On TimeNational Museum of American History

Marking Time
Synchronizing Time
  24-Hour Society
  Organizing Time
  Splitting Seconds
New Time Standard

The Quartz Standard
The Atomic Second
Quartz Accuracy

Splitting Seconds


NIST-F1 is the primary frequency standard for the National Institute of Standards and Technology, Boulder, Colo. This atomic “clock” varies by less than .3 millionths of a second a day and helps set time scales worldwide.
Image courtesy of the National Institute of Standards and Technology

A New Time Standard
  Advertisement, March 1928; by the Synchronome Co., London, from the Horological Journal. William Shortt and F. Hope-Jones developed the first free-pendulum clock.
Traditionally, time has been defined in terms of Earth's rotation relative to the sun and other stars. Early in the 19th century, astronomers defined the second as 1/86,400 of a mean solar day. But in the 1920s, a new kind of mechanical clock—the free-pendulum clock, accurate to within ten seconds a year—enabled scientists to prove that Earth does not keep uniform time. It speeds and slows as it orbits the sun and wobbles on its axis as it spins-a day varies in length by tiny fractions of a second. Scientists began the search for a new standard for defining the second.
The Quartz Standard
  Quartz crystal
  Quartz crystal, an uncut sample; the cut of the quartz determines the number of times per second it will vibrate.
Transfer from National Museum of Natural History, Department of Mineral Sciences
Twentieth-century advances in timekeeping resulted from work done by engineers seeking to improve telephone and radio communication. In 1927, Warren Marrison, a Canadian engineer working at Bell Telephone Laboratories, developed a clock that kept time using the vibrations of a quartz crystal. Quartz clocks were more accurate than any mechanical timekeeper, gaining or losing only a second in three years. In the 1940s and 1950s, quartz standards replaced mechanical ones in some astronomical observatories and scientific laboratories.
The Atomic Second
The quartz clock demonstrated that scientists did not have to rely on the traditional sources of astronomy and mechanical clocks to define tiny slices of time. Physicists began to use the oscillations of certain atoms as precision regulators. Since 1967, the length of a second has been defined by how long it takes the cesium 133 atom to vibrate 9,192,631,770 times when subjected to electromagnetic waves. Superprecise atomic clocks count these unthinkably tiny fragments of a second.
Who Needs Nanoseconds?
Your watch probably doesn't need to keep track of nanoseconds (billionths of a second), but your computer does. A variety of modern technologies—from cellular phones, radio and television broadcasts, and electric power transmissions to the Global Positioning System navigation aid—all depend on such infinitesimal subdivisions of the second. Superprecise quartz and atomic clocks count nanoseconds, even picoseconds (trillionths of a second).
Atomic clock chassis Global Positioning System receiver
Atomic clock chassis, early 1980s; test unit built by Frequency and Time Systems, Inc., Beverly, Massachusetts; identical to ones used in the U.S. military's GPS satellite system. Twenty-four GPS satellites orbit Earth and continuously transmit their exact location based on time derived from cesium and rubidium atomic clocks.
Gift of Datum
  Global Positioning System receiver, 1993–1997; built for the U.S. military, it compares signals from three orbiting satellites to calculate the user's time, velocity, latitude, longitude, and altitude. GPS receivers are now available for civilian use.
Gift of Rockwell International, Collins Avionics and Communications Division
Quartz Accuracy for Everyone
Battery-powered quartz wristwatches give just about everyone access to the split-second accuracy of the quartz time standard that once was available only to scientists and technicians. The first battery-powered quartz watches hit the American marketplace in the early 1970s. Since then, styles have diversified, prices have fallen, and quartz watches outsell mechanical ones by a huge margin.
Wristwatch, 1972; Pulsar, by HMW, Lancaster, Pennsylvania, in cooperation with Electro-Data, Inc., Garland, Texas; the first digital quartz watch with push-button-activated LED (light-emitting diode) display; sold for $2,100
Gift of John M. Bergey
  Wristwatch, 1970; Accuquartz, with a Beta 21 module, by Bulova, Biel, Switzerland
Loan from Butterfield Jewelers
Wristwatch module
Wristwatch module
Wristwatch module, 1969; Seiko Astron 35SQ, by Suwa Seikosha Co., Suwa, Japan, for the first commercially available quartz watch
Gift of Seiko Corporation, Japan
  Wristwatch module, 1972; Pulsar, by HMW, Lancaster, Pennsylvania, in cooperation with Electro-Data, Inc., Garland, Texas
Gift of Richard Walton
Wristwatch, 1973; Teletime, by Gruen Industries, Inc., New York; the first quartz watch with digital twisted nematic LCD (liquid crystal display) by ILIXCO, Cleveland, Ohio
Gift of Fred E. Whelan
  Wristwatch, 1975; by Texas Instruments, Dallas; the first quartz watch marketed at a very low
price–just $20
Gift of Hubert H. Myers
Wristwatch, 1993; Mega 1, by
Junghans Uhren GmbH, Schramberg, Germany; with radio receiver that resets the watch to a time signal sent from Braunschweig, Germany
Gift of Junghans Uhren GmbH
Smithsonian National Museum of American History