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The atomic clock
N o T E s FROM THE N A T I O N A L B U R E A U OF STANDARDS.* THE ATOMIC CLOCK. AN ATOMIC STANDARD OF FREQUENCY AND TIME.
A b a s i c a l l y new, p r i m a r y s t a n d a r d of f r e q u e n c y a n d t i m e , i n v a r i a n t w i t h age, h a s b e e n d e v e l o p e d a t the N a t i o n a l B u r e a u of S t a n d a r d s ; an atomic clock b a s e d on a constant natural frequency associated with t h e v i b r a t i o n of the a t o m s in the a m m o n i a m o l e c u l e . B a s e d o n a p r i n c i p l e d e v e l o p e d by Dr. H a r o l d L y o n s of the B u r e a u ' s m i c r o w a v e r e s e a r c h l a b o r a t o r y , the n e w c l o c k p r o m i s e s t o s u r p a s s by o n e or two o r d e r s of m a g n i t u d e the a c c u r a c y of the p r e s e n t p r i m a r y s t a n d a r d , t h e r o t a t i n g e a r t h . Dr. L y o n s was a s s i s t e d in the d e s i g n a n d c o n s t r u c t i o n of the c l o c k by B. F. H u s t e n , E. D. H e b e r l i n g , a n d o t h e r m e m b e r s of his staff. T h i s is the f i r s t a t o m i c c l o c k ever b u i l t a n d is controlled b y a cons t a n t f r e q u e n c y d e r i v e d from a m i c r o w a v e a b s o r p t i o n line of a m m o n i a gas, p r o v i d i n g a time c o n s t a n c y of one p a r t in t e n million. T h e o r e t i c a l c o n s i d e r a t i o n s i n d i c a t e a p o t e n t i a l a c c u r a c y of one p a r t in a b i l l i o n or even ten b i l l i o n , d e p e n d i n g on the type of a t o m i c s y s t e m a n d s p e c t r u m line u s e d . The p r e s e n t c r o w d i n g of the r a d i o f r e q u e n c y s p e c t r u m h a s i m p o s e d s e v e r e l i m i t a t i o n s , both n a t i o n a l l y a n d internationally, on the e x p a n d i n g u s e of r a d i o for i n d u s t r y and c o m m u n i c a t i o n s . T h e a t o m i c c l o c k m a y be e x p e c t e d t o b e n e f i t g r e a t l y the c o m m u n i c a t i o n s i n d u s t r i e s a n d the m i l i t a r y s e r v i c e s , for it w i l l , in effect, p r o v i d e a d d i t i o n a l r o o m in the r a d i o f r e q u e n c y r a n g e for m o r e c o m m u n i c a t i o n s t a t i o n s of all t y p e s . T h e p r e s e n t " r a d i o s p a c e " a l l o w s for a d r i f t i n g of e a c h s t a t i o n ' s freq u e n c y , so t h a t a b r o a d " r a d i o s p a c e " is r e q u i r e d if i n t e r f e r e n c e with o t h e r s t a t i o n s is t o be a v o i d e d . T h e m a x i m u m u t i l i z a t i o n of a v a i l a b l e s p a c e in the r a d i o s p e c t r u m d e p e n d s on the a c c u r a c y with w h i c h t h e f r e q u e n c y of a n i n d i v i d u a l s t a t i o n can be c o n t r o l l e d , e s p e c i a l l y a t the h i g h e r f r e q u e n c i e s w h e r e q u a r t z c r y s t a l s c a n n o t be u s e d as f r e q u e n c y controlling e l e m e n t s . T h e s e f r e q u e n c i e s , u s e d by r a d a r , t e l e v i s i o n r e l a y s , a n d m i c r o w a v e e q u i p m e n t in g e n e r a l , c o u l d be c o n t r o l l e d by a t o m i c e l e m e n t s . S u c h c o n t r o l w o u l d also m a k e p o s s i b l e the perm a n e n t e s t a b l i s h m e n t of r a d i o c h a n n e l s on s u c h a n e x a c t b a s i s t h a t t u n i n g c o u l d be m a d e a s a u t o m a t i c a s the d i a l i n g of a t e l e p h o n e n u m b e r . T h e i m p r o v e m e n t s in f r e q u e n c y a n d time m e a s u r e m e n t offered by the a t o m i c c l o c k a r e also of f u n d a m e n t a l i m p o r t a n c e in m a n y f i e l d s of science. An a b s o l u t e time s t a n d a r d will be of s p e c i a l i m p o r t a n c e in a s t r o n o m y , w h e r e p r e s e n t t i m e s t a n d a r d s l e a v e m u c h t o be d e s i r e d . * Communicated by the Director.
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T h e a t o m i c c l o c k a n d the m e t h o d r e p r e s e n t i m p o r t a n t t o o l s of r e s e a r c h a n d d e v e l o p m e n t in e v e r y t e c h n i c a l field w h e r e p r e c i s e m e a s u r e m e n t s of time a n d f r e q u e n c y are c r u c i a l - - f o r e x a m p l e , in l o n g - r a n g e r a d i o n a v i g a t i o n s y s t e m s , in the u p p e r r a n g e of the m i c r o w a v e r e g i o n w h e r e a t o m i c s y s t e m s can s e r v e a s electronic c o m p o n e n t s , a n d in b a s i c res e a r c h in m i c r o w a v e s p e c t r o s c o p y and m o l e c u l a r s t r u c t u r e . T h e p r e s e n t time a n d f r e q u e n c y s t a n d a r d s are b a s e d on a s t r o n o m i c a l d e t e r m i n a t i o n s of the p e r i o d of r o t a t i o n of the e a r t h . H o w e v e r , the e a r t h is very g r a d u a l l y s l o w i n g down in r e s p o n s e t o the f o r c e s of t i d a l friction in s h a l l o w s e a s . In a d d i t i o n , t h e r e are i r r e g u l a r v a r i a t i o n s - s o m e of t h e m r a t h e r s u d d e n - - i n the p e r i o d of rotation, the r e a s o n s for w h i c h are u n k n o w n . T h e s e two c a u s e s are r e s p o n s i b l e for c h a n g e s in m e a n s o l a r time a n d t h e r e f o r e in the f r e q u e n c y of a n y p e r i o d i c or v i b r a ting s y s t e m s m e a s u r e d in t e r m s of s u c h time s t a n d a r d s . In r e c e n t y e a r s , v i b r a t i o n s of a t o m s in m o l e c u l e s - - o r w h a t are m o r e specifically t e r m e d s p e c t r u m l i n e s o r i g i n a t i n g in transitions bet w e e n e n e r g y l e v e l s of t h e s e a t o m i c s y s t e m s - - h a v e b e e n f o u n d in the m i c r o w a v e r e g i o n of the r a d i o s p e c t r u m . It h a s been p o s s i b l e t o m a k e very p r e c i s e m e a s u r e m e n t s of t h e s e l i n e s by r a d i o m e t h o d s u s i n g allelectronic e q u i p m e n t of u n p r e c e d e n t e d sensitivity a n d r e s o l u t i o n . When it b e c a m e evident that such spectrum lines m i g h t eventually p r o v i d e new p r i m a r y f r e q u e n c y s t a n d a r d s , scientists a t the N a t i o n a l B u r e a u of S t a n d a r d s b e g a n s e e k i n g a m e a n s of u t i l i z i n g one of t h e s e l i n e s t o c o n t r o l an o s c i l l a t o r w h i c h in turn c o u l d be used t o d r i v e a clock. B e c a u s e the r e s u l t i n g e q u i p m e n t , the a t o m i c c l o c k , is controlled by the i n v a r i a b l e m o l e c u l a r s y s t e m of a m m o n i a gas, it is i n d e p e n d e n t of a s t r o n o m i c a l d e t e r m i n a t i o n s of t i m e . T h e N a t i o n a l B u r e a u of S t a n d a r d s a t o m i c c l o c k c o n s i s t s e s s e n t i a l l y of a c r y s t a l oscillator, a f r e q u e n c y m u l t i p l i e r , a f r e q u e n c y d i s c r i m i n a t o r , and a f r e q u e n c y d i v i d e r , all h o u s e d in two v e r t i c a l - t y p e c a b i n e t r a c k s , on the top of w h i c h are m o u n t e d a s p e c i a l 50-cycle c l o c k a n d a w a v e g u i d e a b s o r p t i o n cell. A m m o n i a g a s u n d e r a p r e s s u r e of 10 o r 15 m i c r o n s is m a i n t a i n e d in this cell, a r e c t a n g u l a r ½ by } - i n . c o p p e r t u b e w o u n d in a c o m p a c t 3 0 - f o o t s p i r a l a b o u t the c l o c k . T h e n e w d e v e l o p m e n t u s e s an a b s o r p t i o n f r e q u e n c y of a m m o n i a t o hold a m i c r o w a v e s i g n a l f i x e d . If the m i c r o w a v e s i g n a l o u t p u t of a g e n e r a t o r differs in f r e q u e n c y from the a m m o n i a a b s o r p t i o n l i n e , then the c o n t r o l c i r c u i t s g e n e r a t e a n " e r r o r s i g n a l " w h i c h b r i n g s the m i c r o w a v e s i g n a l b a c k t o the f r e q u e n c y of the s p e c t r u m llne. T h e o s c i l l a t o r g e n e r a t i n g the m i c r o w a v e s i g n a l is t h u s c o n t r o l l e d , and the s e t t i n g of the c l o c k w h i c h i t d r i v e s can be c o m p a r e d with a n a s t r o n o m i c a l c l o c k . T h e m i c r o w a v e s i g n a l is i n i t i a t e d by a 100-kilocycle q u a r t z - c r y s t a l oscillator or a n y o t h e r o s c i l l a t o r w h i c h , for p u r p o s e s of c o n v e n i e n c e a n d a c c u r a c y , is d e s i g n e d for a h i g h d e g r e e of s t a b i l i t y . By m e a n s of v a c u u m - t u b e c i r c u i t s a n d silicon-crystal d i o d e s , this f r e q u e n c y is
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multiplied to provide output signals throughout the microwave range. These signals are compared with the frequency of a microwave spectrum line, in this case of a m m o n i a gas, by suitable control circuits, often called frequency discriminator or "servo" circuits. If the quartzcrystal oscillator drifts a f t e r the microwave signal at the u p p e r end of the multiplier chain has been exactly tuned to the frequency of the spectrum line, the discriminator circuit generates an o u t p u t signal which, through the proper control circuits, can be applied to the oscillator at the bottom of the multiplier chain to bring it back to the proper frequency. By means of a frequency divider, the 100 kilocycles may be reduced to any desired frequency for driving a clock; e.g. one thousand cycles or 50 cycles. PRINCIPLES AND OPERATION.
Frequency-discriminator or servo-mechanism control circuits for atomic clocks m i g h t be developed in many different forms. The electronic control circuit in the present a t o m i c clock is one successful form of several being developed by the National Bureau of Standards. It is now being refined to give even greater time-keeping accuracy. The fundamental frequency signal generated by the 100-kilocycle oscillator is first multiplied up to 270 megacycles per second (abbreviated Mc) by a frequency-multiplying chain using standard low-frequency tubes. In the next step, the multiplying chain is continued up to 2970 Mc by means of a frequency-multiplying klystron, which is also modulated by an FM oscillator generating a signal at 13.8 4- 0.12 Mc. This m a k e s the frequency-modulated output of the klystron 2983.8 4-0.12 Mc. A f t e r further amplification, the frequency-modulated signal is multiplied in a silicon crystal rectifier to 23,870.4 4- 0.96 Mc, and fed to the ammonia absorption cell. As the frequency of this modulated control signal sweeps across the absorption line frequency of the ammonia vapor, the signal reaching the silicon crystal detector a t the end of the absorption cell dips because of the absorption, thus giving a negative output pulse. A second pulse is generated when the output of the frequencymodulated oscillator at 13.8 4- 0.12 Mc is fed to a mixer (or radio receiver) into which is also fed a 12.5-Mc signal from the quartz-crystal multiplying chain. When the signal sweeps across the proper frequency to be tuned in (12.5 Mc plus the 1.39 Mc intermediate frequency of the receiver, or 13.89 Mc), an output pulse is generated. The time interval between the two pulses--that from the absorption cell, caused by the absorption line, and that from the receiver or m i x e r 4 i s a measure of the degree to which the frequency-multiplying chain is tuned to the absorption line. The two pulses can therefore be made to control a discriminator circuit which will give zero output when the time interval is right (that is, when the circuit is t u n e d to the absorption line) and will
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generate a control signal when the time interval is wrong. If the quartz-crystal oscillator drifts in frequency to higher values, the time interval between the two pulses increases; for frequencies which are too low, the interval decreases. The control signals thus generated are fed to a reactance tube, which then forces the quartz-crystal circuit to oscillate at the correct frequency to tune to the absorption line. The quartz-crystal oscillator is thus locked to the ammonia line. Frequency dividers then divide the precise 100-kilocycle signal down to 50 cycles to drive an ordinary synchronous m o t o r clock, and also down to 1000 cycles to d r i v e a special synchronous-motor clock, w h i c h is designed for e x a c t adjustment and comparison with astronomical time to within 5/1000 of a second. Control of the quartz-crystal circuit depends on the relative duration of the positive and negative portions of a square-wave signal generated by the discriminator. In the discriminator, the two pulses between which the time interval is to be measured turn a trigger circuit or squarewave generator on and off. When the time interval is correct, the onoff cycle generates no output signal from the positive and negative peak detectors driven by the square-wave signal. The detectors or rectifiers draw current on the positive and negative p e a k s of the square-wave, but when the positive and negative portions of the square wave are of equal duration, they balance and give no direct current output. However, if the time interval between the two i n p u t driving pulses gets longer or shorter, the relative duration of the positive and negative p a r t s of the square-wave changes so that a resultant direct-current o u t p u t is generated. This o u t p u t is positive or negative, depending on the change in the time interval. Thus, no control voltage is generated when the quartz-crystal oscillator is on the proper frequency to agree, through the frequency-multiplying chain, with the ammonia line ; but a positive or negative control voltage is produced for correcting the oscillator circuit when it drifts one way or the other from its proper value. One great advantage of this particular clock circuit lies in the inherent short-time stability of the quartz-crystal oscillator, which makes it unnecessary for the discriminator circuits to a p p l y correcting control signals to the oscillator at a very r a p i d rate. The crystal and multiplier circuits bridge the gap between the frequency of the clock and that of the absorption line. Recording equipment and a frequency m e t e r are used in checking the accuracy of the clock. For this purpose, the frequency of the clock's crystal oscillator is compared to the frequency of the Bureau's primary frequency standards, a group of precision, 100-kilocycle quartz-crystal oscillators calibrated in terms of the U. S, N a v a l Observatory time signals. These oscillators maintain constant frequency with respect to each other to an accuracy of one part in a billion for intervals up to
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10 hours and better than one part in 100 million per day. They can therefore be used to measure the constancy of the atomic clock to this accuracy. This is done by beating the signals from the two sources together at a frequency of 12.5 Mc to obtain greater measurement sensitivity. A c h a n g e of one cycle per second in the frequency of the beat note, as recorded .on the frequency m e t e r or on an automatic recorder, indicates a frequency variation of one part in 12.5 million. In recent tests the clock maintained a constancy of one part in ten million for several hours. These tests show that the clock will lock accurately to the a m m o n i a line even when a perturbing signal is applied to the reactance tube in the a t t e m p t to force the clock to c h a n g e its rate. ULTIMATE ACCURACY.
The ultimate accuracy of an atomic clock depends on many factors, of which the most important are t h o s e governing the w i d t h of the spectrum line. Spectrum lines are not infinitely narrow but have a finite w i d t h covering a considerable frequency range, since a t o m s or molecules do not emit or absorb radiation at only one frequency but rather over a narrow band of frequencies. The ratio of a line frequency to its w i d t h at the half-power points is called the Q of the line, in analogy to the Q (quality) factor of resonant circuits used in standard radio technique. The Q is a measure of the sharpness of the line and therefore determines its usefulness as an accurate frequency and time standard. In the case of ammonia, the natural line w i d t h determined by the uncertainty principle of quantum mechanics gives a Q of a b o u t 10is (a billion billion). If a line w i d t h were determined only by the natural life time of an excited s t a t e in the a m m o n i a molecule, giving a Q of l0 Is, frequency and time could be determined to better than one part in a billion billion (1,000,000,000,000,000,000). However, the line is broadened by o t h e r factors which lower the Q to a value of from 50,000 to 500,000, depending on the temperature and pressure of the gas. This may be compared to Q values of roughly 50,000 for a good cavity resonator in a microwave circuit and values of 1,000,000 or so for the best quartz crystals. The a m m o n i a spectrum line thus has a Q approximating that of the best quartz crystals, though much more constant and stable. The ammonia molecules in the absorption cell are moving rapidly in random thermal motion at an average speed of almost 2000 ft. per second at room temperature. When a gas molecule in an absorption cell is approaching or receding from the source of an electromagnetic wave because of its heat motion, its absorption frequency is different from that w h i c h it w o u l d have if it were standing still. This g i v e srise to a "Doppler broadening" of the absorption line, analogous to the c h a n g e in pitch of sound as its source approaches, passes, and leaves an observer. Thus, the line w i d t h can be reduced slightly by lowering the temperature
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of the gas (or by using a heavier molecule). Doppler broadening lowers the Q of the a m m o n i a line to a b o u t 330,000 at room temperatures. Molecular collisions also broaden the absorptionline. This broadening occurs because the collisions abruptly terminate the absorption process, causing the molecules to absorb wave trains whose lengths vary in a random way determined by the distribution of time intervals between collisions. A frequency analysis of t h e s e wave trains shows a corresponding random distribution of absorbed frequencies, all centering tabour a mean value determined by the n u m b e r of collisions per second. In ammonia gas at a pressure of 10 microns t h e r e are a b o u t 120,000 collisions per second, giving an experimentally measured Q of 45,000 for the absorption line used. (This is the line known to spectroscopists as the 3,3 line, for which the quantum numbers J and K are each e q u a l to 3.) Actually, t h e r e are more collisions effectively interrupting the absorption process in a m m o n i a than the kinetic theory of gases would indicate. Further broadening of the line results from collisions of the molecules with the walls, and even near misses between molecules cause interaction strong e n o u g h to i n t e r r u p t absorption. The n u m b e r of collisions per second, and thus the collision broadening, can be reduced by lowering the gas pressure. This process, if not carried too far, does not reduce absorption in the gas, because the decrease in n u m b e r of molecules absorbing e n e r g y is offset by the increase in absorption per molecule resulting from the increase in Q. However, when the pressure is reduced too much, a phenomenon known as saturation of the line sets in, caused by an excess of radiation. Too few molecules are then left in the proper e n e r g y states to absorb the microwave radiation coming into the cell. Many molecules, which normally would be in the proper e n e r g y s t a t e to absorb the incoming radiation, are in an excited s t a t e as a result of previous absorption. Eventually t h e s e molecules will emit the quanta which they have absorbed, returning to the normal level where absorption is a g a i n possible. However, as this process is slow, the molecule usually returns to the ground level in a collision with another molecule, converting the absorbed radiation into h e a t . As the gas pressure is lowered, the n u m b e r of collisions is greatly reduced, and not e n o u g h molecules return to ground levels. The excessive incoming radiation then weakens and broadens the absorption line through saturation. The broadening r e s u l t s because saturation occurs earlier at the peak of the line than out at its wings. Saturation can be eliminated by reducing the strength of the incoming radiation. However, as the gas pressure and radiation i n t e n s i t y are both lowered, a condition will finally be met for which the signal strength will be down in the natural electrical noise level of the circuits used to detect the signal. Circuit noise then sets the ultimate limitation on the reduction of collision and saturation broadening. It is estimated
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that a Q of 300,000 to 400,000 can be attained at pressures of about one micron--still a long way from the Q of the natural line width. Assuming that effective Q values of 400,000 can be obtained with ammonia, an accuracy of one part in 100 million or better should be possible since a measurement of the center of the absorption line to within 1/250 of the width of the line could be made. APPLICATIONS AND SIGNIFICANCE.
Improvement of the accuracy of the atomic clock will make it useful in several fields of pure and applied science. The lengths of the mean solar day, used in astronomical measurements, fluctuate as much as one part in 20 to 30 million, because of variations in the rate of rotation of the e a r t h on its axis. The variation in present time standards, due to these fluctuations, causes errors in the location of heavenly bodies and in studies of t h e i r orbits and motions. The atomic clock offers the possibility of an invariant master clock against which the variation in the earth's time-keeping could be measured. An absorption cell on an atomic clock could, for some purposes, take the place of an astronomical observatory. Broadcasts of standard frequency are of importance in keeping all kinds of radio, radar, and electronic equipment properly tuned throughout the world. This service is required in international transportation and communications so that, for example, an airplane with radio navigational equipment will be using the right frequency wherever it is in the world and whatever airport it is using. At present, the National Bureau of Standards Station, WWV, broadcasts standard frequency and time signals on several transmitter frequencies to all the world. The Navy Department also uses quartz-crystal clocks to broadcast time signals for navigational purposes. These quartz-crystal clocks drift slightly in frequency and have to be adjusted to keep them in agreement with the basic astronomical time signals. Clocks of this type could be kept constant automatically by means of absorption lines. Maintenance of transmitter frequency to within close limits is also necessary to utilize the available r a d i o spectrum efficiently. The use o f long-distance standard frequency broadcasts is complicated by a large reduction in accuracy due to ionospheric effects. A long distance, short-wave signal travels around the e a r t h by reflection from the u p p e r ionized regions of the atmosphere, known as the ionosphere. E v e r y morning a t sunrise the ionosphere moves downward, and every evening at sunset it rises. This d aily variation in height causes a Doppler shift of the frequency of the reflected wave and, together with o t h e r as yet unknown causes, is responsible for a reduction by a factor of 25 or more in the accuracy of the frequency of the received signal. Thus, the Bureau's standard frequency broadcast agrees with astronomical time signals to one part in one hundred million at the transmitter but
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m a y be k n o w n t o only one p a r t in f o u r m i l l i o n a f t e r t r a n s m i s s i o n o v e r long d i s t a n c e s . T h i s difficulty c a n be p a r t l y o v e r c o m e in s e v e r a l w a y s . O n e is the p r o v i s i o n of a local, p r e c i s e f r e q u e n c y s t a n d a r d c a l i b r a t e d b y m e a n s of r e c e i v e d s t a n d a r d time s i g n a l s also t r a n s m i t t e d by r a d i o . H o w e v e r , this p r o c e s s , w h i c h r e q u i r e s a d a y or m o r e , c o m p l i c a t e s the e q u i p m e n t problem and introduces additional errors, m a k i n gimpractical the u s e of s t a n d a r d f r e q u e n c y b r o a d c a s t s for i n s t a n t a n e o u s or c o n t i n u o u s f r e q u e n c y c a l i b r a t i o n s of the h i g h e s t precision. A t the l a s t I n t e r n a t i o n a l R a d i o C o n f e r e n c e held in A t l a n t i c City in 1947, p l a n s w e r e f o r m u l a t e d t o p r o v i d e s t a n d a r d f r e q u e n c y and time b r o a d c a s t s from m a n y s t a t i o n s l o c a t e d t o r e n d e r good s e r v i c e t h r o u g h out the w o r l d . T h e s e s e r v i c e s m a y be i m p r o v e d o r simplified by m e a n s of a t o m i c c l o c k s a n d f r e q u e n c y s t a n d a r d s . S u c h c l o c k s c o u l d c o n t r o l the s t a n d a r d f r e q u e n c y e m i s s i o n s of the v a r i o u s s t a t i o n s w i t h o u t c h e c k ing and m o n i t o r i n g by a s t r o n o m i c a l time s i g n a l s . The D o p p l e r freq u e n c y s h i f t s c o u l d then be e l i m i n a t e d by l i m i t i n g t r a n s m i s s i o n dist a n c e s t o s h o r t r a n g e s . A l s o , e q u i p m e n t a n y w h e r e in the w o r l d c o u l d be c h e c k e d a g a i n s t a n a b s o r p t i o n line with the c e r t a i n t y of o b t a i n i n g a p r e c i s i o n c a l i b r a t i o n a g a i n s t a n a b s o l u t e s t a n d a r d a n d w i t h o u t depending on a standard frequency broadcast. O n e a d v a n t a g e of the r o t a t i n g e a r t h as the b a s i c t i m e - k e e p e r is t h a t it n e v e r s t o p s r o t a t i n g or b r e a k s d o w n . L i k e w i s e , a n y m a n - m a d e c l o c k m u s t not b r e a k down but m u s t be k e p t r u n n i n g f o r e v e r if it is t o keep t r a c k of time from some a r b i t r a r y i n s t a n t c h o s e n a s a s t a r t i n g point. W i t h the p r e s e n t q u a r t z - c r y s t a l c l o c k s , this difficulty is m e t by u s i n g a l a r g e n u m b e r of s i m i l a r c l o c k s c o n s t a n t l y i n t e r c o m p a r e d so t h a t b r e a k d o w n of one does n o t m e a n a loss of t i m e - k e e p i n g r e c o r d s . W h i l e this p r o c e d u r e c o u l d also be u s e d with a t o m i c c l o c k s , it w o u l d n o t be n e c e s s a r y for u s e of the c l o c k as a f r e q u e n c y s t a n d a r d or for d e f i n i n g a s t a n d a r d of t i m e - i n t e r v a l s s i n c e t h e s e a p p l i c a t i o n s do n o t r e q u i r e cont i n u o u s o p e r a t i o n of the a t o m i c c l o c k . T h e a t o m i c c l o c k s h o u l d p e r m i t i m p r o v e m e n t in a s t r o n o m i c a l t i m e s t a n d a r d s in a w a y i m p o s s i b l e with e l e c t r i c - p e n d u l u m o r q u a r t z - c r y s t a l c l o c k s . I t t h u s o p e n s the p o s s i b i l i t y of i m p r o v i n g the p r e c i s i o n of k n o w l e d g e of the l e n g t h of the y e a r , t h a t is, the time i t t a k e s the e a r t h t o r e v o l v e once in its o r b i t a r o u n d the sun. This is i n d e p e n d e n t of the t i m e it t a k e s the e a r t h t o r o t a t e once on its a x i s - - t h e m e a n s o l a r day. M e a s u r e m e n t s c o u l d then d e t e r m i n e w h e t h e r the m e a n s i d e r a l y e a r is m o r e c o n s t a n t t h a n the m e a n s o l a r d a y , a s some a s t r o n o m e r s b e l i e v e m a y be the c a s e . A l t h o u g h the u s e of a t o m i c time p r e s e n t s a d v a n t a g e s in m a n y f i e l d s of s c i e n c e , i t will a l w a y s be n e c e s s a r y for some p u r p o s e s t o h a v e a s t r o n o m i c a l time s t a n d a r d s . This is b e c a u s e the p o i n t i n g of a t e l e s c o p e d e p e n d s o n the o r i e n t a t i o n of the e a r t h a t the i n s t a n t of o b s e r v a t i o n , in o t h e r w o r d s , on a s t r o n o m i c a l time m e a s u r e m e n t s w h i c h d e r i v e from the m o t i o n of the e a r t h .
A b a s i c a l l y new, p r i m a r y s t a n d a r d of f r e q u e n c y a n d t i m e , i n v a r i a n t w i t h age, h a s b e e n d e v e l o p e d a t the N a t i o n a l B u r e a u of S t a n d a r d s ; an atomic clock b a s e d on a constant natural frequency associated with t h e v i b r a t i o n of the a t o m s in the a m m o n i a m o l e c u l e . B a s e d o n a p r i n c i p l e d e v e l o p e d by Dr. H a r o l d L y o n s of the B u r e a u ' s m i c r o w a v e r e s e a r c h l a b o r a t o r y , the n e w c l o c k p r o m i s e s t o s u r p a s s by o n e or two o r d e r s of m a g n i t u d e the a c c u r a c y of the p r e s e n t p r i m a r y s t a n d a r d , t h e r o t a t i n g e a r t h . Dr. L y o n s was a s s i s t e d in the d e s i g n a n d c o n s t r u c t i o n of the c l o c k by B. F. H u s t e n , E. D. H e b e r l i n g , a n d o t h e r m e m b e r s of his staff. T h i s is the f i r s t a t o m i c c l o c k ever b u i l t a n d is controlled b y a cons t a n t f r e q u e n c y d e r i v e d from a m i c r o w a v e a b s o r p t i o n line of a m m o n i a gas, p r o v i d i n g a time c o n s t a n c y of one p a r t in t e n million. T h e o r e t i c a l c o n s i d e r a t i o n s i n d i c a t e a p o t e n t i a l a c c u r a c y of one p a r t in a b i l l i o n or even ten b i l l i o n , d e p e n d i n g on the type of a t o m i c s y s t e m a n d s p e c t r u m line u s e d . The p r e s e n t c r o w d i n g of the r a d i o f r e q u e n c y s p e c t r u m h a s i m p o s e d s e v e r e l i m i t a t i o n s , both n a t i o n a l l y a n d internationally, on the e x p a n d i n g u s e of r a d i o for i n d u s t r y and c o m m u n i c a t i o n s . T h e a t o m i c c l o c k m a y be e x p e c t e d t o b e n e f i t g r e a t l y the c o m m u n i c a t i o n s i n d u s t r i e s a n d the m i l i t a r y s e r v i c e s , for it w i l l , in effect, p r o v i d e a d d i t i o n a l r o o m in the r a d i o f r e q u e n c y r a n g e for m o r e c o m m u n i c a t i o n s t a t i o n s of all t y p e s . T h e p r e s e n t " r a d i o s p a c e " a l l o w s for a d r i f t i n g of e a c h s t a t i o n ' s freq u e n c y , so t h a t a b r o a d " r a d i o s p a c e " is r e q u i r e d if i n t e r f e r e n c e with o t h e r s t a t i o n s is t o be a v o i d e d . T h e m a x i m u m u t i l i z a t i o n of a v a i l a b l e s p a c e in the r a d i o s p e c t r u m d e p e n d s on the a c c u r a c y with w h i c h t h e f r e q u e n c y of a n i n d i v i d u a l s t a t i o n can be c o n t r o l l e d , e s p e c i a l l y a t the h i g h e r f r e q u e n c i e s w h e r e q u a r t z c r y s t a l s c a n n o t be u s e d as f r e q u e n c y controlling e l e m e n t s . T h e s e f r e q u e n c i e s , u s e d by r a d a r , t e l e v i s i o n r e l a y s , a n d m i c r o w a v e e q u i p m e n t in g e n e r a l , c o u l d be c o n t r o l l e d by a t o m i c e l e m e n t s . S u c h c o n t r o l w o u l d also m a k e p o s s i b l e the perm a n e n t e s t a b l i s h m e n t of r a d i o c h a n n e l s on s u c h a n e x a c t b a s i s t h a t t u n i n g c o u l d be m a d e a s a u t o m a t i c a s the d i a l i n g of a t e l e p h o n e n u m b e r . T h e i m p r o v e m e n t s in f r e q u e n c y a n d time m e a s u r e m e n t offered by the a t o m i c c l o c k a r e also of f u n d a m e n t a l i m p o r t a n c e in m a n y f i e l d s of science. An a b s o l u t e time s t a n d a r d will be of s p e c i a l i m p o r t a n c e in a s t r o n o m y , w h e r e p r e s e n t t i m e s t a n d a r d s l e a v e m u c h t o be d e s i r e d . * Communicated by the Director.
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T h e a t o m i c c l o c k a n d the m e t h o d r e p r e s e n t i m p o r t a n t t o o l s of r e s e a r c h a n d d e v e l o p m e n t in e v e r y t e c h n i c a l field w h e r e p r e c i s e m e a s u r e m e n t s of time a n d f r e q u e n c y are c r u c i a l - - f o r e x a m p l e , in l o n g - r a n g e r a d i o n a v i g a t i o n s y s t e m s , in the u p p e r r a n g e of the m i c r o w a v e r e g i o n w h e r e a t o m i c s y s t e m s can s e r v e a s electronic c o m p o n e n t s , a n d in b a s i c res e a r c h in m i c r o w a v e s p e c t r o s c o p y and m o l e c u l a r s t r u c t u r e . T h e p r e s e n t time a n d f r e q u e n c y s t a n d a r d s are b a s e d on a s t r o n o m i c a l d e t e r m i n a t i o n s of the p e r i o d of r o t a t i o n of the e a r t h . H o w e v e r , the e a r t h is very g r a d u a l l y s l o w i n g down in r e s p o n s e t o the f o r c e s of t i d a l friction in s h a l l o w s e a s . In a d d i t i o n , t h e r e are i r r e g u l a r v a r i a t i o n s - s o m e of t h e m r a t h e r s u d d e n - - i n the p e r i o d of rotation, the r e a s o n s for w h i c h are u n k n o w n . T h e s e two c a u s e s are r e s p o n s i b l e for c h a n g e s in m e a n s o l a r time a n d t h e r e f o r e in the f r e q u e n c y of a n y p e r i o d i c or v i b r a ting s y s t e m s m e a s u r e d in t e r m s of s u c h time s t a n d a r d s . In r e c e n t y e a r s , v i b r a t i o n s of a t o m s in m o l e c u l e s - - o r w h a t are m o r e specifically t e r m e d s p e c t r u m l i n e s o r i g i n a t i n g in transitions bet w e e n e n e r g y l e v e l s of t h e s e a t o m i c s y s t e m s - - h a v e b e e n f o u n d in the m i c r o w a v e r e g i o n of the r a d i o s p e c t r u m . It h a s been p o s s i b l e t o m a k e very p r e c i s e m e a s u r e m e n t s of t h e s e l i n e s by r a d i o m e t h o d s u s i n g allelectronic e q u i p m e n t of u n p r e c e d e n t e d sensitivity a n d r e s o l u t i o n . When it b e c a m e evident that such spectrum lines m i g h t eventually p r o v i d e new p r i m a r y f r e q u e n c y s t a n d a r d s , scientists a t the N a t i o n a l B u r e a u of S t a n d a r d s b e g a n s e e k i n g a m e a n s of u t i l i z i n g one of t h e s e l i n e s t o c o n t r o l an o s c i l l a t o r w h i c h in turn c o u l d be used t o d r i v e a clock. B e c a u s e the r e s u l t i n g e q u i p m e n t , the a t o m i c c l o c k , is controlled by the i n v a r i a b l e m o l e c u l a r s y s t e m of a m m o n i a gas, it is i n d e p e n d e n t of a s t r o n o m i c a l d e t e r m i n a t i o n s of t i m e . T h e N a t i o n a l B u r e a u of S t a n d a r d s a t o m i c c l o c k c o n s i s t s e s s e n t i a l l y of a c r y s t a l oscillator, a f r e q u e n c y m u l t i p l i e r , a f r e q u e n c y d i s c r i m i n a t o r , and a f r e q u e n c y d i v i d e r , all h o u s e d in two v e r t i c a l - t y p e c a b i n e t r a c k s , on the top of w h i c h are m o u n t e d a s p e c i a l 50-cycle c l o c k a n d a w a v e g u i d e a b s o r p t i o n cell. A m m o n i a g a s u n d e r a p r e s s u r e of 10 o r 15 m i c r o n s is m a i n t a i n e d in this cell, a r e c t a n g u l a r ½ by } - i n . c o p p e r t u b e w o u n d in a c o m p a c t 3 0 - f o o t s p i r a l a b o u t the c l o c k . T h e n e w d e v e l o p m e n t u s e s an a b s o r p t i o n f r e q u e n c y of a m m o n i a t o hold a m i c r o w a v e s i g n a l f i x e d . If the m i c r o w a v e s i g n a l o u t p u t of a g e n e r a t o r differs in f r e q u e n c y from the a m m o n i a a b s o r p t i o n l i n e , then the c o n t r o l c i r c u i t s g e n e r a t e a n " e r r o r s i g n a l " w h i c h b r i n g s the m i c r o w a v e s i g n a l b a c k t o the f r e q u e n c y of the s p e c t r u m llne. T h e o s c i l l a t o r g e n e r a t i n g the m i c r o w a v e s i g n a l is t h u s c o n t r o l l e d , and the s e t t i n g of the c l o c k w h i c h i t d r i v e s can be c o m p a r e d with a n a s t r o n o m i c a l c l o c k . T h e m i c r o w a v e s i g n a l is i n i t i a t e d by a 100-kilocycle q u a r t z - c r y s t a l oscillator or a n y o t h e r o s c i l l a t o r w h i c h , for p u r p o s e s of c o n v e n i e n c e a n d a c c u r a c y , is d e s i g n e d for a h i g h d e g r e e of s t a b i l i t y . By m e a n s of v a c u u m - t u b e c i r c u i t s a n d silicon-crystal d i o d e s , this f r e q u e n c y is
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multiplied to provide output signals throughout the microwave range. These signals are compared with the frequency of a microwave spectrum line, in this case of a m m o n i a gas, by suitable control circuits, often called frequency discriminator or "servo" circuits. If the quartzcrystal oscillator drifts a f t e r the microwave signal at the u p p e r end of the multiplier chain has been exactly tuned to the frequency of the spectrum line, the discriminator circuit generates an o u t p u t signal which, through the proper control circuits, can be applied to the oscillator at the bottom of the multiplier chain to bring it back to the proper frequency. By means of a frequency divider, the 100 kilocycles may be reduced to any desired frequency for driving a clock; e.g. one thousand cycles or 50 cycles. PRINCIPLES AND OPERATION.
Frequency-discriminator or servo-mechanism control circuits for atomic clocks m i g h t be developed in many different forms. The electronic control circuit in the present a t o m i c clock is one successful form of several being developed by the National Bureau of Standards. It is now being refined to give even greater time-keeping accuracy. The fundamental frequency signal generated by the 100-kilocycle oscillator is first multiplied up to 270 megacycles per second (abbreviated Mc) by a frequency-multiplying chain using standard low-frequency tubes. In the next step, the multiplying chain is continued up to 2970 Mc by means of a frequency-multiplying klystron, which is also modulated by an FM oscillator generating a signal at 13.8 4- 0.12 Mc. This m a k e s the frequency-modulated output of the klystron 2983.8 4-0.12 Mc. A f t e r further amplification, the frequency-modulated signal is multiplied in a silicon crystal rectifier to 23,870.4 4- 0.96 Mc, and fed to the ammonia absorption cell. As the frequency of this modulated control signal sweeps across the absorption line frequency of the ammonia vapor, the signal reaching the silicon crystal detector a t the end of the absorption cell dips because of the absorption, thus giving a negative output pulse. A second pulse is generated when the output of the frequencymodulated oscillator at 13.8 4- 0.12 Mc is fed to a mixer (or radio receiver) into which is also fed a 12.5-Mc signal from the quartz-crystal multiplying chain. When the signal sweeps across the proper frequency to be tuned in (12.5 Mc plus the 1.39 Mc intermediate frequency of the receiver, or 13.89 Mc), an output pulse is generated. The time interval between the two pulses--that from the absorption cell, caused by the absorption line, and that from the receiver or m i x e r 4 i s a measure of the degree to which the frequency-multiplying chain is tuned to the absorption line. The two pulses can therefore be made to control a discriminator circuit which will give zero output when the time interval is right (that is, when the circuit is t u n e d to the absorption line) and will
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generate a control signal when the time interval is wrong. If the quartz-crystal oscillator drifts in frequency to higher values, the time interval between the two pulses increases; for frequencies which are too low, the interval decreases. The control signals thus generated are fed to a reactance tube, which then forces the quartz-crystal circuit to oscillate at the correct frequency to tune to the absorption line. The quartz-crystal oscillator is thus locked to the ammonia line. Frequency dividers then divide the precise 100-kilocycle signal down to 50 cycles to drive an ordinary synchronous m o t o r clock, and also down to 1000 cycles to d r i v e a special synchronous-motor clock, w h i c h is designed for e x a c t adjustment and comparison with astronomical time to within 5/1000 of a second. Control of the quartz-crystal circuit depends on the relative duration of the positive and negative portions of a square-wave signal generated by the discriminator. In the discriminator, the two pulses between which the time interval is to be measured turn a trigger circuit or squarewave generator on and off. When the time interval is correct, the onoff cycle generates no output signal from the positive and negative peak detectors driven by the square-wave signal. The detectors or rectifiers draw current on the positive and negative p e a k s of the square-wave, but when the positive and negative portions of the square wave are of equal duration, they balance and give no direct current output. However, if the time interval between the two i n p u t driving pulses gets longer or shorter, the relative duration of the positive and negative p a r t s of the square-wave changes so that a resultant direct-current o u t p u t is generated. This o u t p u t is positive or negative, depending on the change in the time interval. Thus, no control voltage is generated when the quartz-crystal oscillator is on the proper frequency to agree, through the frequency-multiplying chain, with the ammonia line ; but a positive or negative control voltage is produced for correcting the oscillator circuit when it drifts one way or the other from its proper value. One great advantage of this particular clock circuit lies in the inherent short-time stability of the quartz-crystal oscillator, which makes it unnecessary for the discriminator circuits to a p p l y correcting control signals to the oscillator at a very r a p i d rate. The crystal and multiplier circuits bridge the gap between the frequency of the clock and that of the absorption line. Recording equipment and a frequency m e t e r are used in checking the accuracy of the clock. For this purpose, the frequency of the clock's crystal oscillator is compared to the frequency of the Bureau's primary frequency standards, a group of precision, 100-kilocycle quartz-crystal oscillators calibrated in terms of the U. S, N a v a l Observatory time signals. These oscillators maintain constant frequency with respect to each other to an accuracy of one part in a billion for intervals up to
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10 hours and better than one part in 100 million per day. They can therefore be used to measure the constancy of the atomic clock to this accuracy. This is done by beating the signals from the two sources together at a frequency of 12.5 Mc to obtain greater measurement sensitivity. A c h a n g e of one cycle per second in the frequency of the beat note, as recorded .on the frequency m e t e r or on an automatic recorder, indicates a frequency variation of one part in 12.5 million. In recent tests the clock maintained a constancy of one part in ten million for several hours. These tests show that the clock will lock accurately to the a m m o n i a line even when a perturbing signal is applied to the reactance tube in the a t t e m p t to force the clock to c h a n g e its rate. ULTIMATE ACCURACY.
The ultimate accuracy of an atomic clock depends on many factors, of which the most important are t h o s e governing the w i d t h of the spectrum line. Spectrum lines are not infinitely narrow but have a finite w i d t h covering a considerable frequency range, since a t o m s or molecules do not emit or absorb radiation at only one frequency but rather over a narrow band of frequencies. The ratio of a line frequency to its w i d t h at the half-power points is called the Q of the line, in analogy to the Q (quality) factor of resonant circuits used in standard radio technique. The Q is a measure of the sharpness of the line and therefore determines its usefulness as an accurate frequency and time standard. In the case of ammonia, the natural line w i d t h determined by the uncertainty principle of quantum mechanics gives a Q of a b o u t 10is (a billion billion). If a line w i d t h were determined only by the natural life time of an excited s t a t e in the a m m o n i a molecule, giving a Q of l0 Is, frequency and time could be determined to better than one part in a billion billion (1,000,000,000,000,000,000). However, the line is broadened by o t h e r factors which lower the Q to a value of from 50,000 to 500,000, depending on the temperature and pressure of the gas. This may be compared to Q values of roughly 50,000 for a good cavity resonator in a microwave circuit and values of 1,000,000 or so for the best quartz crystals. The a m m o n i a spectrum line thus has a Q approximating that of the best quartz crystals, though much more constant and stable. The ammonia molecules in the absorption cell are moving rapidly in random thermal motion at an average speed of almost 2000 ft. per second at room temperature. When a gas molecule in an absorption cell is approaching or receding from the source of an electromagnetic wave because of its heat motion, its absorption frequency is different from that w h i c h it w o u l d have if it were standing still. This g i v e srise to a "Doppler broadening" of the absorption line, analogous to the c h a n g e in pitch of sound as its source approaches, passes, and leaves an observer. Thus, the line w i d t h can be reduced slightly by lowering the temperature
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of the gas (or by using a heavier molecule). Doppler broadening lowers the Q of the a m m o n i a line to a b o u t 330,000 at room temperatures. Molecular collisions also broaden the absorptionline. This broadening occurs because the collisions abruptly terminate the absorption process, causing the molecules to absorb wave trains whose lengths vary in a random way determined by the distribution of time intervals between collisions. A frequency analysis of t h e s e wave trains shows a corresponding random distribution of absorbed frequencies, all centering tabour a mean value determined by the n u m b e r of collisions per second. In ammonia gas at a pressure of 10 microns t h e r e are a b o u t 120,000 collisions per second, giving an experimentally measured Q of 45,000 for the absorption line used. (This is the line known to spectroscopists as the 3,3 line, for which the quantum numbers J and K are each e q u a l to 3.) Actually, t h e r e are more collisions effectively interrupting the absorption process in a m m o n i a than the kinetic theory of gases would indicate. Further broadening of the line results from collisions of the molecules with the walls, and even near misses between molecules cause interaction strong e n o u g h to i n t e r r u p t absorption. The n u m b e r of collisions per second, and thus the collision broadening, can be reduced by lowering the gas pressure. This process, if not carried too far, does not reduce absorption in the gas, because the decrease in n u m b e r of molecules absorbing e n e r g y is offset by the increase in absorption per molecule resulting from the increase in Q. However, when the pressure is reduced too much, a phenomenon known as saturation of the line sets in, caused by an excess of radiation. Too few molecules are then left in the proper e n e r g y states to absorb the microwave radiation coming into the cell. Many molecules, which normally would be in the proper e n e r g y s t a t e to absorb the incoming radiation, are in an excited s t a t e as a result of previous absorption. Eventually t h e s e molecules will emit the quanta which they have absorbed, returning to the normal level where absorption is a g a i n possible. However, as this process is slow, the molecule usually returns to the ground level in a collision with another molecule, converting the absorbed radiation into h e a t . As the gas pressure is lowered, the n u m b e r of collisions is greatly reduced, and not e n o u g h molecules return to ground levels. The excessive incoming radiation then weakens and broadens the absorption line through saturation. The broadening r e s u l t s because saturation occurs earlier at the peak of the line than out at its wings. Saturation can be eliminated by reducing the strength of the incoming radiation. However, as the gas pressure and radiation i n t e n s i t y are both lowered, a condition will finally be met for which the signal strength will be down in the natural electrical noise level of the circuits used to detect the signal. Circuit noise then sets the ultimate limitation on the reduction of collision and saturation broadening. It is estimated
March, I949.]
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that a Q of 300,000 to 400,000 can be attained at pressures of about one micron--still a long way from the Q of the natural line width. Assuming that effective Q values of 400,000 can be obtained with ammonia, an accuracy of one part in 100 million or better should be possible since a measurement of the center of the absorption line to within 1/250 of the width of the line could be made. APPLICATIONS AND SIGNIFICANCE.
Improvement of the accuracy of the atomic clock will make it useful in several fields of pure and applied science. The lengths of the mean solar day, used in astronomical measurements, fluctuate as much as one part in 20 to 30 million, because of variations in the rate of rotation of the e a r t h on its axis. The variation in present time standards, due to these fluctuations, causes errors in the location of heavenly bodies and in studies of t h e i r orbits and motions. The atomic clock offers the possibility of an invariant master clock against which the variation in the earth's time-keeping could be measured. An absorption cell on an atomic clock could, for some purposes, take the place of an astronomical observatory. Broadcasts of standard frequency are of importance in keeping all kinds of radio, radar, and electronic equipment properly tuned throughout the world. This service is required in international transportation and communications so that, for example, an airplane with radio navigational equipment will be using the right frequency wherever it is in the world and whatever airport it is using. At present, the National Bureau of Standards Station, WWV, broadcasts standard frequency and time signals on several transmitter frequencies to all the world. The Navy Department also uses quartz-crystal clocks to broadcast time signals for navigational purposes. These quartz-crystal clocks drift slightly in frequency and have to be adjusted to keep them in agreement with the basic astronomical time signals. Clocks of this type could be kept constant automatically by means of absorption lines. Maintenance of transmitter frequency to within close limits is also necessary to utilize the available r a d i o spectrum efficiently. The use o f long-distance standard frequency broadcasts is complicated by a large reduction in accuracy due to ionospheric effects. A long distance, short-wave signal travels around the e a r t h by reflection from the u p p e r ionized regions of the atmosphere, known as the ionosphere. E v e r y morning a t sunrise the ionosphere moves downward, and every evening at sunset it rises. This d aily variation in height causes a Doppler shift of the frequency of the reflected wave and, together with o t h e r as yet unknown causes, is responsible for a reduction by a factor of 25 or more in the accuracy of the frequency of the received signal. Thus, the Bureau's standard frequency broadcast agrees with astronomical time signals to one part in one hundred million at the transmitter but
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m a y be k n o w n t o only one p a r t in f o u r m i l l i o n a f t e r t r a n s m i s s i o n o v e r long d i s t a n c e s . T h i s difficulty c a n be p a r t l y o v e r c o m e in s e v e r a l w a y s . O n e is the p r o v i s i o n of a local, p r e c i s e f r e q u e n c y s t a n d a r d c a l i b r a t e d b y m e a n s of r e c e i v e d s t a n d a r d time s i g n a l s also t r a n s m i t t e d by r a d i o . H o w e v e r , this p r o c e s s , w h i c h r e q u i r e s a d a y or m o r e , c o m p l i c a t e s the e q u i p m e n t problem and introduces additional errors, m a k i n gimpractical the u s e of s t a n d a r d f r e q u e n c y b r o a d c a s t s for i n s t a n t a n e o u s or c o n t i n u o u s f r e q u e n c y c a l i b r a t i o n s of the h i g h e s t precision. A t the l a s t I n t e r n a t i o n a l R a d i o C o n f e r e n c e held in A t l a n t i c City in 1947, p l a n s w e r e f o r m u l a t e d t o p r o v i d e s t a n d a r d f r e q u e n c y and time b r o a d c a s t s from m a n y s t a t i o n s l o c a t e d t o r e n d e r good s e r v i c e t h r o u g h out the w o r l d . T h e s e s e r v i c e s m a y be i m p r o v e d o r simplified by m e a n s of a t o m i c c l o c k s a n d f r e q u e n c y s t a n d a r d s . S u c h c l o c k s c o u l d c o n t r o l the s t a n d a r d f r e q u e n c y e m i s s i o n s of the v a r i o u s s t a t i o n s w i t h o u t c h e c k ing and m o n i t o r i n g by a s t r o n o m i c a l time s i g n a l s . The D o p p l e r freq u e n c y s h i f t s c o u l d then be e l i m i n a t e d by l i m i t i n g t r a n s m i s s i o n dist a n c e s t o s h o r t r a n g e s . A l s o , e q u i p m e n t a n y w h e r e in the w o r l d c o u l d be c h e c k e d a g a i n s t a n a b s o r p t i o n line with the c e r t a i n t y of o b t a i n i n g a p r e c i s i o n c a l i b r a t i o n a g a i n s t a n a b s o l u t e s t a n d a r d a n d w i t h o u t depending on a standard frequency broadcast. O n e a d v a n t a g e of the r o t a t i n g e a r t h as the b a s i c t i m e - k e e p e r is t h a t it n e v e r s t o p s r o t a t i n g or b r e a k s d o w n . L i k e w i s e , a n y m a n - m a d e c l o c k m u s t not b r e a k down but m u s t be k e p t r u n n i n g f o r e v e r if it is t o keep t r a c k of time from some a r b i t r a r y i n s t a n t c h o s e n a s a s t a r t i n g point. W i t h the p r e s e n t q u a r t z - c r y s t a l c l o c k s , this difficulty is m e t by u s i n g a l a r g e n u m b e r of s i m i l a r c l o c k s c o n s t a n t l y i n t e r c o m p a r e d so t h a t b r e a k d o w n of one does n o t m e a n a loss of t i m e - k e e p i n g r e c o r d s . W h i l e this p r o c e d u r e c o u l d also be u s e d with a t o m i c c l o c k s , it w o u l d n o t be n e c e s s a r y for u s e of the c l o c k as a f r e q u e n c y s t a n d a r d or for d e f i n i n g a s t a n d a r d of t i m e - i n t e r v a l s s i n c e t h e s e a p p l i c a t i o n s do n o t r e q u i r e cont i n u o u s o p e r a t i o n of the a t o m i c c l o c k . T h e a t o m i c c l o c k s h o u l d p e r m i t i m p r o v e m e n t in a s t r o n o m i c a l t i m e s t a n d a r d s in a w a y i m p o s s i b l e with e l e c t r i c - p e n d u l u m o r q u a r t z - c r y s t a l c l o c k s . I t t h u s o p e n s the p o s s i b i l i t y of i m p r o v i n g the p r e c i s i o n of k n o w l e d g e of the l e n g t h of the y e a r , t h a t is, the time i t t a k e s the e a r t h t o r e v o l v e once in its o r b i t a r o u n d the sun. This is i n d e p e n d e n t of the t i m e it t a k e s the e a r t h t o r o t a t e once on its a x i s - - t h e m e a n s o l a r day. M e a s u r e m e n t s c o u l d then d e t e r m i n e w h e t h e r the m e a n s i d e r a l y e a r is m o r e c o n s t a n t t h a n the m e a n s o l a r d a y , a s some a s t r o n o m e r s b e l i e v e m a y be the c a s e . A l t h o u g h the u s e of a t o m i c time p r e s e n t s a d v a n t a g e s in m a n y f i e l d s of s c i e n c e , i t will a l w a y s be n e c e s s a r y for some p u r p o s e s t o h a v e a s t r o n o m i c a l time s t a n d a r d s . This is b e c a u s e the p o i n t i n g of a t e l e s c o p e d e p e n d s o n the o r i e n t a t i o n of the e a r t h a t the i n s t a n t of o b s e r v a t i o n , in o t h e r w o r d s , on a s t r o n o m i c a l time m e a s u r e m e n t s w h i c h d e r i v e from the m o t i o n of the e a r t h .