The last unit standing

The kilogram is the last artefact based SI unit – something that has taken the best part of 20 years to change. Dr Ian Robinson takes us on a journey from lump of alloy to Planck constant  

The International System of Units (the SI) provides the basis upon which we measure the world around us, ensuring that measurements made anywhere in the world are comparable. It forms the basis of trade, allowing goods to be valued and exchanged with confidence, and underpins every aspect of scientific research.

The metre was defined by the distance between marks on a particular metal bar which, by comparison, all other such standards could be traced back to

Without it, customers wouldn’t know quite what they were buying, research results would be difficult to compare, and we wouldn’t have many of the technologies we take for granted. Originally, the units of the SI were defined by objects. The metre was defined by the distance between marks on a particular metal bar which, by comparison, all other such standards could be traced back to. But as we got better at measuring accurately, we realised that these objects were difficult to use and were affected by the world around them. The metal metre bar, for example, would shrink or expand depending on the temperature of its environment. So, the search began to find more reliable definitions for these units; ones that were repeatable and reliable.

[caption id="attachment_61065" align="alignnone" width="620"] The International Prototype Kilogram has been stored in France since its creation in 1889.[/caption]

That is how the metre came to be defined as the distance travelled by the speed of light in a vacuum. A number of SI units have been redefined in terms of fundamental constants, but the unit of mass is an exception. The kilogram is still defined by a lump of platinum iridium alloy, known as the International Prototype Kilogram (IPK) which has been kept, very carefully, in the basement of the Bureau International des Poids et Mesures (BIPM) in Paris since 1889.

A weighty problem

The IPK is extremely well made and replacing it requires a technique that can measure mass in terms of a fundamental constant at an uncertainty of two parts in 100 million. This is not a simple task and so, although efforts to do this have been underway since the end of the last century, it has taken a considerable time to get close to redefinition.

The use of a single physical artefact, the IPK, as the basis of worldwide mass measurement introduces a number of problems, however

In the present SI system, all SI mass measurements are traceable to the IPK. This is achieved through regular comparisons with the official 'copies' of the kilogram held in National Measurement Institutes. The United Kingdom's national standard of mass, known as Kilogram 18, is held at the National Physical Laboratory (NPL) and is the basis of the mass scale, or traceability hierarchy, in the UK. NPL participates in a wide range of international comparisons to ensure that mass measurements made in the UK are equivalent to those made elsewhere. The use of a single physical artefact, the IPK, as the basis of worldwide mass measurement introduces a number of problems, however. Due to its unique position in the SI the IPK is used to calibrate other masses very rarely – about once every 50 years – which means that in the intervening time the world has to ‘flywheel’ thereby depending on the stability of other mass standards. The stability of the IPK is not known as there is no way of measuring it within the SI.

As the need for accurate, traceable, measurements in science and industry is increasing, the need has arisen for a definition of the kilogram in terms of a fundamental constant, which offers the guarantee of long-term stability and universal applicability. But before such a definition can be put in place a mechanism, called a primary realisation, is needed to measure the chosen constant (the Planck constant) in terms of the mass of the IPK, to ensure the continuity of the SI.

Watt a solution

Currently, there are two experiments operating at the level of uncertainty required for a primary realisation: the Kibble balance and the Avogadro experiment. The Avogadro experiment determines the number of atoms in a silicon sphere. By measuring the mass of the sphere the technique measures the Avogadro constant (NA) which can be converted to the Planck constant with no significant loss in accuracy. The Kibble balance method works by comparing measurements of virtual electrical and mechanical power and, by using quantum electrical standards, it relates the Planck constant (h) to the kilogram.

The Kibble balance was first created in 1975, a second version is currently in use in Canada.

Formerly known as the watt balance, the Kibble balance was renamed after Dr Bryan Kibble, who conceived the instrument in 1975. I had the privilege of working with Bryan from 1976 until his passing last year, a period in which we made great advances in the development of the Kibble balance, making it more accurate and accessible. The second apparatus that we built, the NPL Mk II Kibble balance, now operating in Canada, has produced the best results in the world in support of the redefinition. Since his passing, I continue to feel a responsibility to see the redefinition process succeed and also to continue our work in simplifying and spreading the Kibble balance technique to as many countries as possible around the world. NPL is working to make this succeed by providing the resources to construct and test prototype balances that Bryan and I conceived and will be making copies of the final, table top, instrument for use around the world.

Better than the real thing

The redefinition will not change the mass scale or affect routine commercial mass measurements, and there will be no need for recalibration of existing equipment until it reaches its usual recalibration dates (at which time nothing unusual should be noticed in the recalibration). However, the redefinition will have a number of effects that are really beneficial.

Firstly, it will make the mass scale drift-free and applicable directly from atomic to macroscopic scales. Secondly, the responsibility for maintaining the worldwide mass scale will be spread across the world, removing the single point of failure represented by the IPK. Thirdly, as the mass scale will no longer be anchored at a single point, laboratories around the world will be able to realise the kilogram, using any method which makes correct use of the fixed value of the Planck constant. Finally, the value of the Planck constant will finally be constant avoiding the present practice of publishing a new recommended value for it every 4 years. Around the world, researchers are gearing up for the revision of the SI in 2018/19, meaning that time is running out for the world’s last artefact-based SI unit. The IPK will likely enjoy a retirement as an item of historical interest, with occasional measurements of it made to ascertain its drift over time. After decades of work towards redefinition, I can’t wait to see the fruits of our labour next year.

Author: Dr Ian Robinson is a Fellow at the National Physical Laboratory