National Company, of Malden, Massachusetts, which made this cesium-based
Atomichron in the mid-to-late 1950s, began life as a toy manufacturer.
It had an output frequency at the nominal resonance frequency of
cesium - 9192.631830 MHz - and was accurate to better than
a second in 600 years. The unit was 7 feet tall and weighed 500
pounds. Modern cesium standards are more stable and are portable.
As of January 2013, the
NIST-F1 cesium fountain primary frequency standard is accurate
to within one second every 100 million years!
Atomichron - World's
Most Accurate Clock
First practical atomic primary frequency standard with stability
better than 0.5 second in 300 years.
The Atomichron, a multi-purpose frequency producing instrument,
was unveiled recently by National Co., Inc., of Malden, Massachusetts.
The most accurate clock in the world, the Atomichron is the first
atomic beam clock available for commercial use. By maintaining synchronism
with the natural resonant frequency of the cesium atom, the device
is the most accurate primary frequency standard in the world, it
Dr. J. R. Zacharias (left), a key figure in development
of the Atomichron. H. C. Guterman (center). and J. H. Quick
(right), chairman and president of National Company, view
the atomic beam tube.
Front view of the Atomichron, which stands 7 feet high and
weighs about 500 pounds. Unit costs $50,000.
The extreme stability will permit: increased speed
and volume of long distance telephone communications in higher frequencies
of the spectrum; greater volume of industrial communications; extension
of power and pipe line control systems; and increased accuracy in
electronic navigational equipment. In the high frequency spectrum,
the Atomichron will permit the use of radio receivers and transmitting
equipment of unprecedented narrow bandwidths and precise frequency
control, eliminating crowded air waves often resulting in one station
or channel interfering with another. In the area of navigation,
the device is being used by the Air Force in its experimental long-range
"Navarho" navigation system.
How It Works
Electrons, and most sub-atomic particles, act in
many respects like tiny bar magnets. The outer electron in an atom,
like cesium, finds itself in the magnetic field of the nuclear magnet
and tends to align itself just like a compass needle. If the electron
is disturbed, it will vibrate about its position like the needle.
Frequency of the vibration of the analogous compass needle is determined
by the magnet strength of the needle, the field in which it is located,
its weight, and shape. Corresponding quantities for the electron
are fixed, unchanging, and identical for all electrons and cesium
nuclei. It is the quality of not changing which makes the vibration
frequency a primary standard and the Atomichron constantly corrects
an auxiliary vacuum-tube oscillator to operate at the frequency
of this electron resonance.
A reservoir of cesium atoms
is placed at one end of a long, evacuated chamber. As heat is applied,
individual cesium atoms drift away from the pool. In the diagram,
two cesium atoms of different orientations of nucleus and electron
are considered to be given off and to begin drifting through the
atomic beam tube, where they come under the influence of two permanent
magnets and an r.f. field. The orientation of nucleus and electron
in atom #2 is such that it is attracted to the strong pole of the
first magnet, and deflected away from the r.f. chamber. Atom #1
exists in an energy state which causes it to be deflected away from
the magnet and toward the r.f. chamber.
When the r.f. field
is near the cesium resonance frequency, atom #1 will probably emit
a photon and change its energy state to the configuration of atom
#2. If the r.f. field is not near cesium resonance, the atom will
probably remain at its original energy level.
As the atom
passes through the second magnetic field, and if its energy state
is unchanged, it is deflected away from the strong pole of the second
magnet. If it has changed its energy state in the r.f. field, it
is deflected toward the strong pole. In this case, however, deflection
toward the strong pole has the effect of conducting the atom into
a sensing chamber and to a target.
The atom strikes the
target, is ionized, and is attracted to the cathode of an electron
multiplier, which amplifies the cesium input current a million times.
The electron multiplier output current varies with the number and
rapidity of impingements of ionized cesium atoms on the cathode.
As the frequency of the r.f. field varies, the impingements on the
cathode decrease, causing a change in the magnitude of the electron
multiplier output current. This effect is used as the first step
in adjusting the frequency of the r.f. signal to return it automatically
to the standard value.
The frequency of the r.f. signal
which is applied to the atomic beam tube is derived from a 5 mc.
crystal oscillator. The output is multiplied to 9180 mc. Meanwhile,
a synthesizer combines harmonics and subharmonics of the output
of the basic 5 mc. oscillator in such a manner that when the synthesizer
output is combined with the multiplier output in the adder, an output
frequency is produced that is the nominal resonance frequency of
cesium - 9192.631830 mc. The cesium resonance frequency signal is
also phase modulated by the 100 cps output of the modulation oscillator.
The purpose of this modulation is to provide a determination of
the direction of variation whenever the output of the crystal oscillator
atomic beam output signal is amplified for transmission to one winding
of a two-phase motor. The current applied to this winding by the
feedback amplification system is automatically and continuously
compared by the motor to the current supplied directly to the other
winding of the motor by the 100 cps modulation oscillator. If the
r.f. frequency is above cesium resonance, the motor will operate
in a direction which reduces the original error. If it is below
resonance, the motor turns in the opposite direction. This rotation
is then transmitted through a gear box to a variable capacitor which
adjusts the output of the 5 mc. crystal oscillator to bring the
frequency at the atomic beam back to standard.
crystal oscillator is under continuous surveillance for precision,
the basic output is a 5 mc. signal stable and reproducible to 1
part in 1011. Higher frequencies are taken from the multiplier
and sub-multiples are taken from the synthesizer as shown in the
Because of its relatively small size and mobility,
the Atomichron now makes high precision time interval and frequency
control practical for navigation, communications, and engineering
systems without reliance on radio time signals.
Functional block diagram of Atomichron showing
the method of connecting the atomic beam tube into the system.
The output curve shown here represents a device with a "Q" of
almost 50 million.