The weight is over: Redefining the Kilogram

20 November 2018

Interview with

Perdi Williams, NPL

Now speaking of meat, the last time you were weighing out your pork chops, did you spare a thought for how we know what a “kilogram” actually is? Probably not, because it’s not changed for over a century. A “kilo” is the weight of a big lump of metal in London, and that’s a clone of a similar object in Paris. But now scientists have been voting at a conference to bring the kilo into the twenty-first Century. Adam Murphy’s been weighing up why with National Physical Laboratory researcher Perdi Williams...

Adam - It's important to be able to weigh things accurately; from baking cakes properly, to the minute amounts required to get drug doses right. But we all need to be talking about the same thing.
My kilogram has to be the same as your kilogram or one of us is getting a terrible cake, or not enough medicine. The standards were decided in the 19th century in France, something for us all to agree on. Initially one kilogram was the same as one litre of water, but the density of water changes with temperature, with pressure, and with purity. So we came up with a different solution.

Perdi Williams - We've had the kilogram for a long time. The ‘UK Standards of the Kilogram Number 18’ was given to us in 1889.

Adam - That is Perdi Williams, assistant researcher at the National Physical Laboratory where they keep one of the UK’s kilograms - lumps of metal that tell every weighing scale in the UK and Ireland what one kilogram is supposed to be

Perdi - And it's been working incredibly well. It's an amazing thing but it is a single point of failure in the system. Also we need to make sure that the original kilogram, IPK, we need to make sure that's stable.

Adam - That's right. Also known as ‘Le Grand K’, the International Prototype Kilogram is a cylinder of platinum iridium about the size of a golf ball that’s sitting in a vault in Paris. And it is one kilogram, and it's what all national kilograms like NPL’s Kilogram 18 take their cues from on what a kilogram is.
So if I could get into that vault and say hack a bit off, or eat a bit, I would change what one kilogram was - how it was defined. I could invalidate every scale in the world. But the thing is, isn't it going to change with time?

Perdi - Yes. So we believe the kilogram is changing over time but we don't know by how much. IPK could be changing and gaining mass, and ours are all staying the same. We can’t accurately see what's going on. So with the redefinition we'll be able to have a better idea of what's going on with the mass scale and also it will be changed to a constant and that can't change.

Adam - We're changing it hopefully this week. What are we changing it to?

Perdi - So the new definition will be based on Planck constant.

Adam - Planck's constant is a number used in quantum physics. It's a zero-point-thirty-four-more-zeros and a 6. It's incredibly small.

Perdi - And this will be done using Kibble balance. Basically, it's making an electric kilogram. So we're balancing out the force from the kilogram with an electric force. So its main concept is a magnet and a coil within the magnetic field. Once you pass a current through the coil it produces a force and moves the platform up and down, and using the planck constant we can get mass from that.

Adam - And is our whole world about tilt on its axis? Is everything going to change with this vote?

Perdi - We've worked incredibly hard to make sure that the general public and end users don’t see any change. A kilogram is a kilogram, it’s just going to be defined differently

Adam - And at the end of the day, why does this matter?

Perdi - Measurement is so incredibly important. Everything can be measured, so we need to be as accurate as possible with the way we measure things and how we define things. People have been obsessed with measurements since the beginning of time - in ancient Egypt they used used the ‘cubit’, which was the length of the Pharaoh's elbow to the end of his fingers, and that was how they did measurement. Even back then they still were worried about things not fitting together or being scammed with how many seeds they were buying. We’re just getting better at defining things.

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