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The paradox of the primary standard of length

Citation for published version (APA):

Koning, J. (1975). The paradox of the primary standard of length. (TH Eindhoven. Afd. Werktuigbouwkunde, Laboratorium voor mechanische technologie en werkplaatstechniek : WT rapporten; Vol. WT0353). Technische Hogeschool Eindhoven.

Document status and date: Published: 01/01/1975 Document Version:

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RRK

01

1)'1

P R

WT 0353

Ein

dh

oven

University

of

Technolog

y

Department of

mechanical

en

gineering

THE PARADOX OF

THE

PRIMARY STANDARD OF LENGTH

J.

KONING

REPORT

WT

0353

EI~DHOVEN

UNIVERSITY PRESS (1975)

Divi

s

ion

of

producti

on technolo

gy

Eindhoven

(3)

THE PARADOX OF THE PRIMARY STANDARD OF LENGTH

J. Koning, Lab. for Metrology, Dept. of Mechanical Engineering, Div. of Production Engineering, Eindhoven, University of Technology, Eindhoven, The Netherlands

Summary: The trend in primary length standards; possible definition of this standard in the near future. For practical metrology no consequences are to be expected as the accuracy of measurement will - as before - be 1 imited by the accuracy of the temperature scale.

(4)

It is wei 1 known that the primary standard never is J5ed for

oractical iIi8()SUrCfnent. Since P\~rard in 1972 (1) for the first

tir::e in hist"lrY ilieasureJ a <Jar; block optical interferometry, all Drecise cal ibrations have probably been done by interfero-metry, using secondary wavelen h standa ds.

After the changeover to the Engelhard kry on lamp was made in 1 0 (2), it was in principle possible to use the primary stan-dard For such cal ibrations. However, the experimental effort needed to operate the krypton lamp makes it use impractical for everyday use.

One might speculate upon the nature of the physical process to be ted in the near future for a primary standard of length. Popular believe has it that a laser can be used for accurate

lenq h measurements, but it is well known ha the long-term in-t bi i ity of eVen the best commercially aI/a 1 ble la er is tvJO orjers of magnitude greater than that of he krypton standard.

~evertheless, recent development

(3)

have opened LWO possible

ways to define a metre standard:

1\'1) a I aser stabil ised on an 12 absorbtion line.' (4) is now ope-rative in a number of metrology laboratories and such a laser promises an inaccuracy which is at least an order of

(5)

n1d',jnilUde c'[:ldll r than thCl of the kJyploll ::,landara.

l2) it h nm'J possi:;]e to determine the frequency of an infrared laser by Jirect counting, thus

wavelength standards and to de an exact value for the velocl mary standard of the second.

it i sible to cal ibrate inc the Gctre by specifying of 1 ight and using the

pri-Evidently there is a principal di rence in these Lwo procedures. ,'\joption of the first method resulls :1 ifferent (if Inore

accu-rate) values for the velocity of J i every time the cXDerimental techniques involved in the measurementS::1 refined. I f the

secQnd method is adopted, the length of the metre \iJi II in that

case ~e changing.

The paradox referred to in the title of this per is that_,6-though it is possible to define the metre to at least) 10 ,

this is an empty accuracy as all length measurement in physical and techn i ca 1 sys tel,Xls will as befor>e 1; i ted to an inaccuracy of the order of 10-6 cau ed by an indeterminacy of rature

(4).

As before, practical length measurE.~ent has no usc for the primary standard.

There seems to be strong arguments in favour of a fixed value of the velocity of 1 ight for the purpose of precise astronomical length measurement which is in fact ti -fl ight method. Therefore, one mig t guess that the future definition of the metre wi 11 be by the thod 2, a fixed value for the velocity of

J ight in vacuum.

Cornptes Rendues 1 ( 191 1798

(2) In reality the 1960 definition of the metre i more

SODhIS-ticated than is suggested nc . See Metrologia, 4-147-(1968)

(3)

Metrologia,

10-75-(1974)

(4) Hanes G.R. and K.M.

ird

Metrol ia

5-

-(1969)

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