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
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.
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
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
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.
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
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
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.
Functional block diagram of Atomichron showing
the method of connecting the atomic beam tube into the system.
output curve shown here represents a device with a "Q" of almost 50