A Century of Time Measurement: From Pendulum to Optical Clocks

Michael Lombardi NIST Time and Frequency Division

NCSLI 2011

For most of recorded history, the most accurate measurements of time

involved dividing the period of a day into smaller parts. For example, the

solar second was measured by dividing the solar day in 86,400 parts. The

rotational rate of the Earth was known to be the best clock of all, the

ultimate reference for timekeeping.

This began to change when mechanical clocks first appeared in the 14th

century. Mechanical clocks measured time by counting the oscillations of a

repetitive event, such as the swings of a pendulum or balance wheel. This

was a fundamental change in timekeeping, because instead of dividing

days to get seconds, they multiplied seconds to get days. Early

mechanical clocks were not very accurate, and for about 600 years,

mechanical clocks were calibrated with astronomical clocks. About a

century ago, technology finally improved to the point where man made

clocks were known to be more stable and accurate than the Earth’s

rotation.

This presentation begins at that point, and takes a very brief look at

developments in clock technology during the past 100 years.

Introduction

• Pendulum Clocks

• Quartz Clocks

• Atomic Clocks and the Redefinition of the Second

• Optical Clocks

• Radio Controlled Clocks

Outline

Riefler Pendulum, 1904

Manufactured by the Clemens Riefler Company of Munich, Germany.

Served as the U. S. national

standard for time interval from 1904 to 1929.

Accurate to tens of

milliseconds per day (parts in 107).

Shortt Pendulum, 1921

A mainstay of astronomical observatories in the 1920s and 1930s, the Shortt pendulum was advertised as “The Perfect Clock”.

Designed by the British railroad engineer William H. Shortt.

Used two pendulums. The master pendulum was disturbed as little as possible.

Accurate to 1 s per year (a few parts in 108). It was so accurate that it suggested for the first time that the Earth was not a perfect timekeeper.

Briefly used as the U. S. national standard for time interval, during part of 1929.

Quartz Clocks

The Curie Brothers demonstrated the piezoelectric effect in quartz and other crystals in 1881. It remained a scientific curiosity for years.

The first application of piezoelectricity was detecting enemy submarines in World War I, with independent work conducted in France and the U. S.

Walter Cady, an American physicist, worked on submarine detection systems during the war. After the war he focused on building a standard for radio frequency, and patented the first quartz oscillator circuit in 1920.

General Radio Type 275, 1924

The first commercially available quartz oscillator, it sold for $145.

Cady and George Pierce (who improved upon Cady’s basic circuits) were involved in the design.

Used by radio engineers to calibrate transmitters, and was soon followed by more accurate quartz standards.

U. S. National Frequency Standard, 1929

A group of four 100-kHz quartz oscillators manufactured by Bell Telephone Laboratories.

Accurate to about 1 × 10-7. Even though this was no better than the pendulum clocks it replaced, it could serve as a standard for both radio frequency and time interval, something that the pendulum could not do.

First Quartz Clock, 1927

• Designed by William Marrison and Joseph Horton of Bell Telephone Laboratories • Used a 100 kHz oscillator and perhaps the first frequency divider, which reduced the frequency to 1 kHz • The 1 kHz frequency controlled the speed of a synchronous motor that moved the hands • Accurate to about 2 parts per million, slightly better than a typical quartz watch of today

Rohde & Schwarz Model CFQ, 1938

Hamilton Electric 500, 1957

The first battery powered watch, it had the unique styling of a 1950s automobile.

Ran at the same frequency as a mechanical watch, 5 Hz, but it derived its frequency from tiny electrical contacts that opened and closed five times per second.

Was not particularly reliable or accurate, but was very popular, showing the large demand for a watch that never needed to be “wound”.

Bulova Accutron Spaceview, 1962

Designed by the Swiss engineer Max Hetzel, the Bulova Accutron was first introduced in 1960

Its tuning fork oscillator

ran at 360 Hz, as opposed to 5 Hz for mechanical watches

Accurate to 2 seconds per day (2 × 10-5), a factor of 10 improvement over the best mechanical watches

“Hummed” instead of “Ticked”

First Quartz Watch Oscillator Circuit, 1966

• Designed by Armin Frei of the Centre Electronique Horloger (CEH) in Switzerland in response to the threat posed to the Swiss watch industry by the Bulova Accutron.

• Ran at 8192 Hz. Thirteen binary flip-flops divided the frequency to 1 Hz. • Was included in the first quartz watch prototype (July 1967), but was never used in a commercially

available watch.

Seiko Astron, 1969

Introduced on Christmas day in 1969, beating the Swiss watchmakers to the market.

Sold for $1250, about the same price as an economy car. It had nearly 200 analog parts that had to be hand soldered (the Swiss had designed ICs).

Ran at 8192 Hz with 13 binary divider stages. By 1972, nearly all quartz watches used 15 divider stages and ran at 32768 Hz, which became the standard.

Atomic Clocks

The Scottish physicist James Clerk Maxwell suggested that atoms could be used to keep time as early as 1879.

The first atomic clock experiments took place some 60 years later at Columbia University in New York, conducted by a team led by Isidor Rabi.

Rabi publicly discussed his plans for an atomic clock during a lecture at Columbia in 1945, and the New York Times (left) covered the story.

The concept was actually simple. Because all atoms of a specific element are identical, they should produce the exact same frequency when they absorb or release energy. An atom, then, could potentially serve as a “perfect” oscillator.

First Atomic Clock, 1949

• Based on the ammonia molecule, it was unveiled in January 1949, designed by a

team led by Harold Lyons at the National Bureau of Standards.

• It never worked well enough to be used as a standard or reference. Its best

reported uncertainty was about 2 x 10-8, less accurate than the quartz oscillators

then used as the national frequency standard. But it provided a glimpse of what

the future would bring ……

NBS-1, Cesium Prototype, 1952

The NBS team, led by Harold Lyons and Jesse Sherwood, had a large early lead in the race to build the first cesium clock. They began work in 1950 and reported their first results in 1952.

NBS interrupted the program in 1953, for budgetary and other reasons. By 1955, both Lyons and Sherwood had left NBS.

The clock was taken apart and moved to the new NBS labs in Boulder, Colorado where it was reassembled. It finally became the national frequency standard in 1959, but by then NPL in England had operated a cesium standard for several years.

First Cesium Time Standard, NPL, 1955

The Atomic Second, 1967 In 1956, the second was defined as 1/31,556,925.9747 of the tropical year 1900.

The ephemeris second was nearly impossible to use as a time reference and of little use to metrologists or engineers.

Ephemeris time was determined by measuring the position of the Moon with

respect to several surrounding stars. The best Moon observations had been recorded at the United States Naval Observatory (USNO) in Washington, DC by the astronomer William Markowitz.

Louis Essen and Jack Parry of NPL compared their new cesium clock to a quartz clock steered to ephemeris time at the USNO. Because the two clocks were located across the Atlantic from each other, they simultaneously compared each clock to radio signals that could be received at both laboratories, a measurement technique now known as common-view time transfer.

Four different solutions were made to determine the effects of using different data. The final result was the average of the four solutions, and was published as 9 192 631 770 cycles/s in August 1958. In 1967, the second was finally redefined 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 caesium 133

atom."

NBS/NIST constructed seven Cesium Beam Primary Frequency Standards from 1959 to 1998.

NBS-1 NBS-2 NBS-3

NBS-4

NBS-5

NBS-6

NIST-7

NIST-F1

laser-cooled

fountain

standard

“atomic

clock”

A cesium fountain frequency

standard that provides the best

possible realization of the SI

second.

Current accuracy (uncertainty):

• 3 x 10-16

• 26 trillionths of a second per day.

• 1 second in 105 million years.

Equivalent to measuring distance from earth to sun (1.5 x 1011 m or 93

million miles) to uncertainty of about 45 mm (less than thickness of

human hair).

NIST-F1 Atomic Fountain Clock

Improvements in Primary Frequency Standards at NBS/NIST

1940 1950 1960 1970 1980 1990 2000 2010 2020

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-16

10-17

10-18

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-16

10-17

10-18

Fre

qu

en

cy U

nce

rta

inty

Year

NBS-1

NIST-F1

Today

More than 50

Years of Progress

in Atomic Clocks

NIST-F1

Initial

NIST-7 NBS-6

NBS-5

NBS-4 NBS-3

NBS-2

5

10

15

0

0 1010

10

microwave

optical

f

f

Optical Clocks: The Next Generation of Primary Standards

Al+ 1124 THz (1124 x 1012 Hz)

Hg+ 1064 THz

Yb 520 THz

Ca 456 THz

Cs 0.0092 THz

Optical

Microwave

Optical clocks “tick faster” than microwave clocks. For example, the mercury ion

clock resonates at a frequency more than 100,000 times higher than a cesium

clock. This is comparable to using a second, rather than a day, as the base unit of

time.

In principle, faster “ticks” means better accuracy and stability.

Single

mercury ion.

Laser-cooled

calcium atoms.

Single mercury ion trap.

• Optical clocks have the potential for accuracy at the 10-18 level, >100 times better than NIST-F1.

• Likely to take many years to realize that potential.

Improvements in Primary Frequency Standards: Optical Clocks

Ytterbium atoms

in optical lattice.

Improvements in Primary Frequency Standards: Optical Clocks

1940 1950 1960 1970 1980 1990 2000 2010 2020

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-16

10-17

10-18

10-9

10-10

10-11

10-12

10-13

10-14

10-15

10-16

10-17

10-18

Fre

qu

en

cy U

nce

rta

inty

Year

NIST-F2

Optical

Standards

NBS-1

NIST-F1

Junghans MEGA 1, 1990

The first radio controlled wristwatch, manufactured in Germany. The antenna was hidden inside the wrist band.

Synchronized to time signals broadcast by radio station DCF77 on 77.5 kHz. This station was synchronized to the German time standard maintained by PTB.

Low Frequency (LF) Radio Controlled Clocks

Low frequency time signal stations operate at frequencies ranging from about 40 to 80 kHz.

The pictured watch can synchronize to transmitters in the United States, England, Germany, Japan, or China.

The U. S. transmitter is radio station WWVB, operated by NIST. More than 50 million WWVB radio controlled clocks are believed to be in operation.

GOES Satellite Clock, 1976

The first clocks controlled by satellites received NBS time signals from the GOES geostationary satellites. These clocks appeared about three years before the launch of the first GPS satellite.

Designed by a team led by Dick Davis, these clocks could remove most of the path delay between the clock and the satellite and were accurate to less than 100 microseconds. The picture shows a GOES clock built to commemorate the U. S. bicentennial.

GPS Clocks

Best known as a positioning and navigation system, GPS is also the main system used to distribute accurate time and frequency worldwide.

A constellation of as many as 32 satellites can deliver time accurate to less than 1 microsecond (typically 100 ns) anywhere on Earth. This has revolutionized timekeeping, and made many new technologies possible.

Many metrology labs use GPS

disciplined oscillators as their standard for frequency.

Mobile Phones are Radio Controlled Clocks

The clocks on mobile phones are usually very accurate. Many phones synchronize to GPS clocks located at cellular base stations, with only a few microseconds of additional delay added.

Unlike LF radio controlled clocks, mobile phones automatically correct when you change time zones.

A recent study indicates that 1 out of 7 people have stopped wearing watches, mostly because of mobile phones. That figure is twice as high among 15 to 24 year olds.

Summary

• The uncertainty of time measurements has improved by about 10 orders of magnitude during the past century, from parts in 106 to parts in 1016. Optical clocks should further reduce uncertainties by at least two more orders of magnitude.

• In everyday life, time-of-day clocks that are accurate to within 1 second will become more common, and should eventually be the norm rather than the exception. Many technologies could contribute to this trend, including LF radio signals, satellite signals, mobile phone signals, Internet time codes, and miniature atomic clocks.

• If you are interested in reading more about the history of time measurements, see the five-part series now being published in IEEE Instrumentation and Measurement Magazine (the first installment is in the August 2011 issue).