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Physics in Canada, 74, 4, pp. 193-196, 2016
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Quantifying our world
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e h in d v irtu a lly e v e ry th in g th a t th e p u b lic u se s a n d c o n s u m e s is th e s c ie n c e o f m e tr o lo g y , w h ic h is c o n s ta n t ly e v o lv in g w ith s o c ie t y ’s n e e d s . i n th is s c ie n c e , w e m e a s u r e p h y s ic a l q u a n titie s re la te d to e v e ry d a y e x p e rie n c e . I f y o u h a v e e v e r b e e n la te fo r a m e e tin g , h a d to ru n to w o rk , g o to th e g y m o r tu r n o n th e a ir c o n d itio n in g , y o u h a v e e x p e rie n c e d th e re s u lts o f th e w o rk o f m e tro lo g is ts : tim e , d ista n c e , w e ig h t a n d te m p e ra tu re , a re all stu d ied .T a k e tim e , fo r in sta n c e . I n s te a d o f b e in g re la te d to th e siz e o f th e e a rth as it o n c e w a s, th e m e tre is n o w m o re a c c u ra te ly d e fin e d a s h o w far lig h t tra v e ls in a c e rta in a m o u n t o f tim e . S in c e th e sp e e d o f lig h t is a fu n d a m e n ta l c o n s ta n t, it w ill n o t c h a n g e b a s e d o n e n v iro n m e n ta l fac to rs. A s e c o n d o f tim e — c e n tra l to m o s t a s p e c ts o f life — is d e fin e d in te rm s o f a n a to m ic tra n s itio n in c e siu m . H o w e v e r, as o u r m e a s u re m e n t c a p a b ilitie s im p ro v e a n d a re tra n s fo rm e d , th e m e th o d s o f re a liz in g th e u n its also c h a n g e . I n so m e c a se s, th e c h a n g e s a re r e v o lu tio n a ry as th e y w e re for th e Jo se p h so n v o lt a n d th e q u a n tu m H all stan d a rd fo r re sis ta n c e . N o rm a lly , su c h c h a n g e s in h o w th e u n its a re d e fin e d a re n o t s o m e th in g th a t th e g e n e ra l p u b lic w o u ld n o tic e , b u t, u ltim a te ly , it d o e s a ffe c t e v e ry o n e .
S a te llite s s e n sin g th e e n v iro n m e n t, m u s t a c c u ra te ly m e a s u re h o w m u c h s u n lig h t w e c a tc h to g a u g e its in flu e n c e o n g lo b a l w a rm in g . S to c k e x c h a n g e s m u s t tim e -s ta m p th e ir tra d e s w ith in c re a s in g a c c u ra c y . T h e T o ro n to S to c k E x c h a n g e r e c e n tly a s k e d th e N a tio n a l R e s e a rc h C o u n c il (N R C ) fo r d ire c t a c c e s s to its d e ta ile d tim e s ig n a l— b a s e d o n a to m ic c lo c k s — so th e y c a n m o re p re c is e ly tim e -s ta m p th e ir in c re a s in g ly r a p id tra n s a c tio n ra te s.
P e rh a p s th e m o s t v is ib le e x a m p le o f th e w o rk m e tro lo g ists p e rfo rm is in th e fo rm o f a 1965 re q u e s t fro m th e n P rim e M in is te r L e s te r P e a rs o n , w h o e n tru s te d N R C w ith th e re s p o n s ib ility o f e s ta b lis h in g c o lo u r sp e c ific a tio n s fo r
S
u mma r y T h e s c ie n c e o f m e tr o lo g y is c o n s ta n tly e v o l v in g to m e e t th e p h y s ic a l n e e d s o f s o c ie ty . O v e r th e p a s t c e n tu r y N R C h as le d th e w a y in m a n y a re a s o f p rim a ry m e tro lo g y , w h ic h h a v e b e n e fitte d C a n a d ia n s o c ie ty c o n s id e r a b ly - a n d w ill c o n tin u e to d o s o . C a n a d a ’s n a tio n a l flag . T o d a y , C a n a d a ’s r e d is a so u rc e o f in te rn a tio n a l p rid e th a t c a n w ith s ta n d th e te s ts o f th e e lem e n ts.M e tr o lo g y is a ls o c r itic a l to e c o n o m ic a n d in d u s tria l d ev e lo p m e n t. Im p ac t stu d ies in d ic ate th a t it lo w ers tra n sa c tio n co s ts, c o n trib u te s to e n e rg y c o n s e rv a tio n , in c re a se s r e s e a rc h a n d d e v e lo p m e n t (R & D ) e ffic ie n c y a n d p ro d u c t q u a lity , a n d e n a b le s n e w m a rk e ts. E v e n th o u g h m o s t n e v e r d ire c tly se e th e im p a c t o f th is r e s e a rc h d ire c tly , th e fin a n c ia l r e tu rn o n th e r e s e a rc h in v e s tm e n t is a n y w h e re fro m 5 to 100 tim e s. In o th e r w o rd s , it lite ra lly p a y s to b e p rec ise!
N R C b e g a n e x p a n d in g its m e tro lo g y a c tiv itie s in th e 1 9 3 0 s [1], a n d is n o w h o m e to o n e o f th e w o r ld ’s m o s t re s p e c te d s ta n d a rd s la b o ra to rie s. A lm o s t fro m its in c e p tio n , N R C h a s p la y e d th e ro le o f C a n a d a ’s h ig h e s t-le v e l m e tro lo g y in s titu tio n , a n d o v e r th e y e a rs h a s b e c o m e o n e o f th e g lo b a l le a d e rs in m e a s u re m e n t s c ie n c e . M o st c o u n trie s h a v e th e ir o w n N a tio n a l M e tro lo g y In stitu te s (N M Is), w ith th e in te rn a tio n a l c o o rd in a tin g b o d y fo r su c h h ig h -le v e l m e tro lo g y b e in g a t th e B u re a u In te rn a tio n a l d e s P o id s e t M e s u re s (B IP M ) in F ra n ce .
A t N R C , w e e n s u re th a t C a n a d a ’s m e a s u re m e n ts c a n b e tra c e d b a c k to th e In te rn a tio n a l S y s te m o f u n i t s (SI), th r o u g h o u r o w n in d e p e n d e n t r e a liz a tio n o f th e u n its . T h is p r o to c o l is c o m m o n to m o s t c o u n trie s a n d in te r c o m p a ris o n e s ta b lis h e s th e le v e l o f e q u iv a le n c e a m o n g n a tio n a l m e a s u re m e n t sy stem s.
R E C A L IB R A T IN G M E A SU R E M E N T S
T h e S I c o m p ris e s se v e n b a s e u n its (se e F ig . 1a a n d T a b le 1): m e tre (m ), k ilo g ra m (k g ), se c o n d (s), a m p e re (A ), k e lv in (K ), c a n d e la (cd ) a n d m o le (m o l), all o f w h ic h n e e d to b e s ta b le a n d r e a liz a b le e v e ry w h e re . A s lo n g ag o as 1900, M a x P la n c k n o te d th a t, i f b a s e d o n fu n d a m e n ta l c o n s ta n ts o f n a tu re , th e u n its w o u ld “n e c e s s a rily re ta in th e ir v a lid ity fo r all tim e s a n d c u ltu re s, e v e n e x tra te rre s tria l a n d n o n h u m a n .” A lth o u g h h e c o u ld n o t h a v e fo re s e e n th e c h a n g e s th a t n o w d riv e th e r e d e fin itio n o f so m e u n its , h e w a s q u ite c o rre c t— p re s c ie n t e v e n — in h is a s se rtio n .
It is n o w lik e ly th a t in 2 0 1 8 , c o u n trie s a ro u n d th e w o rld w ill a g re e to c h a n g e s in th e d e fin itio n s o f th e S I b a s e u n its . It is p ro p o s e d th a t th e k ilo g ra m , a m p e re , k e lv in a n d D a v id J. L o c k w o o d < D a v id .L o c k w o o d @ n r c - c n r c .g c .c a > a n d N e ls o n L. R o w e ll < N e ls o n .R o w e ll@ n r c - c n r c .g c .c a > M e a s u r e m e n t S c ie n c e a n d S ta n d a rd s , N a tio n a l R e s e a r c h C o u n c il C a n a d a , 1 2 0 0 M o n tre a l R d, O tta w a , O N K 1 A 0 R 6
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a n a d a / V o l. 7 2 , N o . 4 ( 2 0 1 6 ) ·193
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Fig. 1 (a) The diagram depicts the current SI in 2016. The arrows show the dependence o f base unit definitions on other base units (for example, the m etre is defined in term s o f the distance travelled by light in a specific fraction o f a second). For definition o f units see Table 1. (b) Proposed SI for possible adoption in 2018. The de pendence o f base unit definitions is on physical cons tants with fixed num erical values and on other base units that are derived from the same set o f constants.
m o le b e re d e fin e d b y c h o o s in g e x a c t n u m e ric a l v a lu e s for, re s p e c tiv e ly , th e P la n c k c o n s ta n t, th e e le m e n ta ry c h a rg e , th e B o ltz m a n n c o n s ta n t a n d th e A v o g a d ro c o n s ta n t (s e e F ig . 1 b a n d T a b le 1). In th is e n d e a v o u r, N R C s c ie n tists p ro v id e d s ig n ific a n t c o n trib u tio n s a n d /o r le a d e rs h ip to th e d e v e lo p m e n t o f th e n e w k ilo g ra m , k e lv in a n d m o le .
N E W K IL O G R A M
F o r 126 y e a rs , L e G r a n d K , a p la tin u m a n d irid iu m c y lin d e r h o u s e d a t B IP M in P a ris (se e F ig . 2 ), h a s d e fin e d th e k ilo g ra m , b e in g th e o n ly S I u n it re m a in in g b a s e d o n a p h y s ic a l o b je ct.
S in c e th e c y lin d e rs w e re m a n u fa c tu re d in 1 889, m a s s d if fe r e n c e s as la rg e as 50 m ic ro g ra m s h a v e b e e n d e te c te d b e tw e e n th e L e G r a n d K a n d its six o ffic ia l co p ies. S u c h v a ria tio n s are c e rta in ly w ith in p re s e n t m e a s u re m e n t re s o lu tio n , a n d c a s t in to d o u b t th e s ta b ility o f th e m a s s o f L e G r a n d K, a n d h e n c e th a t o f
Fig. 2 N ested within several bell jars, rarely handled and stored in a controlled environm ent secure in a vault near Paris, Le G ra nd K is the artifact th at presently defines the official SI unit o f mass.
th e in te rn a tio n a l m a s s u n it. A s a re s u lt, it w as p ro p o s e d th a t th e sta n d a rd k ilo g ra m b e d e fin e d n o t in te rm s o f a p h y s ic a l a rtifa c t, b u t th ro u g h a p h y s ic a l e x p e rim e n t, r e la tin g th e k ilo g ra m to th e v a lu e o f th e P la n c k c o n s ta n t. In th is n e w d e fin itio n , th e k ilo g ra m w ill also b e d e p e n d e n t o n th e s e c o n d a n d th e m e tre .
C o u n tin g a to m s, as in th e A v o g a d ro P r o je c t[2], a n d w e ig h in g u sin g w a tt b a la n c e s [3] are th e tw o d istin ct p h y sic al m e th o d s p ro p o se d to estab lish a m a ss u n it th a t is n o t b a s e d o n a n a rtifa ct. It is re q u ire d th a t b o th m e th o d s b e so p rec ise th a t th e ir m e asu rem en t u n ce rtain ties are less th a n 20 p arts p e r b illio n (ppb).
In th e in te rn a tio n a l A v o g a d ro P ro je c t, N R C h a s m a d e a s ig n ific a n t c o n trib u tio n b y e s ta b lis h in g th e re la tiv e is o to p e ra tio s
T A B L E 1
P
h y s i c a l c o n s t a n t s a n d b a s e u n i t s e m p l o y e d i n t h e c u r r e n t a n d p r o p o s e dSI.
P hysical constants B ase unitsC onstant Sym bol SI quantity U n it S ym bol
V elo city o f lig ht c L ength m etre m
P lan ck constant h M ass k ilo g ram k g
H yperfine tran sitio n freq u en cy in cesiu m Δ ν (Cs) T im e second s
E lem en tary electric charge e C urrent am pere A
B o ltzm an n constant k T em peratu re k elv in K
L um in ous efficacy Kcd L ig ht can d ela cd
A vogadro constant Na M o lecu lar w eig h t m o le m ol
(a)
How to get an accurate value for Planck’s constant- a Watt balance
B C o il P o s itio n S e rv o e d to z = 0
H e N e L a s e r
Fig. 3 NRC has recently com pleted a critical experi m ent - m easuring the Planck constant to high accuracy using a w att balance, such that the m easurem ent uncertainty is now below 14 ppb. The results are crucial to the redefinition of the S i kilogram. (a) in the upper panel, above, is a schematic of the w att balance in which the gravitational force due to the presence of a mass, m, is shown balanced by the electrom ag netic force caused by the current through a coil. (b) in the lower panel, above, is a photograph of the NRC w att balance, with its vacuum cover removed.
in th e s ilic o n sp h e re s n o m in a lly c o m p o s e d o f a sin g le iso to p e (m a ss 2 8 ), u s e d in th e a to m -c o u n tin g m e a s u re m e n ts o f th is p ro je c t. A c c u ra te k n o w le d g e o f th e tra c e a m o u n ts o f th e o th e r is o to p e s p ro v id e d b y N R C a llo w e d th e A v o g a d ro P ro je c t te a m to im p ro v e th e ir v a lu e fo r A v o g a d r o ’s c o n s ta n t, w h ic h th e y c o n v e rte d in to a v a lu e fo r th e P la n c k c o n s ta n t th a t w a s m o re a c c u ra te th a n th e ir p re v io u s o n e . A w a tt b a la n c e is u s e d to p ro d u c e a v a lu e fo r P la n c k ’s c o n s ta n t b y w e ig h in g a te s t m a s s c a lib ra te d a g a in s t a n e le c tro m a g n e tic fo rc e (see F ig . 3a). N R C ’s m e tro lo g y la b o ra to ry in O tta w a h a s th e w o rld -le a d in g w a tt b a la n c e e x p e rim e n t (S ee F ig . 3b), p re s e n tly p ro v id in g v a lu e s fo r th e P la n c k c o n s ta n t w ith th e lo w e s t u n c e rta in ty ( < 15 p p b ) o f a n y su c h e x p e rim e n ts. T h e N R C te a m h a s c o n tin u a lly r e d u c e d th e u n c e rta in tie s in th e ir re s u lts b y a n a ly z in g a n d lo w e rin g s y ste m a tic u n c e rta in tie s .
C u rre n tly , a n d sig n ific a n tly , N R C ’s re s u lts h a v e th e lo w e s t u n c e rta in tie s .
In a 2 0 1 4 -1 5 e v a lu a tio n o f N R C ’s p ro g ra m s , th e r e p o rt sta te d th a t o th e r N M Is re c o g n iz e d N R C a s “h a v in g m a d e a n o te w o r th y c o n trib u tio n to k e y m e tro lo g y re s e a rc h d e v e lo p m e n ts , s p e c ific a lly th e re d e fin itio n o f th e k ilo g ra m . u s i n g e x p e rim e n ts b a s e d o n th e w a tt b a la n c e , N R C h a s b e e n a b le to a c h ie v e th e m o s t p re c is e d e te rm in a tio n o f th e P la n c k c o n s ta n t to d a te .” A s s ta te d b y o n e in te rn a tio n a l N M I in te rv ie w e e , “u n e q u iv o c a lly , th is h a s b e e n a m a jo r c o n trib u tio n to fu n d a m e n ta l m e tro lo g y .” In r e c o g n itio n o f its e x p e rtis e in m e tro lo g y , N R C h a s re c e n tly b e e n in v ite d to jo in th e C o n s u lta tiv e C o m m itte e fo r u n its .
W h ile th e id e a o f re d e fin in g th e k ilo g ra m in te rm s o f e le c tric a l q u a n titie s h a s b e e n a ro u n d fo r s o m e tim e , th e a c c u ra c y a n d stab ility o f e lec trica l m e asu rem en ts h as im p ro v e d trem en d o u sly o v e r th e y e a rs , ta k in g a d v a n ta g e o f q u a n tu m p h e n o m e n a in th e J o s e p h s o n a n d v o n K litz in g effec ts. B e c a u s e e le c tric a l q u a n ti tie s a re n o w b a s e d o n fu n d a m e n ta l c o n s ta n ts , w e k n o w th e y are v e ry a c c u ra te a n d sta b le . S u c h a d v a n c e s in m e a s u re m e n t s c ie n c e a llo w u s to b u ild a m a s s u n it b a s e d o n e le c tric a l q u a n titie s , so th a t w e c a n re tire th e k ilo g ra m a r ti f a c t[4].
N E W K E L V IN
M o s t p ra c tic a l te m p e ra tu re m e a s u re m e n t a ro u n d th e w o rld is b a s e d o n a d e fin e d sc ale , th e In te rn a tio n a l T e m p e ra tu re S cale o f 1990 (IT S -9 0 ), w h ic h re lie s o n th e b e s t te m p e ra tu re v a lu e s a t fix e d p o in ts (e.g ., m e ltin g o r fre ez in g ). T h is a p p ro a c h to e v a lu a tin g te m p e ra tu re n o w h a s a n u m b e r o f issu es. W e k n o w th a t, w ith b e tte r m e a s u re m e n t m e th o d s n o w a v a ila b le , th e te m p e ra tu re v a lu e s d e riv e d u s in g IT S -9 0 a n d th e a c tu a l (th e rm o d y n a m ic ) te m p e ra tu re s are n o t e x a c tly th e sam e. W h ile th e s e d iffe re n c e s w o u ld n o t b e n o tic e d b y — n o r lik e ly a ll th a t im p o rta n t to — th e a v e ra g e p e rs o n , so m e m o re s p e c ia liz e d a p p lic a tio n s d o n e e d to u s e e x c e p tio n a lly a c c u ra te te m p e r a tu re s, m o re so th a n th o s e a ffo rd e d b y IT S -9 0 , e s p e c ia lly a w a y fro m th e fix e d p o in ts.
A s a n e x a m p le o f c a lib ra tio n a c c u ra c y a n d its s ig n ific a n c e to so c ie ty , w e k n o w th a t c lim a te -m o n ito rin g sa te llite s a re in te n d e d to p ro v id e u s w ith a c tu a l (th e rm o d y n a m ic ) te m p e ra tu re q u ite a c c u ra te ly in th e e n v iro n m e n ta l ra n g e b e tw e e n a p p ro x i m a te ly —7 0 a n d 50 d e g re e s C e lsiu s . A s th e lo w e r p a rt o f th is ra n g e is o n e o f th e p la c e s w h e re th e re a re s ig n ific a n t d e v ia tio n s b e tw e e n IT S -9 0 a n d th e rm o d y n a m ic te m p e ra tu re , m e a s u r e m e n ts r e fe re n c e d to c a lib ra tio n d e v ic e s in te rn a l to th e sa te llite s u s in g IT S -9 0 h a v e to b e c o rre c te d b e fo re th e re s u lts a re u se d , fo r e x a m p le , in p h y s ic a l m o d e ls.
T h e d e fin itio n o f th e k e lv in is u n d e rg o in g a fu n d a m e n ta l ch a n g e . R a th e r th a n u s in g th e trip le p o in t o f w a te r to fix th e te m p e ra tu re sc ale , it w ill b e b a s e d o n a fix e d v a lu e fo r th e B o ltz m a n n c o n s tan t; h o w e v e r, th e te m p e ra tu re a t th e trip le p o in t o f w a te r w ill re m a in th e s a m e [5]. N R C , th ro u g h its
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)
m a s te ry o f p rim a ry th e rm o m e try , is n o w o n e o f th e w o rld le a d e rs in th is area.
A n o th e r illu s tra tio n o f th e im p o rta n c e o f c o n tro llin g c a lib ra tio n u n c e rta in ty w a s n o te d in ra d io m e try a n d ra d ia tio n th e r m o m e try s e v e ra l y e a rs a g o w ith th e c o n tro v e rs y a b o u t th e a m o u n t o f su n lig h t (to ta l so la r irra d ia n c e ) in c id e n t o u tsid e th e a tm o s p h e r e [6]. O v e r th e y e a rs , b e g in n in g in th e la te 1970s, s p a c e -b a s e d m e a s u re m e n ts o f so la r irra d ia n c e w e re m a d e o n a c o n tin u o u s b a s is w ith a se rie s o f ra d io m e tric in s tru m e n ts . T h e re s u lts c o n ta in e d s ig n ific a n t d iffe re n c e s b e tw e e n th e ra d io m e te rs , g e n e ra tio n to g e n e ra tio n , d u e to a sm a ll b u t o v e rlo o k e d so u rce o f c a lib ra tio n u n c e rta in ty . T h is r e s u lte d in in te r in s tru m e n t v a ria n c e s o f a fe w w a tts p e r sq u a re m e tre , w h ic h , a lth o u g h o n ly 0 .3 % o f th e to ta l, w e re s ig n ific a n t re la tiv e to th e o th e r ra d ia tio n c o m p o n e n ts th a t c o m p ris e th e c u rre n t g lo b a l ra d ia tiv e e n e rg y im b a la n c e . T h is p a rtic u la r c a lib ra tio n u n c e r ta in ty w a s c o rre c te d w ith m o re a c c u ra te m e tro lo g y , w h ic h w ill a lso b e re q u ire d to e v a lu a te th e re lia b ility o f th e m e a s u re m e n ts in th e lo n g te rm .
T IM E (T H E S E C O N D )
T im e m e a s u re m e n t, th e m o s t a c c u ra te m e tro lo g y , h a s alw a y s b e e n b a s e d o n th e id e a o f a n o s c illa to r, e.g ., a s in th e p e n d u lu m . E v e n a to m ic c lo c k s m e a s u re th e n a tu ra l m ic ro w a v e o s c illa tio n o f a n a to m ic tra n s itio n in ce siu m . F ro m th e 1950s o n w a rd , N R C m e tro lo g is ts w e re e a rly in n o v a to rs (see F ig . 4 ) o f th is ty p e o f te c h n o lo g y [7,8,9], b u ild in g s o m e o f th e firs t - a n d fin e s t c e s iu m c lo c k s w ith h ig h q u a lity fre q u e n c y a n d tim e -m e a s u re -m e n t c a p a b ilitie s [10].
T o d a y , a lth o u g h th e p re c is io n o f c e s iu m c lo c k s h a s b e e n s o m e w h a t s u rp a s s e d b y o p tic a l fre q u e n c y s ta n d a rd s, th e u n it o f tim e is s till b a s e d o n c e s iu m sta n d ard s. N o n e th e le s s , w e sh o u ld still b e a w a re th a t th e p re s e n t c e s iu m a to m ic c lo c k s h a v e e x tre m e ly lo w u n c e rta in tie s (less th a n o n e p a r t in 1016). T h is a m o u n t is le ss th a n a n a n o s e c o n d p e r c e n tu ry , so th a t i f su c h
Fig. 4 In 1965, NRC employees Allan Mungall, Herman Daams and Ralph Bailey check out a chart record ing o f the Ram say fringes originating from N R C ’s prim ary, atomic beam, cesium clock of the day.
a c lo c k w e re ru n n in g c o n tin u o u s ly , it w o u ld ta k e a b o u t 100 m illio n y e a rs to b e o u t b y a se co n d . F u rth e r im p ro v e m e n ts in p re c is io n c o u ld , h o w e v e r, s till b e s ig n ific a n t in v a rio u s areas.
C O N C L U SIO N
A lth o u g h its m e tro lo g y p ro g ra m s are re la tiv e ly sm a ll b y in te r n a tio n a l s ta n d a rd s, N R C c o n tin u e s to b e o n e o f th e w o r ld ’s to p N M Is , a n d h a s c o n trib u te d s ig n ific a n tly to th e d e v e lo p m e n ts le a d in g to w a rd th e n e w SI. O v e r th e p a s t c e n tu ry N R C h a s le d th e w a y in m a n y a re a s o f p rim a ry m e tro lo g y , w h ic h h a v e b e n e f i t e d C a n a d ia n s o c ie ty c o n s id e ra b ly — a n d w ill c o n tin u e to d o so. P e rh a p s, b y th a t s ta n d a rd , w e w o u ld n e e d to re v is it h o w w e m e a s u re th e siz e o f m e tro lo g y p ro g ra m s . I f su c h a re d e fin itio n w e re to ta k e p la c e , N R C w o u ld n o d o u b t b e a t th e fo re fro n t, re a d y to ta c k le th e c h a lle n g e .
R E F E R E N C E S
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6. G. K opp an d J.L . L ean, “A new , lo w er v alu e o f to ta l solar irradiance: E v id en ce an d clim ate sig n ifican ce” , G eo p h y s. R es . L e t t , 38, L 01706 (2011).
7. S.N. K alra, R. B ailey, an d H. D aam s, “ C esiu m B eam S tandard o f F req u en cy ” , C a n a d ia n J. P h y s ., 34, 1442-1443 (1958). 8. S.N. K alra, R. B ailey, an d H. D aam s, “ C an ad ian C æ siu m -B eam S tan d ard o f F req u en cy ” , N a tu r e , 183, 575-576 (1959). 9. A. G. M ungall, R. B ailey an d H. D aam s, “T h e C an ad ian C esiu m B eam F requency S tandard” , M e tr o lo g ia , 16, 98-104 (1966). 10. P.A . R edhead, “ T he N atio n al R esearch C o u n c il’s Im pact on C an ad ian P hy sics”, P h y s ic s in C a n a d a , 56, 109-121 (2000).
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