You might have the most expensive watch in the world, but if it's set to the wrong time to begin with, it's no use to you at all. Even
really good quartz clocks struggle to keep time to better than a
second a day; if they wander out by just a couple of seconds in 24
hours (an amazing accuracy of 99.998 percent), and the errors don't cancel out,
that could add up to a minute a month or almost a quarter of an hour a year. That's why most people regularly check their watches against a reliable time signal—like the ones
you hear before news broadcasts on radio stations. Now wouldn't it be
neat if your watch could listen to those broadcasts and set itself to
the right time automatically without you ever needing to worry?
That's the basic idea behind radio-controlled clocks and watches,
which set their time by super-accurate atomic clocks.
Let's take a closer look at what these things are and how they work!
Artwork: Watches and clocks synchronized using radio signals mean anyone can own a watch as accurate as an atomic clock. Radio-controlled clocks and watches were popularized by such companies as Junghans in the late 1980s and early 1990s. Today, many different manufacturers make them and there are millions in use all over the world.
An ordinary clock or watch is a time-counting device that
adds up the number of seconds, minutes, hours, and days that have
passed. But it doesn't actually know what time it is until you tell
it: it's not a time-keeping device unless you set it to the
right time to start with. A radio-controlled clock (RCC) is different. It's similar to an
ordinary electronic clock or watch but it has two extra components:
an antenna that picks up radio signals and a circuit that
decodes them. The circuit uses the radio signals to figure out the
correct time and adjusts the time displayed by the clock or watch
accordingly. Unlike an ordinary clock or watch, an RCC always knows what
time it is—you never have to tell it!
Photo: The basic concept of RCC radio-controlled clocks: a radio transmitter hundreds or
thousands of km/miles from your home (represented here by the ordinary silver radio) beams regular signals to your quartz clock or watch to keep it in time.
The radio signals come from a unique radio "station" that
doesn't broadcast any words or music. There's no DJ and no irritating
advertisements for car insurance. All the station broadcasts is the
time—over and over again—in the form of a special code that only
radio-controlled clocks can understand. In the United States, these
time signals are broadcast by a station called
WWVB operated by the
National Institute of Standards and Technology (NIST) from a base near Fort
Collins, Colorado. (Other countries have equivalent radio stations.
In the UK, for example, the station is called
MSF and operated by the National
Physical Laboratory, while China's station is called BPC and
broadcast by the National Time Service Center.) The NIST time code
contains the basic time and date, whether it's a leap-year, whether
it's daylight-saving time, and so on and takes about a minute to
broadcast in its entirety.
Most RCCs synchronize themselves with a time broadcast signal once
a day, at night, although some check themselves every few hours.
Generally, that gives them an accuracy of better than plus or minus a
half second (±0.5s) a day. Another advantage is that they automatically
correct themselves for daylight-saving time, leap years, months with
different numbers of days, and so on.
It's pretty obvious that an RCC is only going to be as accurate as
the time signals it uses to regulate itself. How can you be sure
those are accurate? The time-signal radio stations operated in
different countries broadcast UTC (Coordinated Universal Time), the officially agreed time used worldwide that's informally known as GMT
(Greenwich Mean Time). UTC is maintained by hundreds of atomic clocks
(the world's most accurate timekeeping devices) around the world, all
of which are synchronized with one another. It's because RCC radio
signals are based on time kept by atomic clocks that you'll sometimes
see RCC manufacturers describing their products as "atomic" clocks and
watches (even though they're really no such thing).
What are atomic clocks?
Atomic clocks are actually quartz clocks—just like the ones you
have at home. The difference is that an ordinary quartz clock relies
purely on the oscillations of its quartz crystal to count
seconds. As we've already seen, the rate at which quartz vibrates is
affected by things like ambient temperature, so although a quartz
clock is generally very accurate, it doesn't necessarily keep time as
well as you might think. By contrast, an atomic clock has an extra
mechanism—pulsating atoms—that it uses to keep an ordinary
quartz clock to time.
Photo: Before atomic clocks were developed, highly accurate quartz clocks like this one were used
as "time standards" to which other clocks could be synchronized.
This is the 1941 Quartz Crystal Time Standard, used in the United States until 1949, including during World War II, when it helped to synchronize military forces all around the world.
Photo courtesy of National Institute of Standards and Technology Digital Collections, Gaithersburg, MD 20899.
Cesium atomic clocks
This atomic mechanism is based on the idea that atoms have
electrons in particular energy states. When an atom absorbs energy,
electrons leap to higher energy states and become unstable. They then
give out the same energy as photons of light (or some other kind of
electromagnetic radiation such as X rays or
radio waves), returning
to their original or ground state. The cesium atoms used in many
atomic clocks have 55 electrons arranged in orbitals. The very
outermost electron can oscillate between two different energy states
by spinning in two slightly different ways. When it shifts from the
higher to the lower of these states, it gives out a photon that
corresponds to microwaves with a frequency of exactly 9,192,631,770
Hz (roughly 9.2 billion hertz or 9.2 gigahertz). That means it can
be stimulated from its lower to its higher state by exactly the
We can use this neat fact to keep a quartz clock to very precise time.
In a cesium atomic clock, there's a quartz oscillator tuned to
exactly the same frequency, 9,192,631,770 Hz, which makes microwaves
and fires them at a bunch of cesium atoms. If its frequency is
correct, and hasn't drifted at all, these microwaves will have
exactly the right amount of energy to shift the electrons in the
atoms to their higher energy state. A magnetic detector in the clock
measures how many atoms are in the higher and lower energy states. If
most are in the higher state, it means most have been excited by the
waves from the quartz oscillator. And that means those waves are exactly the right frequency, so
the quartz oscillator must be telling time correctly. However, if the atoms are mostly in the
lower state, it means the oscillator has drifted away from its
correct frequency and isn't giving out the right amount of energy to
promote electrons in the cesium atoms. A feedback mechanism in the
clock detects this and adjusts the frequency of the oscillator so it's
correct again. In this way, the quartz oscillator is constantly
regulated so it's always exactly set to 9,192,631,770 Hz. An
electronic circuit converts this exact frequency into one-per-second
pulses that can be used to drive a relatively ordinary quartz clock
mechanism with amazing accuracy. "Amazing" in this case means
just that: the best atomic clocks are accurate to within 2
nanoseconds per day, or one second in 1.4 million years!
Other types of atomic clocks
Other atomic clocks work in broadly the same way but using atoms
of different gases to regulate the quartz oscillator. In a hydrogen
clock, atoms of hydrogen gas are stimulated with a
microwave-frequency laser (maser), but they're less practical because
hydrogen is a fairly hard gas to contain. Rubidium clocks are
simpler, and therefore more compact and portable; they use microwaves
to excite the atoms in rubidium glass. The world's most advanced
atomic clocks, such as NIST-7 at the National Institute of Standards
and Technology in Boulder, Colorado, use what are called atomic
fountains. They use six laser beams to contain cesium atoms, cool
them almost to absolute zero, bounce them upward, and let them fall
back down through gravity (hence the name "atomic fountain").
This process makes them oscillate between two precise energy states
that can be measured, in a broadly similar way to how we explored
above, and used to keep a quartz clock to time.
How do atomic clocks work?
At one end of the clock, an oven (red) heats a lump of cesium metal so cesium atoms boil off it. The atoms are either in their unexcited ground state (orange) or their excited state (yellow).
A magnetic filter at one end of the oven allows only unexcited atoms (orange) to pass through.
The unexcited atoms enter a chamber called a microwave cavity. Here, they are bombarded with microwaves controlled by a quartz oscillator (green, 6), theoretically tuned to the magic frequency of 9,192,631,770 Hz.
If the quartz oscillator is working at exactly this frequency, most of the unexcited cesium atoms will be converted to their excited state. Otherwise, far fewer will be excited. A second magnetic filter lets only the excited atoms pass through.
A detector measures the number of excited atoms.
The detector feeds back a signal to the microwave oscillator (green), constantly adjusting its frequency to ensure that a maximum number of atoms are excited. This ensures that the frequency of the oscillator is always as close as possible to 9,192,631,770 Hz.
An electronic divider circuit (blue) converts this high frequency signal down to a lower frequency signal that can drive a fairly ordinary quartz clock mechanism.
A digital display (gray) connected to the circuit shows the precise atomic time.
Who invented radio-controlled clocks?
According to Michael Lombardi of the NIST, one of the world's authorities on radio-controlled clocks: "There is no true consensus on who invented the first RCC that could synchronize to a wireless signal." He suggests the first such device may have been the Horophone invented by Frank Hope-Jones (1887–1950) and sold from 1913 by his Synchronome Company of London, England.
I looked through numerous patents covering RCCs on the US Patent and Trademark Office database and the earliest one I found was filed on March 24, 1921 (granted February 5, 1925) by Thaddeus Casner for the Radio Electric Clock Corporation of New York City. Casner explains that his invention covers a "... mechanism by means of which a clock may be periodically corrected by electrical impulses transmitted through space... [by] Hertzian waves [what we now call radio waves]..." You can read a full description and browse numerous detailed drawings (including the one shown here) in US Patent: # 1,575,096: Mechanism for Synchronizing Clocks (via Google Patents).
Artwork: One of the drawings of Thaddeus Casner's early radio-controlled clock. It keeps time using a traditional gear mechanism (which I've roughly indicated in blue), but also uses electromagnets (red) controlled by radio signals to keep the time correct. Artwork courtesy of US Patent and Trademark Office.
Can clocks really be this accurate?
We can now tell time with an incredible degree of accuracy—a
nanosecond or two each day. While you might think that's wonderful,
it really just swaps one problem for another. In past times, the
problem was that we couldn't tell time well enough to keep up with
the "natural accuracy" of the real world. So while the heavens
turned and the planets whizzed round the Sun, our clocks struggled to
keep time as effectively as the natural clock high in the sky.
Today, ironically, it's just the opposite. We now define time not
in terms of moving planets but using oscillating atoms. Since 1967,
the second has been
defined as 9,192,631,770 oscillations of the atom
cesium-133 between two energy states (for reasons we saw up above).
Keep time with an atomic clock (as the world's various national
standards organizations now do) and you'll find that the stars and
planets gradually get out of step—because they're not moving
accurately enough! Earth's rotational speed isn't constant, for
example: it has random blips and it's gradually slowing down. All
this means that we have to "correct" our super-accurate atomic
clocks, from time to time, so they keep step with the much less
accurate world around us. We do that by adding occasional
seconds" to the official, scientifically measured world time
(International Atomic Time, TAI) so that it always agrees with the
official time that people actually use (Coordinated Universal Time,
A brief history of atomic time
Photo: An early electromechanical mechanism for synchronizing a clock by radio control, developed by
U.S. Dunmore Bureau of Standards. Photo by Harris & Ewing
courtesy of US Library of Congress.
1879: British physicist Lord Kelvin (William Thomson) suggests
that the energy transitions of sodium and hydrogen atoms might be
used for telling time.
1937: American physicist Isidor Rabi pioneers a technique called atomic beam magnetic resonance (ABMR), which uses magnetism
to measure properties of an atom. The work earns him the 1944 Nobel Prize in Physics.
1945: Rabi suggests how a practical atomic clock can be built.
1949: Rabi's student and collaborator Norman Ramsey builds on and improves Rabi's ABMR method with discoveries that gradually lead to the development of the cesium atomic clock.
For this work, Ramsey is later awarded the 1989 Nobel Prize in Physics.
1949: The US National Bureau of Standards (the official US
standards body, renamed the National Institute of Standards and
Technology, NIST, in 1988), builds the world's first atomic clock
using ammonia gas and a maser (microwave laser). It was not very
accurate, but it proves that atomic clocks can be built.
1952: NBS builds a prototype cesium atomic clock, NBS-1. Political
problems halt further research.
1953: Meanwhile, at the National Physical Laboratory in the UK
(the British equivalent of NBS/NIST), Louis Essen and Jack Parry
begin work on atomic timekeeping.
1955: Essen and Parry build the first highly accurate cesium
atomic clock, Cesium-1.
1956/1958: Atomichron, the first commercial atomic clock, goes on
1959: Essen and Parry's improved clock, Cesium-2, can tell time to
an unheard accuracy of one second in 2000 years.
1967: The International System of Units (SI) is revised to define
the second as "the duration of 9,192,631,770 periods of the
radiation corresponding to the transition between the two hyperfine
levels of the ground state of the cesium-133 atom." For the first
time in human history, the measurement of time is no longer based on
the movement of the stars and planets.
1993: NIST build NIST-7, a cesium beam atomic clock used for
official timekeeping in the United States until 1999.
1999: NIST builds
NIST-F1, a replacement for NIST-7 that is 10 times more accurate. Based on cesium fountain technology, it is
accurate to about one second in 100 million years.
Radio Controlled Clocks by
Michael A. Lombardi, National Institute of Standards and Technology (NIST): This is a longer, more detailed, and more interesting introduction to how RCCs work. It traces the technology all the way back to Marconi's radio experiments in the early 20th century! (PDF format, approx 1 MB).
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