Mass standard research in New Zealand

Metrologists (measurement scientists) at the Measurement Standards Laboratory of New Zealand (MSL) have developed a novel research approach to the international development of a new mass standard. Their system – nicknamed the ‘Kiwi Kibble balance’ – has attracted the attention of fellow metrologists from Canada, China, France, Switzerland and the USA.

For more than a century, the kilogram was defined by a small cylinder of platinum-iridium, stored in a vault below the home of the SI system – the International Bureau of Weights and Measures (BIPM) in Paris. This carefully stored block of metal was the last remaining physical artefact in the world’s measurement system. But thanks to a huge collaboration between scientists scattered across the globe, that all changed on 20 May 2019.

Two independent methods were developed that could measure a fundamental constant of nature – Planck’s constant (h) – with high precision. This number links the amount of energy a photon carries with the frequency of its electromagnetic wave, and its value never changes. By linking the kilogram back to h, mass measurements no longer need to rely on the international prototype kilogram.

Mass research at MSL has focused on developing and refining one of these measurement techniques – the Kibble balance – which links mechanical power to electrical power.

What is a Kibble balance?

A Kibble balance relates mechanical power to electrical power by comparing the gravitational force on a mass with the electromagnetic force on a current-carrying coil in a magnetic field. Taking a measurement on a Kibble balance requires two steps:

• Weighing mode: Everything is stationary – the gravitational force on a given mass is balanced by an electromagnetic force produced by an electric current flowing through a coil.
• Moving mode: The same coil is moved through a magnetic field to produce a voltage. This acts as an important calibration step that removes some of the complexity in the resulting measurements, particularly the strength of the magnetic field and geometry of the coil.

In the weighing experiment, a mass (m) and a coil of length (L) are suspended from a balance. The coil is placed in a magnetic field of strength (B), and when a current (I) is sent through it, an electromagnetic force is generated, pushing the coil downwards. This downwards force balances the gravitational force on the mass suspended on the other arm of the balance. When balanced, the force generated by the mass is equal to the electromagnetic force.

In the moving experiment, the coil is moved at a constant speed (v) through the magnetic field. This action generates a voltage (U) across the coil.

If the properties of the coil (L) and the magnetic field (B) do not change between the two experiments, the results can be combined and the equations are simplified. From this comparison, we can infer the link between mechanical power and electrical power.

When the electrical quantities U and I are measured using special quantum electrical effects (known as the Josephson effect and the quantum Hall effect), a direct link can be established between mass and Planck’s constant.

Because the unit of power is the watt, this measuring device was initially called the watt balance. It was renamed after its inventor, Dr Bryan Kibble, shortly after his death in 2016.

MSL’s novel approach

A Kibble balance is relatively simple in principle but the real challenge is to perform the experiment with sufficient accuracy. The most challenging aspects of the experiment are:

• moving the coil strictly vertically, without any horizontal movement
• aligning the axes of the coil and the magnet so they align exactly with gravity.

For these reasons, most existing Kibble balance experiments require correction systems. While they might fix the problem, they can also add complexity and potentially introduce ‘noise’ into the system, reducing its accuracy and precision. The late Dr Chris Sutton set out to create a novel version of the Kibble balance that would not only overcome these challenges but ideally do it within a smaller, simpler experimental set-up. Chris’s knowledge of gas-operated pressure balances led him to wonder if they could be a suitable alternative to the horizontal beams that are usually used to weigh the mass and move the coil.

Chris and his MSL colleagues developed a pair of pressure balances that could act as a very precise mass comparator – a key step along the route to a working Kiwi Kibble balance. Chris passed away in late 2018, but his legacy lives on through his design, which is now under construction at MSL.

A key enabling technology for this aspect of the Kiwi Kibble balance is the ability to measure the oscillatory voltage (U) with high speed and accuracy. Dr Laurie Christian from MSL, an expert in voltage measurement, is developing a quantum sampling voltmeter based on the Josephson effect for this measurement. It’s through measurement of this effect that the link to Planck’s constant (h) can be established. Laurie has established a collaboration with the National Institute of Standards and Technology (NIST) – one of the few laboratories in the world that can make the superconducting Josephson integrated circuits required for this sampling voltmeter.

The combination of these ideas along with various successful preliminary trials has attracted international recognition for the work of the MSL Kibble balance team. Work is ongoing.