Technical Blog

World's Roundest Object

The perfect sphere placed upon Heason Technology's 5-axis manipulator is, supposedly, the world's roundest object. Its purpose is to help solve the longest running problem in measurement; defining the kilogram.

It is made out of a single crystal of silicon-28 atoms, with a roundness delta of less than 50 nano-metres over a 93.6 mm diameter. Each sphere costs an estimated £2.5m each and have to be handcrafted by a master lens maker.

The spheres have to be extensively analysed for their crystal perfection and measured to determine the sphere diameter and resulting volume. The sphere's surface is so smooth that if they were blown up to the size of Earth, the distance between the tallest mountain and deepest ocean would be around 10-15 feet.

Defining the Kilogram

As mentioned above, the purpose of the sphere is to help define the kilogram, the longest-running problem in measurement.

A kilogram is no longer defined as what it once was, with the definition changing many times since its creation. The definition of a kilogram way back in 1799 was the mass of one litre of water at four degrees Celsius. As of 2016, the kilogram is defined by the mass of a platinum-iridium sphere, a physical artefact called the International Prototype Kilogram or IPK.

Avogadro Project

The Avogadro Project, originating in Australia, has been trying to solve the kilogram problem for many years, with researchers at the Centre for Precision Optics crafting the spheres.

In the spheres, the number of atoms can be calculated using a laser to measure the diameter. The number of atoms in the sphere can then be used to define Avogadro’s number, which in turn could define the kilogram.

Heason Technology

Heason Technology has recently completed the design and supply of a UHV compatible and completely non-magnetic 5-axis manipulator. Commissioned by ITRI, it will be used to micro-position the silicon sphere that could play a significant role in the redefinition of the unit of mass for the International System of Units (SI).

Over time, Heason have developed an in-house design and build service, curating a large portfolio of proven application successes for motion system installations in UHV for synchrotron sources and many other equally demanding research and manufacturing environments. Our Project Gallery showcases a selection of interesting and diverse examples where we have helped our customers across the industry to fulfil their challenging motion control requirements.

If you would like to find out more information about the project please contact us on +44 (0) 1403 792 300 or email


Article published on: 29/08/2019