Since 1889 the kilogram has been defined by a small metal cylinder stored in a vault in France. But now, with the help of a perfect silicon sphere recently acquired by the Museum, that’s all about to change.

What’s in a kilogram?

Under the metric system, a gram is defined as the weight of one cubic centimetre of water at a temperature of 4°C, with a kilogram being equal to one thousand grams. But due to the impracticability of a definition based on a volume of water, it was soon realised that a physical standard would be required.

The first standard kilogram was the Kilogram des Archives, a platinum cylinder made in 1799. Then in 1889, as the metric system gained acceptance, a new standard was created. This artefact, known as the International Prototype Kilogram (the IPK, or ‘Big K’ for short) is a cylinder made from an alloy of platinum and iridium. It measures just 39 millimetres in both diameter and height – and it is the kilogram. Our entire system of mass rests on this one object.

Changing with the times

Along with the IPK, there were a number of copies made, which were distributed around the world. Every 40 years the IPK is brought out of its vault to compare its mass to that of the copies. These comparisons have been made just twice just since the creation of the IPK – first in 1948 and again 1989 – but have found that the mass of the IPK is gradually diverging from the copies. And that means that a kilogram may not actually be a kilogram after all!

In the meantime, there has been an effort to redefine other base units – such as the metre, second, ampere, and kelvin – in terms of ‘fundamental constants’ of nature, rather than physical objects. As the name suggests these quantities are constants – they can be measured from nature, do not change with time, and have the same value anywhere in the Universe. For example, in 1983 the metre was redefined in terms of the speed of light, replacing the standard metre stick. The deviation in the Big K’s mass from its copies prompted scientists to redefine the kilogram in this way too.

The Avogadro Project the and perfect spheres

Several different methods for redefining the kilogram have been proposed – including the ‘Watt balance’, which would relate the kilogram to electric charge, and the ‘Avogadro Project’, which would instead define the kilogram in terms of atomic properties.

The Avogadro project uses a series of perfect spheres to measure the value of something called the Avogadro constant (one of the fundamental constants). The spheres are made out of a single crystal of pure silicon, with atoms arranged in a regular, repeating pattern. Using precision techniques, scientists can ‘see’ the crystal structure to determine the exact number of atoms in the sphere, and thus the value of the Avogadro constant. The new definition, based this constant, would allow the kilogram to be reproduced anywhere in the world by following a specific procedure, removing the need for a physical kilogram standard.

The roundest objects in the world

The spheres are made right here in Sydney, at the Australian Centre for Precision Optics, CSIRO, Lindfield – currently the only place in the world capable of achieving the required level of precision. The process begins with a silicon ingot ‘grown’ in the lab, which is then cut and ground down to shape. The maker of the spheres is Achim Leistner, who uses a series of custom-made tools to manually grind and polish the spheres until they are perfectly round. As he polishes, Leistner uses his hands to feel for imperfections in the surface of the sphere. The result is a sphere which is perfect to within 30 or 40 billionths of a metre.

And now we have one of these amazing objects for our collection. In September 2016, Leistner generously donated his own sphere (one of the prototypes made for the project) to the Museum. It was given to him by CSIRO, at the conclusion of the project, in recognition of his contribution. After 16 years taking pride of place in his living room, Leistner made the decision to donate his sphere to the Museum, where it can be appreciated by everybody.

You can catch the sphere until Sunday 16 July, in its last days on display as part of the exhibition MicroGravity or read more about it on our online collection but with an object this beautiful we’re sure it won’t be long until we get it out on display again!

Written by Sarah Reeves