Electronic Kilogram

The artifact mass standard for the kilogram unit is suffering from 120 years of wear and contamination, so its value over time is becoming uncertain at several parts in 108. As a potential replacement, the Electronic Kilogram Project compares the energy in power generated by mechanical and electrical means. Einstein’s famous equation E=mc2 can relate this energy measurement to mass. Except for the kilogram, the standard units involved are all quantum-based: the second (atomic clocks), meter (laser wavelength), volt (Josephson effect), and ohm (quantum Hall effect). These are all unchanging in time. The result is a measured value for the Planck constant, h, relative to the mass artifact standard, or vice versa, if the international community redefines mass in terms of a value for h.

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.

• 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