MSL LEGO Kibble balance

Dr Rebecca Hawke and Dr Yin Hsien Fung are research scientists at the Measurement Standards Laboratory (MSL). MSL is building the southern hemisphere’s only Kibble balance – an exceptionally accurate weighing machine. Rebecca and Fung explain how the simpler yet similar LEGO version of the Kibble balance works.

Transcript

REBECCA HAWKE

Hi, we are scientists from the Measurement Standards Laboratory, New Zealand’s national metrology institute.

YIN HSIEN FUNG

In 2019, the International System of Units has undergone some major changes, and that includes a redefinition of the kilogram, which is the unit for mass.

REBECCA HAWKE

One way the kilogram can be realised is via a system called a Kibble balance.

YIN HSIEN FUNG

Here, we have a LEGO version of the Kibble balance, although it is not as accurate as the actual Kibble balance, but it can show you how it works.

REBECCA HAWKE

Kibble balances have two measurement modes – weighing and moving. In weighing mode, we place an unknown mass on one pan. We then provide a balancing force using electricity and magnets. How do we do that? We run an electrical current through the coil, and that becomes a magnet. This interacts with a permanent magnet, which is hiding underneath in here. The beam holding the two pans is free to pivot around a central point. A laser pointer and a light sensor track the beam so that we can tell when it’s balanced. When the beam is balanced, the current in the coil is proportional to the mass on the pan.

YIN HSIEN FUNG

In moving mode, we take the mass off the pan and we turn off the current and then we move the coil back and forth through the magnetic field at a constant velocity. We drive this movement by using an electromagnetic force generated by a similar set-up of a coil and magnet on the other side of the balance beam. This induces a voltage in the coil.

REBECCA HAWKE

Comparing the electrical current from weighing mode with the induced voltage of moving mode leaves a simple equation that allows us to measure mass.

YIN HSIEN FUNG

In a real Kibble balance, electrical measurements are linked to the Planck’s constant, which is a fundamental constant in nature.

REBECCA HAWKE

By making a connection between the kilogram and Planck’s constant, stable accurate mass measurements can be made anywhere in the world without relying on a physical artefact.

YIN HSIEN FUNG

Here at the Measurement Standards Laboratory, we are building our own Kibble balance to realise the national primary kilogram for New Zealand.

Acknowledgements

The LEGO Kibble balance video was filmed on site at the Measurement Standards Laboratory (MSL) in Lower Hutt with Yin Hsien Fung and Rebecca Hawke from MSL.

As part of the International System of Units (SI) redefinition on 20 May 2019, MSL developed a LEGO Kibble balance to demonstrate how the kilogram (of the unit of mass) will be determined in terms of quantum physics and Plank’s constant. MSL has developed a Kibble balance (not made of LEGO) to realise the mass standard.

Filming and editing by Ricky Situ at ZHU Creative.