NIST logo

Electronic Kilogram


This project provides the basis for a new definition of mass, based on unchanging quantum properties of nature rather than a physical artifact.  This is accomplished through a high-accuracy comparison of power as measured in mechanical (force and velocity) and electrical (voltage and resistance) units.  Because of the quantum nature of all but the kilogram mass unit (laser length, atomic time, Josephson effect voltage, and Quantum Hall effect resistance), the two measurements provides a high-accuracy determination of the Planck Constant relative to the kilogram artifact.  The goal is to establish a consistent measured value that can be adopted as an exact value for the Planck constant, similar to what was done for the second and the speed of light.  Following such a redefinition, this experiment would realize the kilogram mass unit in terms of purely quantum units - the meter, second, volt and ohm.  This would complete the conversion to a measurement system where all units are derived from unchanging quantum properties of nature, instead of being tied to the properties of a single physical artifact.


NIST has led the world since 1998 with three consistent results for the determination of h. The most recent NIST value reported in 2005 has the lowest uncertainty so far, at 36 parts in 109. Two results from the National Physical Laboratory (NPL) in England and one from the Avogadro international consortium have varied by 300 and 1000 parts in 109, respectively. This shows that significant progress has been made since the first NPL result in 1988, but also shows how difficult and complicated this experiment is.
Four laboratories besides NIST are working with the same technique. In the experiment, a mass balance uses a precisely aligned induction coil in a high stability magnetic field, where a current applied to the coil provides an electromagnetic force in opposition to the gravitational attraction on a mass standard. As a necessary and complementary measurement, the coil is moved to generate a voltage while recording the coil velocity. The reference quantities of mass, time, length, voltage, and resistance are based on in-lab peripheral systems: mass artifacts, GPS, laser interferometer, Josephson array volt system, and quantum Hall calibrated standard resistor.

Precise measurement, calibration, and other instrumentation issues must be well characterized, which is why this type of experiment is best and only performed at national standards laboratories. The International Committee for Weights and Measures (CIPM) has recommended a redefinition when the experiment proves its feasibility with several labs reproducing a consistent result within the present NIST uncertainty. With the next opportunity occurring in 2011, the race is on to improve the experiments and trigger a change in the International System (SI) of units that will provide a quantum-based matrix of units for the foreseeable future, permanently stabilizing both scientific measurements and commercial standardization in production and trade of electrical, mass, and force measurement equipment.

Major Accomplishments:

  • Lowest uncertainty of 36 x 10-9 reported on value of Planck constant in 2005
  • New values consistent with system rebuilt after 1998 
  • Apr 2005 Metrologia paper sparked CIPM decision to consider redefinition of kilogram
  • Participating in annual discussions between the other metrology laboratories working on similar experiments
  • Collaborating with MEL on the Advanced Mass Competence project
Artifact International Prototype Kilogram (IPK).
Artifact International Prototype Kilogram (IPK). Photo: Courtesy BIPM

Start Date:

January 1, 1980

End Date:


Lead Organizational Unit:


Facilities/Tools Used:

  • Non-magnetic facility Bldg 238
  • Watt balance assembly and vacuum chamber
  • RF screen room
  • Josephson volt array
  • absolute gravity meter
  • relative gravity meter
  • computer systems and custom software for: data acquisition/analysis, coil servo/mass placement, X-Y position damping, superconducting solenoid current control


Richard Steiner
Darine el Haddad
Ruimin Liu
Edwin Williams

Related Programs and Projects:


Richard Steiner
(301) 975-4226 Telephone

100 Bureau Drive, MS 8170
Gaithersburg, MD  20899-8170