| | | In Europe during most of the Middle Ages (roughly 500 CE to 1500 CE), technological advancement virtually ceased. Sundial styles evolved, but didn't move far from ancient Egyptian principles.
During these times, simple sundials placed above doorways were used to identify midday and four "tides" (important times or periods) of the sunlit day. By the 10th century, several types of pocket sundials were used. One English model even compensated for seasonal changes of the Sun's altitude.
Then, in the first half of the 14th century, large mechanical clocks began to appear in the towers of several large Italian cities. We have no evidence or record of the working models preceding these public clocks, which were weight-driven and regulated by a verge-and-foliot escapement. Variations of the verge-and-foliot mechanism reigned for more than 300 years, but all had the same basic problem: the period of oscillation of the escapement depended heavily on the amount of driving force and the amount of friction in the drive. Like water flow, the rate was difficult to regulate.
Another advance was the invention of spring-powered clocks between 1500 and 1510 by Peter Henlein of Nuremberg. Replacing the heavy drive weights permitted smaller (and portable) clocks and watches. Although they ran slower as the mainspring unwound, they were popular among wealthy individuals due to their small size and the fact that they could be put on a shelf or table instead of hanging on the wall or being housed in tall cases. These advances in design were precursors to truly accurate timekeeping. |
| Scientists had long realized that atoms (and molecules) have resonances; each chemical element and compound absorbs and emits electromagnetic radiation at its own characteristic frequencies. These resonances are inherently stable over time and space. An atom of hydrogen or cesium here today is (so far as we know) exactly like one a million years ago or in another galaxy. Thus atoms constitute a potential "pendulum" with a reproducible rate that can form the basis for more accurate clocks.
The development of radar and extremely high frequency radio communications in the 1930s and 1940s made possible the generation of the kind of electromagnetic waves (microwaves) needed to interact with atoms. Research aimed at developing an atomic clock focused first on microwave resonances in the ammonia molecule. In 1949, NIST built the first atomic clock, which was based on ammonia. However, its performance wasn't much better than the existing standards, and attention shifted almost immediately to more promising atomic-beam devices based on cesium.
The first practical cesium atomic frequency standard was built at the National Physical Laboratory in England in 1955, and in collaboration with the U.S. Naval Observatory (USNO), the frequency of the cesium reference was established or measured relative to astronomical time. While NIST was the first to start working on a cesium standard, it wasn't until several years later that NIST completed its first cesium atomic beam device, and soon after a second NIST unit was built for comparison testing. By 1960, cesium standards had been refined enough to be incorporated into the official timekeeping system of NIST. Standards of this sort were also developed at a number of other national standards laboratories, leading to wide acceptance of this new timekeeping technology.
The cesium atom's natural frequency was formally recognized as the new international unit of time in 1967: the second was defined as exactly 9,192,631,770 oscillations or cycles of the cesium atom's resonant frequency, replacing the old second that was defined in terms of the Earth's motions. The second quickly became the physical quantity most accurately measured by scientists. As of January, 2002, NIST's latest primary cesium standard was capable of keeping time to about 30 billionths of a second per year. Called NIST-F1, it is the 8th of a series of cesium clocks built by NIST and NIST's first to operate on the "fountain" principle.
Other kinds of atomic clocks have also been developed for various applications; those based on hydrogen offer exceptional stability, for example, and those based on microwave absorption in rubidium vapor are more compact, lower in cost, and require less power.
Much of modern life has come to depend on precise time. The day is long past when one could get by with a timepiece accurate to the nearest quarter-hour. Transportation, communication, financial transactions, manufacturing, electric power and many other technologies have become dependent on accurate clocks. Scientific research and the demands of modern technology continue to drive the search for ever more accurate clocks. The next generation of time standards is presently under development in the USA, in France, in Germany, and in other laboratories around the world. |
| Since 1923, NIST radio station WWV has provided round-the-clock shortwave broadcasts of time and frequency signals. WWV's audio signal is also offered by telephone: dial (303) 499-7111 (not toll-free). A sister station, WWVH, was established in 1948 in Hawaii, and its signal can be heard by dialing (808) 335-4363 in Hawaii.
Broadcast frequencies are 2.5 MHz (megahertz), 5 MHz, 10 MHz, and 15 MHz for both stations, plus 20 MHz on WWV. The signal includes UTC time in both voice and coded form; standard carrier frequencies, time intervals and audio tones; information about Atlantic or Pacific storms; geophysical alert data related to radio propagation conditions; and other public service announcements. Accuracies of one millisecond (one thousandth of a second) can be obtained from these broadcasts if one corrects for the distance from the stations (near Ft. Collins, Colorado, and Kauai, Hawaii) to the receiver. The telephone services provide time signals accurate to 30 milliseconds or better, which is the maximum delay in cross-country telephone lines.
In 1956, low-frequency station WWVB, which offers greater accuracy than WWV or WWVH, began broadcasting at 60 kilohertz. The broadcast power for WWVB was increased in 1999 from about 10 kilowatts to 50 kilowatts, providing much improved signal strength and coverage to most of the North American continent. This has stimulated commercial development of a wide range of inexpensive radio-controlled clocks and watches for general consumer use.
Time signals are an important byproduct of the Global Positioning System (GPS), and indeed this has become the premier satellite source for time signals. The time scale operated by the USNO serves as reference for GPS, but it is important to note that the time scales of NIST and USNO are highly coordinated (that is, synchronized to well within 100 nanoseconds, or 100 billionths of a second). Thus, signals provided by either NIST or USNO can be considered as traceable to both institutions. The agreements and coordination of time between these two institutions are important to the country, since they simplify the process of achieving legal traceability when regulations require it. |
|