Kibble balance

The Kibble balance, also known as the watt balance, measures how much electricity it takes to balance a weight suspended in a magnetic field. The difference between this process and using, for example, water, is that the Kibble balance is tied to universal fundamental constants—principles that dictate how the universe works, in this case the Planck constant—, and so we can factor out such things as atmosphere and other environmental errors.

To compare that to an ordinary balance, the typical balance measures an unknown mass (e.g. you on your bathroom scale) using a scale that one hopes is accurately calibrated.

Why is it important

As unlikely as it seems, the current global standard against which all things are weighed is a cylinder of metal in a vault in a suburb of Paris. This small cylinder of platinum-iridium alloy—a little more than 39 millimetres tall and the same in diameter—is the International Prototype Kilogram, or IPK. It is carefully sealed inside three concentric glass jars lest its mass be altered by dust, damage or fingerprints. It has been removed from its enclosure only four times since it was introduced as the IPK in 1889—to compare the accuracy of the half-dozen official copies that exist.

Scientists are increasingly concerned about the IPK's longer-term accessibility and stability, because its mass is not easily reproducible and gradually changes with time. Of the three fundamental constants that are involved in the Kibble balance experiment, the Planck constant (h) is currently the only one whose value is not fixed by definition and hence, the Kibble balance provides an accurate measurement of h, whose value will redefine the kilogram.

Who is Dr. Bryan Kibble

Dr. Kibble (1938-2016) worked at the UK National Physical Laboratory (NPL) from 1967 to 1998 as an experimental physicist, and was made an NPL Individual Merit Fellow in 1985. He was instrumental in reshaping the International System of Units (SI), and is best known for his conception of the watt balance, one of the measurement approaches proposed for the redefinition of the kilogram.

Kibble balance history

The predecessor of the Kibble balance was the current balance, used to measure the ampere, the SI unit of current. Measurements made using the current balance were limited in accuracy due to difficulties in measuring the equipment itself. For example, it was almost impossible to measure the position of the wires in the coil accurately.

In the early 1970s, Dr. Kibble proposed a new method which would allow for very accurate measurements of the ampere. His proposal was brought to life in 1978, when the first Kibble balance was made.

Over the next 10 years it was used to make measurements which resulted in the setting of the 1990 conventional values of the von Klitzing and Josephson constants which are used throughout the world for voltage and resistance calibration.

By 1990, Dr. Kibble and Dr. Ian Robinson built the Mark II Kibble balance which used a circular coil and operated in vacuum conditions.

In 2008, the NRC took over the Mark II balance project with the goal of obtaining an independent means of realizing the kilogram. A high level of difficulty was anticipated, but the NRC's unique position of having all the required experts on site made for an ideal environment for this project.

Dr. Robinson's insight and knowledge were major contributors to the NRC's success, and Drs. Wood and Sanchez worked alongside him during the setup in order to gain a better understanding of the apparatus.

In 2009, the NRC began refurbishing building M34 to house the Kibble balance experiment. The balance itself arrived in September of that year. It would take until the end of 2011 to fully construct, assemble, evaluate and calibrate all aspects of the experiment, as well as develop software to interface with it.

In 2012, the NRC performed a trial measurement. That year involved the elimination of a major systemic error: mass exchange errors.

2013 provided the time and opportunity to improve the modelling of the alignment of the Kibble balance experiment, as well as gravity determinations and profiling. In late 2013, four Planck constant determinations were made, resulting in the most accurate values ever reported by any previous technique.

Continual improvements were made for the following two years, until late 2016 yielded three more Planck constant determinations that again were the most accurate values ever reported by any technique, including the previous determinations. As recommended by the International Committee for Weights and Measures (CIPM), from that point on, watt balance projects are referred to as Kibble balance projects in honor of the late Bryan Kibble.

In April of 2017, the NRC's value of the Planck constant is included in the CODATA special adjustment to set the value of Planck's constant for redefinition of the SI. The NRC's value has the smallest uncertainty and the most 'weight' of any result in setting the final value of Planck's constant. If finally approved in November 2018, this will fix the value of Planck's constant forever.

What's next

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Number of days until redefinition of the kg: 204

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