IPF 2010: Redefining the kilogram

Standardization is important to science and industry. Ideally you’d like to define units in terms of physical constants, which, as far as we know, are constant. Defining time, for instance, by the frequency of an atomic transition or defining electrical resistance by the quantum Hall effect shifts the uncertainty of measurement into the quantum realm, where accuracy can be high.

The kilogram is the only remaining unit in the International System (SI) still defined in terms of a physical artifact, a chunk of metal, the International Prototype Kilogram (IPK), kept in a bell jar in Paris, with sister duplicates established in various other parts of the world. For some time an effort has been under way to define the kilogram in terms of something else, probably Planck’s constant , denoted by the letter h.

Physicist Richard Steiner at NIST reported on progress in redefining the kilogram. He and his colleague operate a Watt balance, a device whose heart is an electromagnet coil sitting in a background magnetic field. Run one way, the coil acts as a motor; run the other way, it can act as an electrical generator. With that flexible setup the balance can compare mechanical power (counteracting the force of gravity on a test mass) with electrical power.

The power measured with the balance is proportional to h and the mass of the test object. But the value of h is constant with quantum systems involving time, length, voltage, and resistance. Thus mass can be measured now with an uncertainty (in NIST experiments) of about 36 parts per billion, which is smaller than the uncertainty over time in the IPK artifact.

“Very probably within the next 5 years the kilogram will be redefined,” said Steiner. He believes the era when a single ruler, an artifact standard, could provide calibration is over. Why not, since quantum standards are intrinsically quieter, more accurate, and more stable?

“This whole field may undergo a similar transformation to what happened in voltage metrology 25 years ago,” Steiner said. “The world went from using chemical voltage cells that were hard to measure or even move without altering their voltage to measuring improved references directly against Josephson effect quantum devices.”

Industry will benefit, Steiner says, from an improved kilogram standard. Transferring the new standards from national laboratories to an industrial lab setting will be easier since there would involve fewer intermediate mass calibrations (each contributing its own uncertainties to the overall measurement quality) and because a standard can be applied to many more kinds of reference materials, not just the expensive metals used in the current SI protocol. As an example of what he means, Steiner points out that only about 90 of the official IPK masses are available in the world, although there are tens of thousands steel masses used for calibration, subject to error.

Phillip F. Schewe

All the talks at IPF 2010 were recorded and are now available on video .