NIST Improves Accuracy Of 'Watt Balance' Method For Defining The Kilogram

The latest NIST work, described in the October 2005 issue ofMetrologia and published online today,* confirms the institute's 1998results using the same method while reducing the measurementuncertainty by about 40 percent, thanks mainly to improvements in thehardware used in the experiments.

"The fact that we got the same values gives us confidence that theuncertainties we're quoting are probably reasonable," says NISTphysicist Richard Steiner, lead author of the paper.

Scientists at NIST and other institutions around the world have spentyears conducting experiments to find a reliable definition based innature to replace the current international standard for the kilogram,a century-old cylinder of platinum-iridium alloy about the size of aplum. The new results mean that the NIST method, using an apparatuscalled the watt balance or electronic kilogram, is almost accurateenough now to meet the criteria for redefinition.

Any decision about when and how to redefine the kilogram would be madeby an international group, the International Committee for Weights andMeasures, CIPM, and ratified by a General Conference on Weights andMeasures (CGPM), which next meets in 2007. The CGPM likely will delay aredefinition until other groups confirm the new NIST results.

The primary kilogram standard is currently maintained at the BureauInternational des Poids et Mesures (BIPM) near Paris. Although thecylinder is housed in a special vault under controlled conditions, itsmass can drift slightly over time and can change because ofcontamination, material loss from surface cleaning or other effects.Moreover, the standard is accessible only at BIPM and could be damagedor destroyed. By contrast, a property of nature is by definition alwaysthe same and can, in theory, be measured anywhere.

The other six basic units of the international measurement system arethe meter (unit of length), second (time), ampere (electric current),Kelvin (temperature), mole (amount of substance) and candela (luminousintensity). All six are defined in terms of properties of nature andcan be measured at any suitably equipped laboratory.

The NIST watt balance is a two-story-high apparatus designed toredefine mass in terms of fundamental physics and quantum standards. Itmeasures the force required to balance a 1-kilogram mass artifactagainst the pull of Earth's gravity, as well as two electrical values(see graphic). These measurements are used to determine therelationship between mechanical and electrical power, which can becombined with several equations to define the kilogram in terms ofbasic properties of nature.

One of these properties is the Planck constant, the ratio of the energyof radiation to its frequency. This is one of an extensive set of"fundamental constants" used by scientists to predict a wide range ofphenomena. The latest NIST value for the Planck constant reported inthe new paper (6.62606901 x 10 ^-34Joule seconds) is equivalent to the 1998 NIST result and a 1988measurement by the National Physical Laboratory in the United Kingdom,which are the two other most accurate values.

The watt balance is one of two leading approaches for redefining thekilogram. The other approach involves counting how many atoms of aspecific atomic mass equal the mass of 1 kilogram. The latest NISTmeasurements, which have an uncertainty of 0.052 parts per millioncompared to 0.087 parts per million in the 1998 experiments, are farmore precise than any previous results by any research group usingeither approach, according to Steiner. The total uncertainty iscalculated by adding up more than 20 sources of error.

The precision of the latest NIST measurements is roughly equivalent tothe suspected drift in the current kilogram standard's mass over time,Steiner says. The NIST researchers hope to further reduce theuncertainty of the watt balance measurements to 0.02 parts per millionwithin the next year or so, to reach the level of precision needed forcommercial mass measurements in the near future.

The measurements reported in Metrologia are the product of numerousimprovements in NIST's watt balance, including reconstruction of mostof the hardware to eliminate many sources of error. The coil andbalance were enclosed in a fiberglass vacuum chamber, which reduced theneed for air corrections. In addition, the coil was stiffened to reduceflexing that caused excess "noise" in the signals being measured. Noisewas reduced to one-fifth of the level of the 1998 experiments.Researchers also improved the alignment of instruments, temperaturecontrol and the software used for management and analysis of theexperiments.

As an agency of the U.S. Department of Commerce's TechnologyAdministration, NIST develops and promotes measurement, standards andtechnology to enhance productivity, facilitate trade and improve thequality of life.

* R. Steiner, E.R. Williams, D.B. Newell and R. Liu. "Towards anelectronic kilogram: an improved measurement of the Planck constant andelectron mass." Metrologia. (October 2005). Published online Sept. 13,2005.