Researchers at the National Physical Laboratory (NPL), UK, have produced technology capable of accurate measurements of Planck’s constant, the final piece of the puzzle in moving from a kilogram defined by a physical object to a kilogram based on fundamental constants of nature.
The international system of units (SI) is the most widely used system of measurement for commerce and science. It comprises seven base units: meter, kilogram, second, Kelvin, ampere, mole and candela. Ideally, these units should be stable over time and universally reproducible, which requires definitions based on fundamental constants of nature.
The kilogram is the only unit still defined by a physical artifact called the international prototype of the kilogram, kept by the International Bureau of Weights and Measures (IBWM) in Paris, France. Its form is a cylinder with diameter and height of about 39 mm. It is made of an alloy of 90 % platinum and 10 % iridium. The prototype has been conserved at the IBWM since 1889, initially with two official copies. Over the years, one official copy was replaced and four have been added.
In October 2011, the General Conference on Weights and Measures agreed that the kilogram should be redefined in terms of Planck’s constant (h).
In a paper, published today in the journal Metrologia, the researchers describe how this can be done with the required level of certainty.
Planck’s constant is a fundamental constant of nature, which relates the frequency (color) of a particle of light (a photon) to its energy. By using two quantum mechanical effects discovered in the last 60 years: the Josephson effect and the quantum Hall effect, electrical power can be measured in terms of Planck’s constant (and time).
A piece of kit called the watt balance – first proposed by Brian Kibble at the NPL in 1975 – relates electrical power to mechanical power. This allows it to make very accurate measurements of Planck’s constant in terms of the SI units of mass, length and time. The SI units of length and time are already fixed in terms of fundamental and atomic constants. If the value of h is fixed, the watt balance would provide a method of measuring mass.
“The watt balance divides its measurement into two parts to avoid the errors which would arise if real power was measured,” explained Dr. Ian Robinson, leader of the project at the NPL. “The principal can be illustrated by considering a loudspeaker placed on its back. Placing a mass on the cone will push it downwards and it can be restored to its former position by passing a current through the speaker coil. The ratio of the force generated by the current is fixed for a particular loudspeaker coil and magnet and is measured in the second part of the experiment by moving the speaker cone and measuring the ratio of the voltage produced at the speaker terminals to the velocity of the cone.”
“When the results of the two parts of the experiment are combined, the product of voltage and current is equated to the product of weight and velocity and the properties of the loudspeaker coil and magnet are eliminated, leaving a measurement of the weight of the mass which is independent of the particular speaker used.”
Measurements of h using watt balances have provided uncertainties approaching the two parts in one hundred million level, which is required to base the kilogram on Planck’s constant. Thanks to improvements highlighted in the paper, measurements at the National Research Council in Canada, which is now using the NPL equipment, look set to provide considerably greater accuracy.
“This is an example of British science leading the world,” Dr. Robinson concluded. “NPL invented the watt balance and has produced an apparatus and measurements, which will contribute to the redefinition. The apparatus is now being used by Canada to continue the work, and we anticipate their results will have lower uncertainties than we achieved, and the principle is used by the US and other laboratories around the world to make their own measurements.”
“This research will underpin the world’s measurement system and ensure the long term stability of the very top level of mass measurement. Although the man on the street won’t see much difference – you’ll still get the same 1kg bag of potatoes – these standards will ultimately be used to calibrate the world’s weighing systems, from accurate scientific instruments, right down the chain to domestic scales.”