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The 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! That's a tad better than the Atomichron,
non?
Atomichron - World's Most Accurate Clock
First practical atomic primary frequency standard with stability better than
0.5 second in 300 years.
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 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
was said.
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.
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.
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 drifts.
The 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.
Since the 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 diagram.
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.
Posted May 1, 2020
(updated from original post on 6/25/2013)
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