Dr. James Olthoff

Acting Director

NIST Physical Measurement Laboratory

February 6, 2013

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INTERNATIONAL METROLOGY

AND THE REDEFINITION

OF THE KILOGRAM
Bottom Line Up Front:

Replacing the Prototype Kilogram

2

Standard Kilogram prototypes

Shown: International Prototype 

of the Kilogram, kept at BIPM

Electronic Kilogram realization

Shown: Computer rendering of NIST-4 

watt balance, under construction
Survey of the Meridian,

Dunkirk to Barcelona

1792–1799

The Metric System

Version 1.0

• Adopted by international committee in 1799

• Came into common use in 19th century

– In U.S., Metric Act of 1866 permitted its use,

preempting state laws that forbade its use

• Standards kept in the Archives of France

• Basic principles:

– Decimalization

– Based on nature

(not monarchs)

– Open access

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Version 2.0:

Improving the Standards and Governance

4

1870: Paris conference to consider constructing new primary 

metric system standards

– Originals from 1799 were aging and worn

1872: Second conference decided that new meters and new 

kilograms should be constructed to conform with the 

standards in the French Archives, as they were

– Committee appointed to carry out this decision

1873: Committee agrees upon improved materials and

designs for standards

1875: French Government calls diplomatic conference to 

consider how to validate the new meters and kilograms

– Framework for permanent maintenance
5

General Conference on Weights and Measures

Conférence générale des poids et Measures

[CGPM]

Consists of delegates from Member States 

and meets every four years

Member

States of the 

Meter

Convention

International Bureau of Weights and Measures

Bureau international des poids et mesures

[BIPM]

Intergovernmental organization with 

headquarters located in Sèvres, France 

Direction, Laboratories, and permanent staff

National

Metrology

Laboratories

Intl. Committee for Weights and Measures

Comité international des poids et mesures

[CIPM]

Consists of 18 individuals elected by the 

CGPM, charged with the supervision of the 

BIPM and of its activities, meets annually

Technical Staff
State of the Metric System

Late 19th Century

• The “Metric System” was only weights (kilogram) and 

measures (meter)

– Also areas and volumes, derived from length

6

Standard Meter prototypes Standard Kilogram 

prototypes
Court of Honor and Grand Basin of the

1893 World's Columbian Exposition (Chicago, Illinois)

The Columbian Exhibition (1893)

Birthplace of International Electrical Standards

7

Here, in 1893, the International Electrical Congress established the first international 

standards for the measurement of electrical quantities (ampere, ohm, volt, ...) 
Treaty of the Meter

1921 Amendments

8

• Added coordinating the measures of electrical units

• Added establishing and keeping standards of 

electrical units, and their “test copies”

• Added duty to determine the physical constants

• Ratified by the U.S. Senate in 1923
The Beginning of the System of Units

1954: 10th CGPM, Resolution 6: practical system of units

In accordance with the wish expressed by the 9th Conférence

Générale des Poids et Mesures (CGPM) in its Resolution 6 

concerning the establishment of a practical system of units of 

measurement for international use, the 10th CGPM

decides to adopt as base units of the system, the following units:

length meter

mass kilogram

time second

electric current ampere

thermodynamic temperature degree Kelvin

(renamed “kelvin” in 1967)

luminous intensity candela

9

A comprehensive system of measurement units was now complete.
Version 3.0:

The Système International d’Unités (SI)

1960: 11th CGPM adopts the name “Système International 

d’Unités” for the compilation of its work, the compete, 

modernized Metric System

– Units of measurement and rules for usage

1971: 14th CGPM adds the “mole” to SI as the seventh

base unit, as the amount of substance

– The International Union of Pure and Applied 

Physics (IUPAP) and the International Union of 

Pure and Applied Chemistry (IUPAC) did not agree 

until 1960 to base the mole on 12C, rather than 16O.

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The Système International d’Unités (SI)

• Creation of the SI began the process to revise and 

improve the units in a way that benefited the system

as a whole

• Since 1960, rapid improvement of unit definitions and 

realizations

• Examples:

– Changes to definition of the “second”

– Changes to definition of the “meter”

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Definition of the Second

1967: The second is the duration of 9,192,631,770

periods of the radiation corresponding to the 

transition between the two hyperfine levels of 

the ground state of the 133Cs atom.

12

NIST-F1, today’s primary 

atomic frequency standard
Definition of the Meter

1889: International Prototype Meter

1960: Based on wavelength of 86Kr radiation

1983: The meter is the length of the path travelled by 

light in vacuum during a time interval of

1/299,792,458 of a second.

(17th CGPM, Resolution 1)

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Media Attention

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The Two Biggest Headaches in SI Today

1) The artifact kilogram

2) The electrical units

15

Compared to the International Prototype Kilogram, the measured 

masses of prototype kilograms around the world are diverging.
NIST Leadership

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Proposed Solution to Both Problems:

Metric System Version 4.0

• Redefine the:

– kilogram

– ampere

– kelvin

– mole

• By determined through best experiment, then fixing,

the values of:

– Planck constant

– Elementary electric charge (of electron)

– Boltzmann constant

– Avogadro constant

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Converging on Consensus

The CGPM, at its 24th meeting (2011), took note of the intention of 

the CIPM to propose a revision of the SI where:

• the kilogram will continue to be the unit of mass, but its magnitude will be set by 

fixing the numerical value of the Planck constant to be equal to exactly 6.626 

06X ×10-34 when it is expressed in the SI unit m2 kg s-1, which is equal to J s

• the ampere will continue to be the unit of electric current, but its magnitude will be 

set by fixing the numerical value of the elementary charge to be equal to exactly 

1.602 17X ×10-19 when it is expressed in the SI unit s A, which is equal to C

• the kelvin will continue to be the unit of thermodynamic temperature, but its 

magnitude will be set by fixing the numerical value of the Boltzmann constant

to be equal to exactly 1.380 6X ×10-23 when it is expressed in the SI unit

m2 kg s-2 K-1, which is equal to J K-1

• the mole will continue to be the unit of amount of substance of a specified 

elementary entity, which may be an atom, molecule, ion, electron, any other 

particle or a specified group of such particles, but its magnitude will be set by 

fixing the numerical value of the Avogadro constant to be equal to exactly

6.022 14X ×1023 when it is expressed in the SI unit mol-1

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Ref: http://www.bipm.org/en/CGPM/db/24/1/

What remains is to determine the missing digits “X” in each case
Two Approaches to the Kilogram

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The Avogadro Number Approach

Near-perfect sphere of near-perfect crystal of

nearly isotopically pure Silicon-28. Interatomic

spacing, determined by x-ray diffraction, gives

you the number of atoms in the sphere, which

is also the mass standard.

The Watt Balance Approach

Electrical force and power equated to 

mechanical force and power. Electrical

measurements made with quantum

standards, derived from e and h. Local

gravitational field must also be measured.
Worldwide Realizations

20

NPL Mark II, now

at NRC Canada

NIST W-4 (and

NIST W-3) USA

MSL

New Zealand

NIM (Joule)

China

KRISS (Planned)

Korea

MNIJ (Planned)

Japan

IAC

Germany

LNE

France METAS Mark I (& II)

Switzerland

BIPM
Status of the Planck Constant

21

NRC and NIST are working 

together to understand this 

discrepancy
NIST Contribution to the Avogadro Project

• NIST provided independent measurement of

the molar mass of the crystal

– Mass spectrometry / Isotope ratio measurement

– Results to be published

– NIST result tends to agree

with PTB measurement

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Improvements to Watt Balance W-3

23

Velocity measurement system 

revamped, for lower noise

Central knife edge

reinvented and

replaced

Current sources replaced 

with new, ultra-low noise 

systems invented at NIST

Our mass, K85, went back to 

BIPM for calibration and 

vacuum characterization

Voltage reference 

system rebuilt

Temperature control 

in room improved

Electric power system 

revamped to lower noise 

and improve grounding Gravity measurement system 

compared to NRC and NOAA. 

Agreement better than 10 ppb
Watt Balance W-3 Cross Checks

• FY 2012: All primary standards used by NIST watt balance 

upgraded, being cross-checked with NRC

– Voltage (compared to NRC in 2006)

– Resistance (compared to NRC in 2008)

– Time

– Length

• Test masses

– Check transport, handling,

procedural differences

• Contribution of gravity measurements to discrepancies is now

constrained to be < 1 part in 108

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Goal: Converge on h value prior to CCU meeting, June 2013

The Great Gravity Showdown

NIST, NOAA, NRC (Canada)

February 6–10, 2012
Blinded Experiment

• Watt balance group evaluating instrument using 

variety of tests to explore systematic deviations

• Watt balance group making new determination of 

Planck’s constant (h) using a mass with an unknown

(to them) offset

• Mass group knows offset

• When value of h has been determined, offset will be 

added (shifting h)

• New NIST-measured value of h by March 2013

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Timeline

• Nov 2012: Workshop on mise en pratique

• Draft mise en pratique circulating now

• Jan 2013: BIPM decides to not yet use IPK in comparison

– Precludes final adoption of new system at 25th CGPM

• Feb 2013: Meeting of Consultative Committee on Mass

• March 2013: New NIST value from W-3

• Spring/Summer 2013: NIST will work with NRC, PTB

to dry run the mise en pratique

• June 2013: Meeting of Consultative Committee on Units

• Fall 2013: Meeting of the CIPM

• Fall 2014: 25th CGPM

– Decision on next opportunity for adoption

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NIST’s Watt Balances

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You are here

W-4 is here

W-3 is here
Impact on Practical Dissemination of Mass

• Education and training

• Develop W-4, whose primary purpose is the realization of 

the kilogram

– “Ease” of use

• Transfer of standards between

air and vacuum

– New issue

– Need to study stability

of mass artifacts

28

Patrick Abbot inspects the 

upper (vacuum) section of the

vacuum levitation system.
Impact on Practical Dissemination of Mass

• Realizations at arbitrary scale points

– Should improve uncertainties

of measurement of

very small masses

29

Mechanical engineer Jon Pratt 

makes an adjustment to the 

prototype NIST Electrostatic 

Force Balance designed to 

measure nanoscale forces.

Credits

1) Title Slide: Photograph of David Wineland, © Geoffrey Wheeler, 2003; Dawn Cross, NIST, 2011; 

Charles Gibson, © Robert Rathe, 2002; Sae Woo Nam, © Geoffrey Wheeler, 2003

2) International Prototype of the Kilogram, http://www.bipm.org/en/scientific/mass/prototype.html,

© BIPM; NIST W-4 Watt Balance, NIST

3) Survey of the Meridian, from http://www.planetseed.com/node/16531

© Ministère de l'Économie, des Finances et de l'Industrie, France

5) Photograph of Dr. Patrick Gallagher from http://www.nist.gov/director/bios/gallagher.cfm,

and of Dr. Willie May from http://www.nist.gov/director/bios/willie_may.cfm.

Organizational chart, after http://www.bipm.org/utils/en/pdf/structure.pdf.

The Treaty of the Meter is published in The Statutes at Large of the United States of America, 

Vol. 20, pp. 709–724 (1879).

6) Meter bar, from the NIST Museum Collection, http://museum.nist.gov/object.asp?ObjID=37; 

International Prototype of the Kilogram, http://www.bipm.org/en/scientific/mass/prototype.html

7) The Columbian Exhibition, photographer unknown, from 

http://en.wikipedia.org/wiki/World's_Columbian_Exposition

8) The 1921 Amendments to the Treaty of the Meter are published in

The Statutes at Large of the United States of America, Vol. 43 (Part 2), pp. 1686–1694 (1925)

9) More on SI and its history at: http://physics.nist.gov/cuu/Units/bibliography.html,

in particular, NIST Special Publication 330 http://physics.nist.gov/Pubs/SP330/sp330.pdf

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Credits (Continued)

12) NIST-F1 frequency standard, photograph © 2005 Geoffrey Wheeler;

http://www.nist.gov/pml/div688/grp50/primary-frequency-standards.cfm.

See http://physics.nist.gov/cuu/Units/second.html and

NIST Publication SP 330 http://physics.nist.gov/Pubs/SP330/sp330.pdf, Sec. 2.1.1.3.

13) International Prototype Meter, photograph from http://www.nist.gov/pml/wmd/metric/length.cfm

See http://physics.nist.gov/cuu/Units/meter.html and NIST Publication SP 330 

http://physics.nist.gov/Pubs/SP330/sp330.pdf, Sec. 2.1.1.1.

14) International Prototype of the Kilogram, http://www.bipm.org/en/scientific/mass/prototype.html;

data plot from http://www.bipm.org/en/scientific/mass/verifications.html; both © BIPM

15) See http://dx.doi.org/10.1088/0026-1394/42/2/001.

See also http://dx.doi.org/10.1088/0026-1394/43/3/006.

16) Media clips from: http://www.cbsnews.com/8301-501465_162-20021243-501465.html, 

http://online.wsj.com/article/SB10001424052748704062604576106094010476886.html, and 

http://usatoday30.usatoday.com/tech/science/2011-04-18-kilo-weight.htm.

17) See http://www.nist.gov/pml/newsletter/siredef.cfm.

See http://www.bipm.org/utils/common/pdf/si_brochure_draft_ch2.pdf.

18) See http://www.bipm.org/en/CGPM/db/24/1/.

19) NIST watt balance, from http://dx.doi.org/10.1109/TIM.2008.2007060, © 2008 IEEE;

Silicon sphere, from http://www.scienceimage.csiro.au/mediarelease/mr07-108.html.

See also: http://www.csiro.au/en/Outcomes/ICT-and-Services/Data-deluge/ Precision-spheresin-push-to-re-define-kilogram.aspx, both © CSIRO Industrial Physics.

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Credits (Continued)

20) Clockwise from upper left:

NPL/NRC, from http://www.npl.co.uk/engineering-measurements/

mass-force-pressure/mass/research/npl-watt-balance, Crown Copyright;

IAC, from http://www.euramet.org/fileadmin/docs/EMRP/JRP/JRP_Summaries_2011/

SI_JRPs/SIB03_Publishable_JRP_Summary.pdf;

NIM, from http://dx.doi.org/10.1109/CPEM.2010.5544772 , © 2010 IEEE;

MSL, adapted from http://www.bipm.org/ws/CCM/MeP_2012/Allowed/November_2012/ 

09g_3.2.7-MSLwattbalanceresearch_status_OpenAccessVersion.pdf;

METAS, adapted from http://dx.doi.org/10.1109/19.769561, © 1999 IEEE;

BIPM, from http://www.bipm.org/en/scientific/elec/watt_balance/wb_bipm.html, © BIPM;

LNE, from http://dx.doi.org/10.1016/j.precisioneng.2011.06.003, © 2011 Elsevier Inc.; 

NIST W-4 rendering, NIST.

21) See, e.g., http://dx.doi.org/10.1088/0034-4885/76/1/016101.

22) Silicon sphere, from http://www.euramet.org/fileadmin/docs/EMRP/JRP/ 

JRP_Summaries_2011/SI_JRPs/SIB03_Publishable_JRP_Summary.pdf

23) NIST W-3, from http://dx.doi.org/10.1109/TIM.2008.2007060, © 2008 IEEE

24) Photograph of gravity comparison, from http://www.nist.gov/pml/div684/gravity_comparison.cfm

27) Photograph of NIST W-3, rendering of NIST W-4, NIST; Aerial imagery of NIST Gaithersburg, 

MD site, © 2013 Google and Google Map imagery providers: Commonwealth of Virginia, 

DigitalGlobe, GeoEye, U.S. Geological Survey, and/or USDA Farm Service Agency

28) Photograph of Patrick Abbot, from http://www.nist.gov/pml/div684/maglevkg.cfm

29) Photograph of Jon Pratt, from http://www.nist.gov/pml/div684/grp07/sfmet.cfm

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