Kilogram celebrates its 125th birthday

September 25, 2014 by Stuart Davidson, National Physical Laboratory
Kilogram celebrates its 125th birthday
The UK national standard kilogram removed from its protective casing

The IPK has been the global standard for mass for the last 125 years; it was sanctioned at the first General Conference on Weights and Measures (CGPM) on 7-9 September 1889 in Paris. This is likely to be the last landmark birthday of the IPK as current experiments to redefine the kilogram are due for completion as early as 2018. These will relate mass to naturally occurring constants rather than a physical object such as the IPK.

The IPK is made from an alloy of platinum (90%) and iridium (10%). This mixture was chosen because platinum has a high density so the IPK could be made with a small volume and surface area; the addition of iridium improves the hardness. The IPK has a fixed mass approximately equivalent to one litre of water at 4 °C, but its weight varies depending on local gravity.

When the IPK was manufactured, 40 copies were also made from the same -iridium alloy. These were distributed for use as national standards so scientists would not have to go back to the IPK (held in Sèvres, just outside Paris) every time they needed an accurate measurement of mass. These national standards are checked for deviations against the IPK every 40 years. At the last check, in 1989, the maximum deviation was about 50 micrograms. These changes are not fully accounted for and the corresponding lack of stability of the mass scale poses a problem for scientists. NPL scientist Dr Stuart Davidson comments that: "While the current definition of the is fit for purpose, we know that it can't be perfectly stable because all artefacts will change their mass with time. It is a concern that we know that the IPK must be changing, but there is currently no way to actually measure this change."

Two experiments are under way to redefine the kilogram in terms of a fundamental constant. The watt balance establishes the kilogram with relation to quantum realisations of the volt and the ohm, and the Avogadro experiment defines a kilogram in terms of a fixed number of atoms. The watt balance concept was originally developed at NPL in 1975 by Dr Bryan Kibble. NPL worked on developing the watt balance until 2008 when the mark II NPL watt balance was transferred to NRC Canada where it is now producing the most accurate measurements of the Planck constant yet made. NPL is also leading a European research project to creating a practical link between results from the new experiments and the current mass scale defined by the IPK. This is a two-fold process where the Planck constant will be initially fixed against the current mass scale as defined by the IPK. After this the unit will be realised by the watt balance and Avogadro experiments and the mass scale will need to be disseminated from these experiments which realise the unit in vacuum, to practical in air. They are anticipating completing this research by 2015.

The IPK nominally has zero uncertainty as the scale is fixed against it. Relating the scale to naturally occurring constants will assure the long term stability of the scale but, in the short term, it will increase the uncertainty due to the uncertainty in the realisation experiments. There will also be an additional uncertainty of disseminating the scale from vacuum to air. Scientists are aiming to minimise these additional uncertainties to a level of about 3 in 108 which is approximately equivalent to adding the weight of one grain of rice to the overall weight of a car.

Dr Stuart Davidson remarked that: "This anniversary is interesting as it shows just how long this standard for the kilogram has lasted and therefore how good the original choice of material for the IPK was." The kilogram is the last metrological unit to be linked to a physical quantity. Even though the future of the IPK is finite, it should be pleased to have outlasted to other base SI units.

Explore further: Vacuums provide solid ground for new definition of kilogram

Related Stories

Vacuums provide solid ground for new definition of kilogram

October 23, 2013

Of all the standard units currently in use around the world, the kilogram – the official unit of mass in the International System of Units (SI) – is the only one that still relies on a physical object for its definition. ...

How the kilogram has put on weight

January 6, 2013

Using a state-of-the-art Theta-probe XPS machine experts at Newcastle University, UK, have shown the original kilogram is likely to be tens of micrograms heavier than it was when the first standard was set in 1875. And they ...

Redefining the kilogram

February 20, 2012

New research, published by the National Physical Laboratory (NPL), takes a significant step towards changing the international definition of the kilogram – which is currently based on a lump of platinum-iridium kept ...

Redefining the kilogram and the ampere

September 29, 2011

New research using graphene presents the most precise measurements of the quantum Hall effect ever made, one of the key steps in the process to redefine two SI units.

Recommended for you

Trembling aspen leaves could save future Mars rovers

March 18, 2019

Researchers at the University of Warwick have been inspired by the unique movement of trembling aspen leaves, to devise an energy harvesting mechanism that could power weather sensors in hostile environments and could even ...

Quantum sensing method measures minuscule magnetic fields

March 15, 2019

A new way of measuring atomic-scale magnetic fields with great precision, not only up and down but sideways as well, has been developed by researchers at MIT. The new tool could be useful in applications as diverse as mapping ...

Researchers report new light-activated micro pump

March 11, 2019

Even the smallest mechanical pumps have limitations, from the complex microfabrication techniques required to make them to the fact that there are limits on how small they can be. Researchers have announced a potential solution—a ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

Bob Osaka
not rated yet Sep 26, 2014
Ask Americans if theirs is an imperialist system, they'll deny it. As they inch their way burdened by pounds, guzzling by the gallon, three feet in their yards, separated from the world by miles and miles, (except Mexico and Canada) wondering why they aren't understood.
Remember the joint NASA/ESA Mars mission in which a probe was crashed into the Martian atmosphere at the wrong angle and velocity? And rocket scientists used to be so clever. That's a great argument for Europeans to adopt the American system based on some guy's foot.
Happy birthday inanimate object. Let's hope a metric United States joins the world before your 250th.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.