How metres, kilos, and seconds came to be

On the 20th of May, 1875, seventeen nations came together in Paris to throw out centuries of chaos and contradiction, and build a universal language of measurement. Today, we take the idea of a metre, a kilogram, and a second for granted. But before the groundbreaking Metre Convention, units of measurement were defined by monarchs and markets and not by nature itself. 150 years on, though, that quiet revolution still shapes everything - from the smartphone in your pocket to the satellites above your head. To tell us how it happened is Alan Brewin who is the science and engineering director at the National Physical Laboratory…

Alan - Well, if you go way back in time, there was a real problem with science, which is that everybody would measure things in their own ways, they'd use their own measurement systems. And strangely, even use parts of their body as frames of reference. So for example, in ancient Egypt, people would talk about cubics, which is the length of an arm, and lots of people will recognise the term hands for measuring horses. But everybody used their own system or their own way of doing that. And it wasn't until 1875, when that changed, and scientists from around the world came together and agreed to use one set of measurements and a system to support them and to make them work.

Chris - That sounds like a nightmare. The Industrial Revolution spans and predates all that. So you're saying that people were building, say, James Watt's steam engines and steam trains and so on. And they were using their own definitions of units and scales when they did all that engineering.

Alan - Yes, so it wasn't the case that nothing works, right? Lots of things did work. And it worked much of the time because people would communicate, they'd fix errors along the way. So it wasn't that nothing works, but there were much more efficient ways to do it. And so much wastage around changing one unit to another, or fixing errors or wastage in processes. So things didn't always work. And so a unified system was seen as a way of eliminating all that waste and all that risk.

Chris - How did the individuals who came together to come up with that unified system decide what measures they were going to regulate or stipulate to make them international units? How did they decide that list?

Alan - So as you can imagine, it was a monumental committee exercise. Think about the UN or these kind of large committees. So it wasn't quick. I suppose the principle would be that they looked at the units that they needed to essentially describe the physical world in its entirety, right? So the SI system that we think about today, and we don't even think about it actually, you know, it's so ubiquitous. We use it without thinking about it. So if you pick units like the second for measuring time, or the metre for measuring distance, kilogram for mass, it feels like they've always been there, right? But actually, they really had to think about this. There were seven units that they described that when tested, described the physical world in its entirety. So all the measurements that we use today to cover aspects of modern life are covered by those seven units.

Chris - I can understand how you do it for a metre, because you could have a physical thing that you could say, well, there you go, that's a metre. I can understand how you do it for a mass like a kilo and Le Grand Kilo. It's in Paris, isn't it? That huge chunk of metal that is the kilogram. How did you do it for time, though? Because surely the inaccuracies in keeping time must have been quite tricky to surmount. Or were they just really very good metrologists by then?

Alan - Originally, time was measured with reference to celestial bodies and things in nature that human beings could refer to and understood. But of course, there's variation in those. And as technology moved on, we need more and more accurate measurements. Nowadays, we measure time against fundamental properties of nature. But what happens is an ion is trapped in very complicated engineering, and we trap an ion, and we excite that ion, and it gives off radiation at a certain frequency. And that frequency is the same wherever that atom is in the world. And provided you do the right things in the right way, it gives you a very consistent and accurate frequency of radiation. And you can use that. Instead, if you think about the ticking of a clock, you use that atom as the ticking, and that becomes your point of reference. In 1955 at NPO, Louis Essen developed and demonstrated the first accurate caesium atomic clock. So that atom was caesium in that case. And actually that stayed as the point of reference for the second since that point.

Chris - NPL came along in 1900. So that's 25 years after this all got started. What was the motivation for having the National Physical Laboratory? What was its role? And what does it still do?

Alan - NPL was established by Prince Charles at the time, who talked about the interface between science, technology and industry. It's actually 125 years since NPL was established. And we're celebrating that at the moment with some open events. About 2,000 people are going to be coming to NPL to see what we do here. And we're celebrating our past, our present and our future.

Chris - And what will visitors to the Open Day potentially be treated to?

Alan - They'll be able to talk to the scientists and engineers, I think is the primary thing, and really ask open questions and see where the work is done. We're going to be celebrating all sorts of things around our history. So the work that Alan Turing did here with the ACE computing engine, which was the forerunner of modern computers. The atomic clock work that I mentioned previously. Even things like packet switching, which Donald Davis developed here in 1965, which is one of the underpinning principles behind the internet. So there's lots of things around our past. People will be able to see the things that we're doing now and the impact we have now, the accurate atomic clock. So it's a room full of wires and electronic equipment that doesn't look like a clock that you might have in your head. But that's fascinating to see that and speak to the scientists there and understand everything that it takes to make a clock that is that accurate. These clocks now are heading towards an accuracy that if you were to run it since the beginning of the creation of the universe, it would still be accurate to within about a second.