Controlled Clocks
In 1945, Columbia University physics professor Isidor Rabi suggested that a clock could be made from a technique he developed in the 1930s called atomic beam magnetic resonance. By 1949, the National Bureau of Standards (NBS, now the National Institute of Standards and Technology, NIST) announced the world’s first atomic clock using the ammonia molecule as the source of vibrations, and by 1952 it announced the first atomic clock using cesium atoms as the vibration source, NBS-1.
In 1955, the National Physical Laboratory in England built the first cesium-beam clock used as a calibration source. Over the next decade, more advanced forms of the clocks were created. In 1967, the 13th General Conference on Weights and Measures defined the SI second on the basis of vibrations of the cesium atom; the world’s time keeping system no longer had an astronomical basis at that point! NBS-4, the world’s most stable cesium clock, was completed in 1968, and was used into the 1990s as part of the NIST time system.
The atomic clock, which used the oscillations of cesium-133 atoms to tell time, was invented. This clock had an error ratio of 1 second for every 1.4 million years. The atomic clock keeps time via the emanating frequencies of an atom. Even though these timepieces are more precise due to newer technology, they still rely on the basis of a mechanical movement, which functions via power being applied to the mechanism. Recently, in 1999, scientists developed the cesium fountain atomic clock, which is off by only one second every 20 million years. This clock is the most accurate in the world.
Many gadget catalogs and high-tech stores sell "radio-controlled" clocks and wrist watches that are able to receive these radio signals. These clocks and watches truly are synchronizing themselves with the atomic clock in Colorado. This feature is made possible by a radio system set up and operated by NIST -- the National Institute of Standards and Technology, located in Fort Collins, Colorado. NIST operates radio station WWVB, which is the station that transmits the time codes.
WWVB is a very interesting radio station. It has high transmitter power (50,000 watts), a very efficient antenna and an extremely low frequency (60,000 Hz). For comparison, a typical AM radio station broadcasts at a frequency of 1,000,000 Hz. The combination of high power and low frequency gives the radio waves from WWVB a lot of bounce, and this single station can therefore cover all of the continental United States plus much of Canada and Central America as well
The time codes are sent from WWVB using one of the simplest systems possible, and at a very low data rate of one bit per second (for comparison, a typical modem transmits over the phone lines at tens of thousands of bits per second -- imagine receiving a web page at one bit per second!). The 60,000 Hz signal is always transmitted, but every second it is significantly reduced in power for a period of 0.2, 0.5 or 0.8 seconds:
·0.2 seconds of reduced power means a binary zero
·0.5 seconds of reduced power is a binary one.
·0.8 seconds of reduced power is a separator.
The time code is sent in BCD (Binary Coded Decimal) and indicates minutes, hours, day of the year and year, along with information about daylight savings time and leap years. The time is transmitted using 53 bits and 7 separators, and therefore takes 60 seconds to transmit.
A clock or watch can contain an extremely small and relatively simple antenna and receiver to decode the information in the signal and set the clock's time accurately. All that you have to do is set the time zone, and the clock can display a very accurate time. The only thing more accurate that you can carry around easily is a GPS receiver, which derives atomic clock accuracy in real time from the atomic clocks in orbiting GPS satellites.
