How to Keep Time

A millionth of a second is a long, long time in modern science; many spectacular things, such as atomic bomb explosions, happen much faster. Therefore scientists are forever striving for more accurate measurement of time. In the latest Physical Review Letters, Harvard Physicists H. Mark Goldenberg. Daniel Kleppner and Norman F. Ramsey tell about a new electronic clock that they hope will keep time with an accuracy of one part in i million billion. This is equivalent to making an error of only one second in 30 million years.

In the Harvard clock a thin trickle of hydrogen gas flows through an apparatus that splits its two-atom molecules into single atoms. Each of these atoms has one proton and one electron, but some of them have slightly more energy than the others because their electrons are spinning in a different way. When the atom stream shoots through a system of magnets, the low-energy atoms in it are deflected sideways while the high-energy ones converge, pass through a small hole in a 6-in. quartz bulb. The bulb is lined with paraffin which does not affect the atom’s energy state as metal or bare quartz would, so the atoms bounce about inside for as much as a whole second, hitting the walls 10,000 times without losing their extra energy.

If about 4 trillion particles enter the bulb in one second, the agitated gas gets dense enough to support a sort of chain reaction. A few atoms spontaneously lose their extra energy, which they emit in the form of photons (units) of radio microwaves about 21 cm. (8.3 in.) long. The newborn photons hit other hydrogen atoms and make them emit photons too. Then a pulse of microwaves bursts from the bulb and is gathered as high-frequency current by apparatus outside.

This radiation, and the current that it generates, is extraordinarily “stable.” That is, its frequency (of approximately 1,420,405,000 cycles per second) is expected to vary less than that of any other kind of radiation. Like all regular wave motion, it can be used to keep time. The Harvard physicists hope their hydrogen clock will measure small intervals of time with 100,000 times the accuracy that is presently possible.

Personal timekeepers (trade doubletalk for watches) do not use anything as fancy as hydrogen atoms. Since the 17th century they have depended on a delicate hairspring that keeps a balance wheel turning backward and forward at a regular rate. But last week Bulova Watch Co.

Inc. introduced something new: an electronic watch that uses a tuning fork and hums faintly instead of ticking.

The tuning fork lies flat across the watch and carries small, conical magnets on its tines. When the tines vibrate, the magnets poke into tiny coils, generating a very small pulse of electric current that goes to a transistor and triggers it so as to permit a somewhat larger current to flow through the coils from a battery. The energized coils react with the magnets and keep the fork vibrating at a steady 360 cycles per second, giving a musical note a little higher than F above middle C. Each vibration pushes a jewel-tipped spring against a pinhead-sized wheel whose rim has 300 microscopic ratchet teeth. The turning of this wheel moves the hands of the watch through a conventional gear train. Bulova guarantees that its electronic watch, which it calls the Accutron. will not gain or lose more than one minute per month. Price of present-model watches: $175 and up.