0:00:01 > 0:00:05On the morning of the 14th June, 1940,
0:00:05 > 0:00:07several German tank divisions rumbled
0:00:07 > 0:00:09through the streets of Paris.
0:00:11 > 0:00:13The impossible had happened.
0:00:13 > 0:00:17Germany had invaded and France had fallen.
0:00:25 > 0:00:28'But there was one building on the outskirts of Paris that
0:00:28 > 0:00:31'the Nazis never occupied.
0:00:33 > 0:00:38'This chateau has the same status as an independent territory.
0:00:40 > 0:00:43'Its contents are so closely guarded,
0:00:43 > 0:00:46'I have to hand over my passport to gain access.
0:00:49 > 0:00:53'Today, an eminent group of scientists have gathered
0:00:53 > 0:00:57'from all over the world to witness a very special event.
0:01:03 > 0:01:07'Security is tight, with key holders arriving
0:01:07 > 0:01:08'from three different countries.
0:01:11 > 0:01:16'The vault holds one of the most important artefacts in our world.'
0:01:18 > 0:01:21This is a real piece of measurement history.
0:01:21 > 0:01:23Well, I suppose it's not really history at all...
0:01:23 > 0:01:25This is the kilo.
0:01:26 > 0:01:31'Under three layers of protective glass is the kilogram master
0:01:31 > 0:01:33'known as Le Grand K.
0:01:35 > 0:01:40'It's the weight on which all weights have been based since 1889.
0:01:41 > 0:01:44'Its importance is so great that neither the Nazis nor
0:01:44 > 0:01:49'the liberating American forces dared set foot inside here.
0:01:50 > 0:01:53'And the reason we're here today?
0:01:53 > 0:01:56'Well, just to check it's still here.
0:01:58 > 0:02:00'But there's a problem.
0:02:00 > 0:02:04'Tests have revealed that Le Grand K, this scientific celebrity,
0:02:04 > 0:02:10'is losing weight, creating a crisis in the scientific world.
0:02:12 > 0:02:17'It's losing approximately 20 billionths of a gram every year.
0:02:19 > 0:02:24'But why on earth should such a tiny change matter so much?'
0:02:27 > 0:02:30I'm on a journey to investigate the world of measurement,
0:02:30 > 0:02:32and to see how our drive for precision
0:02:32 > 0:02:35has really changed the course of history.
0:02:38 > 0:02:40'Today, we can describe the chaos
0:02:40 > 0:02:45'and complexity of the universe with just seven fundamental units,
0:02:45 > 0:02:48'the building blocks of modern science.
0:02:48 > 0:02:51'And science is obsessed with defining these units
0:02:51 > 0:02:54'with ever-greater precision.
0:02:54 > 0:02:58'In this series, I want to understand why such extreme
0:02:58 > 0:03:01'levels of precision are so important,
0:03:01 > 0:03:03'how we define these units,
0:03:03 > 0:03:06'and how through history each step forward has unleashed
0:03:06 > 0:03:09'a technological revolution.
0:03:12 > 0:03:15'In this programme, we'll explore why being able to measure weight
0:03:15 > 0:03:17'is so important.
0:03:20 > 0:03:24'And how the race to replace the ageing Grand K
0:03:24 > 0:03:28'might hold the key to a new way of understanding our world.'
0:03:35 > 0:03:40This is the story of how we mastered weight.
0:03:56 > 0:03:59"How much do I have?" is a question that has driven trade
0:03:59 > 0:04:02and commerce since the dawn of civilisation.
0:04:06 > 0:04:09And today, weights are still central to all our lives.
0:04:12 > 0:04:15The reason we're so reliant on weights and scales
0:04:15 > 0:04:20is in part down to our own inability to accurately gauge weight.
0:04:20 > 0:04:24We tend to believe our eyes, rather than trusting
0:04:24 > 0:04:26the weight in our hands.
0:04:29 > 0:04:32'And I've come to London's Borough Market to prove the point.'
0:04:35 > 0:04:37Excuse me - wonder whether I could get you to take part
0:04:37 > 0:04:40- in a little experiment? - Of course, yes.
0:04:40 > 0:04:43So, I've got a series of weights here which I've put in order
0:04:43 > 0:04:46of height and what I'd like you to do is to place
0:04:46 > 0:04:50the heaviest weight here, and the lightest one at your end.
0:04:50 > 0:04:52Have a go. See which one you think is the heaviest.
0:04:58 > 0:05:00That's...
0:05:00 > 0:05:02This little guy, that's the heaviest? OK.
0:05:02 > 0:05:04What about the next heaviest?
0:05:06 > 0:05:09I think this one...that's the lightest.
0:05:09 > 0:05:11- The lightest of all?- I think...- OK!
0:05:11 > 0:05:16The really surprising thing is that the one you've put at this end,
0:05:16 > 0:05:19which you think is the lightest, is in fact the heaviest!
0:05:19 > 0:05:22So you thought this one here was the heaviest.
0:05:22 > 0:05:25OK, I'm going to give you both these in your hand - this one is
0:05:25 > 0:05:29actually heavier than that one. Do you believe me?
0:05:29 > 0:05:30Well, it doesn't feel like that.
0:05:30 > 0:05:32No, it doesn't, but let's use the scales.
0:05:32 > 0:05:35So I am going to weigh the one that you thought was the lightest,
0:05:35 > 0:05:39so that comes out about 424 grams.
0:05:39 > 0:05:41OK, let's put your one on.
0:05:41 > 0:05:45You think this one is heavier. It's only 345 grams!
0:05:45 > 0:05:50Isn't that extraordinary? So, even with that knowledge, now try
0:05:50 > 0:05:52and weigh them again, which one is heavier...
0:05:52 > 0:05:56- This one.- I know! And that's why we need a set of weights
0:05:56 > 0:06:00because we're so bad at perception.
0:06:00 > 0:06:03'Like any good scientist, I carried on with the testing.'
0:06:05 > 0:06:07How's that possible?
0:06:07 > 0:06:09'And my random shoppers, to a man and a woman,
0:06:09 > 0:06:12'all chose the same two weights -
0:06:12 > 0:06:14'and they all chose wrong.'
0:06:14 > 0:06:15OK. Wow!
0:06:16 > 0:06:19Seeing if something is big or small massively
0:06:19 > 0:06:22skews our perception of how heavy it is.
0:06:24 > 0:06:26It is a problem our ancestors
0:06:26 > 0:06:30started first grappling with more than 5,000 years ago.
0:06:33 > 0:06:37Our earliest evidence comes from the Middle East and was driven
0:06:37 > 0:06:43by the emergence of the first cities in Mesopotamia around 3,000 BC.
0:06:46 > 0:06:49As populations grew,
0:06:49 > 0:06:52a way of fairly trading goods was urgently needed.
0:06:54 > 0:06:57People demanded a system of weight that everyone could trust.
0:06:59 > 0:07:04Taking their inspiration from nature, they used grain.
0:07:07 > 0:07:11Uniform in size and shape, grain was available to all.
0:07:13 > 0:07:16The world had its first weights.
0:07:18 > 0:07:22Using simple beam balances, which we continue to use today,
0:07:22 > 0:07:27we started to trade goods based on their weight in grains.
0:07:29 > 0:07:33It wasn't perfect, but with grains varying so little in weight,
0:07:33 > 0:07:35the system worked.
0:07:37 > 0:07:40It made the movement and sale of goods possible,
0:07:40 > 0:07:44enabling humans to live together in bigger cities
0:07:44 > 0:07:47and allowing the first economies to grow.
0:07:49 > 0:07:53Empires were no longer being built solely by armies.
0:07:55 > 0:07:56They were being built by trade.
0:08:00 > 0:08:03As commerce developed across the ancient world,
0:08:03 > 0:08:06a faster means of weighing produce was needed.
0:08:06 > 0:08:10After all, if I wanted to buy something that weighed
0:08:10 > 0:08:11700 grains of barley,
0:08:11 > 0:08:14I don't want to have to count out 700 grains each time.
0:08:15 > 0:08:19So, gradually, a standardised system of weights began to emerge.
0:08:19 > 0:08:23First the Mesopotamians, then the Ancient Egyptians developed
0:08:23 > 0:08:25stones and things made out of metals
0:08:25 > 0:08:30and brass in order to represent different weights of grain.
0:08:32 > 0:08:35It was such an efficient system that it began to be copied
0:08:35 > 0:08:39across the civilised world. So here we have standard
0:08:39 > 0:08:40weights from China.
0:08:43 > 0:08:46These hexagons are standard weights used in Sudan.
0:08:48 > 0:08:51And the amazing thing is that, despite all of these
0:08:51 > 0:08:54different weights and measures, they were all related back
0:08:54 > 0:08:56to the weight of a grain,
0:08:56 > 0:08:59because everyone trusted how much a grain would weigh.
0:09:02 > 0:09:06'By Roman times, millions of tonnes of produce were being
0:09:06 > 0:09:08'traded around the world every day.'
0:09:11 > 0:09:14The ability to compare the weights
0:09:14 > 0:09:17or masses of two different kinds of goods
0:09:17 > 0:09:21so that you could work out how to exchange between them,
0:09:21 > 0:09:24that's the key to economic success.
0:09:24 > 0:09:29And so it's the demand for economic comparison that drives weight
0:09:29 > 0:09:32standardisation throughout history.
0:09:35 > 0:09:38By the end of the 13th century, the world had hundreds of different
0:09:38 > 0:09:44weights, and nearly all were based on a fixed number of grains.
0:09:46 > 0:09:51In England, we'd inherited the pound from the Roman Empire.
0:09:53 > 0:09:56It was initially made up of 12 ounces,
0:09:56 > 0:10:01which were equivalent to 437 grains of barley.
0:10:03 > 0:10:05But the problem all rulers faced
0:10:05 > 0:10:09was how to keep weight standardised across a nation.
0:10:11 > 0:10:15It was considered such a big issue that even the Magna Carta,
0:10:15 > 0:10:19the most celebrated legal document in English history,
0:10:19 > 0:10:20tried to deal with it.
0:10:23 > 0:10:26"Let there be one measure of wine throughout our whole realm,
0:10:26 > 0:10:30"and one measure of ale and one measure of corn."
0:10:33 > 0:10:37It all sounded great in theory, but in practice,
0:10:37 > 0:10:40it was virtually impossible to enforce.
0:10:42 > 0:10:44Cheating was such a big problem
0:10:44 > 0:10:48regular trials were held to check merchants' weights and measures.
0:10:50 > 0:10:54Any found to be wrong were immediately destroyed.
0:10:55 > 0:11:00Accurate scales were the only way cheats could be exposed.
0:11:00 > 0:11:03Accuracy was power.
0:11:17 > 0:11:20Scales were not only a great measuring device.
0:11:20 > 0:11:23They also came to symbolise fairness, power,
0:11:23 > 0:11:26the very legal system itself.
0:11:32 > 0:11:37From Ancient Egypt's Feather of Truth to the paintings
0:11:37 > 0:11:42of the great Dutch Masters, scales have featured throughout history.
0:11:47 > 0:11:51As it was written in the Bible, "By weight, measure and number,
0:11:51 > 0:11:53"God made all things."
0:12:04 > 0:12:08Measurement has always been associated in culture with justice
0:12:08 > 0:12:14and law and crime. Because what it does is to establish the equivalence
0:12:14 > 0:12:19between two things that you otherwise could not compare.
0:12:19 > 0:12:22That's what justice means,
0:12:22 > 0:12:26so it's no coincidence that the figure of Justice is shown
0:12:26 > 0:12:29carrying scales, carrying balance pans.
0:12:31 > 0:12:34And for centuries, when you made a weight measurement, you had to
0:12:34 > 0:12:39show your customers what you were doing - partly to avoid
0:12:39 > 0:12:44the possibility of deceit, but also to show how just you were -
0:12:44 > 0:12:48to be just, was precisely to use balance.
0:12:53 > 0:12:56So, with all this moral weightiness flying around, the punishment
0:12:56 > 0:13:00for using false measures could be severe.
0:13:05 > 0:13:10In 1772 BC, the Code of Hammurabi was introduced in Babylonian law,
0:13:10 > 0:13:13which said that any taverner using false weights
0:13:13 > 0:13:15could be served up with a death penalty.
0:13:20 > 0:13:23And in the 18th century, bankers caught cheating
0:13:23 > 0:13:29would have to stand in pillory, and brewers in the dung cart.
0:13:32 > 0:13:36But despite the importance we placed on weight
0:13:36 > 0:13:40and getting it right, it took one remarkable Englishman
0:13:40 > 0:13:44to realise the measurement of weight has a fundamental problem.
0:13:48 > 0:13:52It was the great Sir Isaac Newton who first realised that
0:13:52 > 0:13:56weight changes depending on where and when you are measuring it.
0:14:05 > 0:14:08It was 1665, and Britain was gripped by the Plague,
0:14:08 > 0:14:12so Newton decided to flee his college in Cambridge and
0:14:12 > 0:14:16he came to the safety of his country retreat here at Woolsthorpe Manor.
0:14:17 > 0:14:21And here is the famous apple tree that inspired his observations.
0:14:24 > 0:14:28So much has been written about this apple tree, it really has become
0:14:28 > 0:14:31a symbol for the turning point in our understanding of the universe.
0:14:35 > 0:14:39Newton's eureka moment was witnessed by a friend.
0:14:41 > 0:14:44"After dinner, the weather being warm,
0:14:44 > 0:14:47"we went into the garden and drank tea, under the shade
0:14:47 > 0:14:49"of some apple trees.
0:14:50 > 0:14:53"The notion of gravitation came into his mind.
0:14:55 > 0:14:59"Why should that apple always descend perpendicularly
0:14:59 > 0:15:01"to the ground?"
0:15:03 > 0:15:07Newton realised there must be a force acting on that apple,
0:15:07 > 0:15:09pulling it to the ground, otherwise why wouldn't it just
0:15:09 > 0:15:13float in the air, or move sideways or go upwards?
0:15:13 > 0:15:16He named that force "gravity", after the Latin word
0:15:16 > 0:15:17"gravitas" for heaviness.
0:15:19 > 0:15:23'Newton's law of gravity was to completely change the way
0:15:23 > 0:15:24'we think about weight.'
0:15:26 > 0:15:31We finally understood the subtle but vital difference between weight
0:15:31 > 0:15:37and mass, and it paved the way for modern measurement.
0:15:39 > 0:15:42Now, to show how important Newton's discovery was,
0:15:42 > 0:15:44I've got a piece of metal here
0:15:44 > 0:15:46and an incredibly sensitive set of scales.
0:15:46 > 0:15:54Now, the scales say that this piece of metal weighs 368.7025/4.
0:15:54 > 0:15:57It's kind of flickering between the two, it's so sensitive.
0:15:57 > 0:16:01Now, let's take this piece of metal to the top of this block of flats
0:16:01 > 0:16:03and see how much it weighs up there.
0:16:11 > 0:16:17Now, up here, the metal weighs 368.6 9 grams,
0:16:17 > 0:16:20so I seem to have lost ten milligrams.
0:16:20 > 0:16:22But of course the mass hasn't changed,
0:16:22 > 0:16:24what's changed is the gravity.
0:16:24 > 0:16:26I've got less gravity up here
0:16:26 > 0:16:29than I have got down at the bottom of the block of flats.
0:16:30 > 0:16:32Simply put, mass is measuring
0:16:32 > 0:16:35the amount of stuff there is inside here,
0:16:35 > 0:16:38and that doesn't change whether I'm at sea level or out in space.
0:16:38 > 0:16:39But the weight does.
0:16:42 > 0:16:45In one simple equation,
0:16:45 > 0:16:50Newton's genius revolutionised how we thought about weight and mass.
0:16:54 > 0:16:58'But it would take a real revolution in France to finally create
0:16:58 > 0:17:02the measure of mass that we all use today - the kilogram.
0:17:07 > 0:17:10By the middle of the 18th century, weight measurement,
0:17:10 > 0:17:14like length, was in a total mess and nobody had it worse than the French.
0:17:14 > 0:17:17People were supposed to use the King's measures
0:17:17 > 0:17:19for pounds and ounces.
0:17:19 > 0:17:22But, in reality, every village and town had their own system,
0:17:22 > 0:17:24all slightly different.
0:17:24 > 0:17:27Disputes and arguments were so commonplace that the village took
0:17:27 > 0:17:31to chaining the weights and measures to the wall of the local church.
0:17:35 > 0:17:39Trade was painfully slow and open to corruption,
0:17:39 > 0:17:43and no-one could agree on whose weight was right.
0:17:45 > 0:17:49A new international system of measurement was urgently needed.
0:17:51 > 0:17:54Letters flew between the powers of Europe.
0:17:57 > 0:18:00"Too long have Great Britain and France been at variance
0:18:00 > 0:18:04"with each other, for empty honour or guilty interests.
0:18:04 > 0:18:08"It is time that two free nations should unite their exertions
0:18:08 > 0:18:13"for the promotion of a discovery that must be useful to mankind."
0:18:15 > 0:18:18On the eve of the French Revolution, the great and good
0:18:18 > 0:18:23of the French scientific community approached the doomed Louis XVI for
0:18:23 > 0:18:28permission to create a new system of length, mass and volume measurement.
0:18:31 > 0:18:36'The greatest minds of the day gathered here at the prestigious
0:18:36 > 0:18:40'Academy of Sciences in Paris to brainstorm a solution.
0:18:46 > 0:18:49'They decided to base their new system on something universal
0:18:49 > 0:18:52'and unchanging - the Earth.'
0:18:55 > 0:18:57It was the birth of metrication.
0:19:00 > 0:19:04The first unit they fixed was the metre,
0:19:04 > 0:19:07basing it on one ten millionth of the distance
0:19:07 > 0:19:10between the North Pole and the Equator.
0:19:12 > 0:19:15The next was the kilogram, and the task was given to the
0:19:15 > 0:19:20father of modern chemistry, Antoine Laurent Lavoisier.
0:19:21 > 0:19:24By day, he was a wealthy tax collector. By night,
0:19:24 > 0:19:27he was the greatest chemist in the land.
0:19:29 > 0:19:31The French visionaries behind the metric system
0:19:31 > 0:19:34wanted all the new measurements to be linked,
0:19:34 > 0:19:37so they came up with an elegant solution.
0:19:38 > 0:19:43The new kilogram was to be equal to the weight of one perfect
0:19:43 > 0:19:47cubic decimetre of water...
0:19:47 > 0:19:48a litre.
0:19:55 > 0:19:57The idea was very simple.
0:19:57 > 0:20:02Anybody with a metre ruler and some water could create their own kilo.
0:20:04 > 0:20:08But making a kilo based on the weight of a cubic decimetre
0:20:08 > 0:20:12of water turned out to be much more difficult than they thought.
0:20:15 > 0:20:19Now, I've got two perfect decimetres of water here.
0:20:19 > 0:20:22The trouble is, these don't weigh the same amount.
0:20:22 > 0:20:28The colder water weighs 998 grams, whilst the hotter water
0:20:28 > 0:20:32is 957 grams.
0:20:32 > 0:20:37'Because the hotter water is, the less dense it is.'
0:20:37 > 0:20:40And that's the trouble, the weight depends on the temperature.
0:20:40 > 0:20:43Not only that, it will depend on what impurities are inside the
0:20:43 > 0:20:47water, what the atmospheric pressure is, how far I am above sea level.
0:20:47 > 0:20:50There's a real problem with trying to define the kilo
0:20:50 > 0:20:53based on the weight of water.
0:20:56 > 0:20:59Lavoisier came close to solving the problem of how to accurately
0:20:59 > 0:21:01weigh water.
0:21:01 > 0:21:05But his brilliant career met an abrupt end
0:21:05 > 0:21:09at the hands of the guillotine on the 8th May 1794.
0:21:11 > 0:21:14His tax-collecting day job was his downfall.
0:21:19 > 0:21:23Next to take up the kilo challenge were scientists
0:21:23 > 0:21:26Louis Lefevre-Gineau and Giovanni Fabbronni.
0:21:28 > 0:21:32Four years later, they finally perfected how to measure
0:21:32 > 0:21:36a cubit decimetre of distilled water.
0:21:37 > 0:21:41A master metal kilogram could finally be cast.
0:21:43 > 0:21:46And on the 22nd June, 1799,
0:21:46 > 0:21:49they presented their prototype kilogram to the nation.
0:21:49 > 0:21:52Called the "kilogram des archives",
0:21:52 > 0:21:55it was made out of the new wonder metal, platinum.
0:21:55 > 0:21:59Soon, kilogram clones, as well as copies of the metre bar,
0:21:59 > 0:22:02were being sent to villages and towns across the nation
0:22:02 > 0:22:05to bring uniformity to the French Empire.
0:22:09 > 0:22:11Their vision was brilliant.
0:22:13 > 0:22:14But there was a flaw.
0:22:18 > 0:22:22The trouble was that pure platinum, although resistant to air and water,
0:22:22 > 0:22:25is actually rather soft and prone to damage.
0:22:25 > 0:22:29And that meant bits were easily knocked off,
0:22:29 > 0:22:33gradually rendering the hundreds of cloned kilos inaccurate.
0:22:36 > 0:22:39'The Academy's grand idea was slowly being eroded.
0:22:41 > 0:22:44'It would take nearly 70 years to realise a new,
0:22:44 > 0:22:49'more stable master kilo. And then a set of clones would be needed.'
0:22:49 > 0:22:53London metallurgists Johnson Matthey were given the order to
0:22:53 > 0:22:58produce 250 kilograms of platinum mixed with strength-giving iridium.
0:23:02 > 0:23:06It was a big order, worth £2.2 million at today's prices.
0:23:09 > 0:23:12The man in charge of production, George Mathey,
0:23:12 > 0:23:15the world's leading expert in casting platinum, offered to
0:23:15 > 0:23:18make the kilos at his state-of-the-art furnaces
0:23:18 > 0:23:19at Hatton Garden.
0:23:22 > 0:23:26But French pride intervened, insisting it happened here,
0:23:26 > 0:23:29at the Conservatoire in Paris.
0:23:32 > 0:23:35It was a disaster. The platinum got contaminated by iron,
0:23:35 > 0:23:37rendering the whole consignment useless.
0:23:37 > 0:23:41It was a huge embarrassment, both for French pride and their pockets.
0:23:47 > 0:23:51'But it wasn't the death of the kilo, or the metric system.
0:23:53 > 0:23:57'With international trade booming, the benefits of having one
0:23:57 > 0:24:00'common measurement system were clear for all to see.
0:24:00 > 0:24:08'And in 1875, diplomats from 17 countries met here in Paris
0:24:08 > 0:24:11'and agreed to formally adopt the metric system.'
0:24:13 > 0:24:17With great zeal, a new kilogram master was commissioned.
0:24:19 > 0:24:22The order once again went to Johnson Matthey,
0:24:22 > 0:24:26and this time George Matthey was finally allowed
0:24:26 > 0:24:31to cast the most accurate platinum and iridium kilo ever made.
0:24:34 > 0:24:38Christened "Le Grand K", it was consigned to a specially-made vault
0:24:38 > 0:24:43at a newly established international centre of measurement outside Paris.
0:24:49 > 0:24:53And here it is - the Bureau Internationale des Poids et Mesures.
0:24:53 > 0:24:55The BIPM. In English,
0:24:55 > 0:24:57the International Bureau of Weights and Measures.
0:24:57 > 0:25:00And this is really international territory.
0:25:00 > 0:25:03It's kind of a mark of how important measurement
0:25:03 > 0:25:07is to the world that we've created a UN of measurement.
0:25:11 > 0:25:15'From the beginning, the BIPM's mission was to make sure
0:25:15 > 0:25:18'measurements were consistent throughout the world.
0:25:20 > 0:25:22'This is the building that was once home
0:25:22 > 0:25:26'to all the world's master measurements.'
0:25:30 > 0:25:33Today, most have been retired,
0:25:33 > 0:25:38replaced by new definitions based on the universal and unchanging
0:25:38 > 0:25:41laws of nature, like the speed of light...
0:25:43 > 0:25:45..and the movement of atoms.
0:25:46 > 0:25:52Le Grand K is in fact the only artefact that is still in use.
0:25:52 > 0:25:55A measurement dinosaur.
0:26:06 > 0:26:11Today, here at the BIPM, they're still making clones of that Grand K.
0:26:11 > 0:26:14Fabrice here is polishing this until it exactly matches
0:26:14 > 0:26:19the mass of the Grand K sitting in the vault downstairs.
0:26:20 > 0:26:23'Over half the countries in the world have one of these clones.'
0:26:23 > 0:26:27The next one he's working on is clone number 103 -
0:26:27 > 0:26:30that's going to go to... Well, we're not actually allowed to know
0:26:30 > 0:26:32where it's going to go.
0:26:32 > 0:26:36'Without Le Grand K, our entire global system of mass
0:26:36 > 0:26:38'and weight measurement would crumble.'
0:26:40 > 0:26:44Unfortunately, "crumble" is a little bit of a touchy word
0:26:44 > 0:26:47inside this building because that's what's happening to Le Grand K.
0:26:47 > 0:26:49I mean, it's not literally crumbling,
0:26:49 > 0:26:52but despite the kid-glove treatment it's received
0:26:52 > 0:26:55over the last 150 years, it's believed that it has changed
0:26:55 > 0:26:59by the equivalent of one grain of sand during its lifetime.
0:26:59 > 0:27:02'And that's bad news,
0:27:02 > 0:27:06'because it no longer matches the weight of the world's clones.
0:27:06 > 0:27:10'A new way to define mass is urgently needed.'
0:27:12 > 0:27:16Now the race is on to replace the definition of the kilo with
0:27:16 > 0:27:19something more fitting for the 21st century -
0:27:19 > 0:27:22something based on a universal constant that can be measured
0:27:22 > 0:27:24wherever you are in the universe.
0:27:27 > 0:27:30We've done it for length - that's now tied to the speed of light...
0:27:32 > 0:27:35..for time - that's related to the movement of electrons in the atom.
0:27:37 > 0:27:39Now we want to do it for the kilo.
0:27:42 > 0:27:46It's a head-to-head race between two international teams.
0:27:48 > 0:27:51Each one taking a radically different approach to solving
0:27:51 > 0:27:53the kilo crisis.
0:27:54 > 0:27:58In America, Team Watt Balance are combining the power
0:27:58 > 0:28:04of electricity with scales whose principles date back 5,000 years.
0:28:06 > 0:28:10Their dream? To redefine the kilo based on energy.
0:28:12 > 0:28:166,000 kilometres away in Germany, Team Silicon Sphere
0:28:16 > 0:28:21are trying to count every single atom in a perfect ball of silicon.
0:28:25 > 0:28:28It's an immense task - like covering the Earth in sand
0:28:28 > 0:28:31and trying to count every single granule.
0:28:33 > 0:28:38As the best minds in measurement science fight it out,
0:28:38 > 0:28:42Le Grand K's long and illustrious career could soon be over,
0:28:42 > 0:28:45but its legacy has been staggering.
0:28:53 > 0:28:57From the moment it was adopted, the movement and sale of goods
0:28:57 > 0:29:00became much easier and more efficient.
0:29:02 > 0:29:06The scientific community jumped on the new metric system,
0:29:06 > 0:29:10loving its simplicity and the ease they could split or multiply
0:29:10 > 0:29:12the metre and the kilogram by ten.
0:29:20 > 0:29:23But from the very beginning of its life in the 18th century,
0:29:23 > 0:29:25the public remained less convinced.
0:29:28 > 0:29:32People were just not interested in revolutionising their everyday
0:29:32 > 0:29:36life - what they did when they went shopping, how they exchanged
0:29:36 > 0:29:40and bought - in the name of revolutionary purity.
0:29:42 > 0:29:44The kilo continues to divide opinion.
0:29:48 > 0:29:51In the UK, it was only adopted in the 1960s
0:29:51 > 0:29:55and its arrival was met with outright hostility.
0:29:57 > 0:30:01All we ask is the freedom of choice to record in the native
0:30:01 > 0:30:05and still legal measures of this country instead of these
0:30:05 > 0:30:07cock-eyed kilograms, which make no sense at all.
0:30:07 > 0:30:12But despite the opposition, today all but three nations -
0:30:12 > 0:30:15the United States, Liberia and Myanmar -
0:30:15 > 0:30:17have embraced the kilo and the metric system.
0:30:29 > 0:30:33While the world was moving towards a unified weight measurement
0:30:33 > 0:30:37system, the actual technology of weighing was now lagging behind.
0:30:40 > 0:30:43Variations on ancient Mesopotamian and Egyptian beam balances
0:30:43 > 0:30:48remained our scales of choice right up to the 19th century.
0:30:50 > 0:30:54The problem was they took so long to use.
0:30:55 > 0:31:01In the UK, weighing was made much worse by the Turnpike Act of 1752.
0:31:03 > 0:31:08Eager to tax the movement of goods, the government ordered all towns
0:31:08 > 0:31:14to "erect a crane machine or engine for the weighing carts and wagons."
0:31:15 > 0:31:19At each location, carts had to be unloaded, weighed,
0:31:19 > 0:31:22reloaded and weighed once again.
0:31:24 > 0:31:29And to the add to the daily misery, every key road demanded tolls, too.
0:31:29 > 0:31:32All payable on the weight you were carrying.
0:31:35 > 0:31:39With the birth of the Industrial Revolution, things had to change.
0:31:39 > 0:31:43Factories to forges now needed raw materials
0:31:43 > 0:31:46in unprecedented quantities.
0:31:46 > 0:31:48And they had to be weighed, bought and transported
0:31:48 > 0:31:52with ever-increasing speed and precision.
0:31:57 > 0:32:02A faster, more efficient means of weighing was desperately needed.
0:32:03 > 0:32:06The solution was the weighbridge.
0:32:09 > 0:32:11A technological triumph, the weighbridge,
0:32:11 > 0:32:15with its balance scale hidden beneath the floor, would play
0:32:15 > 0:32:19a key role in driving our industrial revolution onwards.
0:32:22 > 0:32:25Now, loads could be weighed in seconds as they rolled on
0:32:25 > 0:32:27and off the bridge.
0:32:29 > 0:32:32But it would take electricity to drive the next big
0:32:32 > 0:32:35breakthrough in weighing.
0:32:38 > 0:32:40Inventor Charles Wheatstone
0:32:40 > 0:32:44championed the use of electricity in the 1840s.
0:32:46 > 0:32:49Experimenting with simple electrical circuits,
0:32:49 > 0:32:53he devised a way of measuring electrical resistance.
0:32:53 > 0:32:57But it wasn't until a century later that people realised this
0:32:57 > 0:33:01very same technology could be used to measure weight.
0:33:08 > 0:33:12Today, the need for speedy mass measurement drives our world.
0:33:17 > 0:33:21This train is delivering coal to Rugeley Power Station,
0:33:21 > 0:33:25and, as it runs over the track, it's being weighed by load cells,
0:33:25 > 0:33:26which are underneath the track.
0:33:26 > 0:33:31And if we come in here, we can see how much we've weighed so far.
0:33:35 > 0:33:37- So, hi, Andy.- Hi.
0:33:37 > 0:33:40So, this is the first carriage that's gone over,
0:33:40 > 0:33:42so we've got 100 tonnes.
0:33:42 > 0:33:46- Yeah.- So it's much more efficient than weighing by hand.
0:33:46 > 0:33:47Oh, yeah, very much so.
0:33:47 > 0:33:50We can measure at 70 kilometres per hour,
0:33:50 > 0:33:53so we're talking less than a second per wagon, probably.
0:33:53 > 0:33:55Wow, that's extraordinary.
0:33:59 > 0:34:02So how's this piece of track actually weighing the train?
0:34:02 > 0:34:05Well, underneath the track are several of these.
0:34:05 > 0:34:08They're called load cells. And, actually,
0:34:08 > 0:34:11it's this little system of wires on the rod which is doing the weighing.
0:34:11 > 0:34:14But as soon as something runs over the track,
0:34:14 > 0:34:18it compresses the rod and the wires get shorter and fatter.
0:34:18 > 0:34:20The resistance goes down,
0:34:20 > 0:34:23and I get more electrical current running through it.
0:34:23 > 0:34:25And suddenly I'm getting a reading.
0:34:25 > 0:34:28What's amazing is there's a direct mathematical relationship between
0:34:28 > 0:34:33the increase in electrical current and the weight going over the wires.
0:34:33 > 0:34:36So we're using electricity to weigh the train.
0:34:36 > 0:34:39In fact, this thing is so sensitive that even if I step on it,
0:34:39 > 0:34:42I actually can get how much I weigh.
0:34:42 > 0:34:44So let's see.
0:34:45 > 0:34:47So how much do I weigh, Andy?
0:34:47 > 0:34:4984.
0:34:49 > 0:34:51- 84 kilos?!- Yeah.
0:34:51 > 0:34:54I don't weigh 84 kilos. Must be the weight of this...
0:34:59 > 0:35:04Today, load cells are used the world over.
0:35:07 > 0:35:11We've come a long way since the days of the beam balance.
0:35:11 > 0:35:14Now, everywhere, from roadside weigh stations
0:35:14 > 0:35:16to supermarket checkouts, use them.
0:35:16 > 0:35:21Measuring mass with electricity has changed our world.
0:35:21 > 0:35:23We can now weigh, transport
0:35:23 > 0:35:28and deliver billions of tonnes-worth of produce with a speed and accuracy
0:35:28 > 0:35:32our Victorian forefathers would never have dreamt possible.
0:35:32 > 0:35:37Precision mass measurement is key to world commerce.
0:35:41 > 0:35:45Now, it's the turn of the very small to push the limits
0:35:45 > 0:35:47of mass measurement.
0:35:49 > 0:35:53Here in America, I've come to meet a team who've come up with
0:35:53 > 0:35:56a unique approach to measuring some of the smallest living
0:35:56 > 0:35:59things on Earth...cells.
0:36:08 > 0:36:10'Project leader Scott Manalis
0:36:10 > 0:36:14'is using mass to monitor the growth of cells.
0:36:14 > 0:36:18'His work could one day revolutionise our fight
0:36:18 > 0:36:19'against cancer.
0:36:21 > 0:36:26'In his lab, he has built the world's smallest weighing station.
0:36:28 > 0:36:32'Here, inside a microchip just millimetres in size,
0:36:32 > 0:36:35'cells are captured and passed over a sensor.'
0:36:37 > 0:36:40The long, thin section highlighted here,
0:36:40 > 0:36:43acts a bit like a diving board.
0:36:43 > 0:36:45When a cell passes over it,
0:36:45 > 0:36:50it vibrates just like a diving board moves after a diver jumps off it.
0:36:50 > 0:36:52The speed of the vibration
0:36:52 > 0:36:56is directly linked to the weight of the cell.
0:36:56 > 0:36:58So, using simple maths,
0:36:58 > 0:37:01Scott can measure the cell with incredible accuracy.
0:37:01 > 0:37:04This cell is the equivalent of like a white blood cell,
0:37:04 > 0:37:06- in terms of its size.- OK.
0:37:06 > 0:37:08And it weights 100 picograms.
0:37:08 > 0:37:11- Picograms, so that's ten... - To the minus 12.
0:37:11 > 0:37:14All right, OK. So that's a lot of zeros.
0:37:14 > 0:37:16So this is incredibly small.
0:37:16 > 0:37:18So the cell doesn't weigh very much.
0:37:18 > 0:37:21And the precision with which we can weight it with is
0:37:21 > 0:37:23four orders of magnitude below that.
0:37:23 > 0:37:26- Wow, that's incredible. - So that's ten femtograms...
0:37:26 > 0:37:28So a part in a thousand.
0:37:28 > 0:37:31- One part in 10,000.- 10,000!
0:37:31 > 0:37:33We care a lot about these things.
0:37:35 > 0:37:38'We're soon in the domain of extreme numbers,
0:37:38 > 0:37:41'but what's amazing is Scott's measuring the weight
0:37:41 > 0:37:46'of a single cell to within a thousand trillionth of a gram.
0:37:46 > 0:37:51'His work is revolutionising our understanding of how cells grow.
0:37:51 > 0:37:56'And by measuring how cells respond to a drug, it could lead to
0:37:56 > 0:38:00'personalised and far more effective cancer treatment.'
0:38:04 > 0:38:09It's absolutely amazing, the limits we are now pushing mass measurement.
0:38:09 > 0:38:11But scientists are frustrated.
0:38:11 > 0:38:16And it's because we're still trying to tie mass back to that
0:38:16 > 0:38:20ageing lump of metal in Paris, Le Grand K.
0:38:20 > 0:38:24And with Le Grand K's weight unstable, there's a real
0:38:24 > 0:38:30urgency to find a new even more accurate way to define mass.
0:38:31 > 0:38:35Now, a race is being fought across two continents to retire Le Grand K.
0:38:49 > 0:38:53'20 miles north of Washington is one of the world's most
0:38:53 > 0:38:55'accurate sets of scales.'
0:38:57 > 0:39:01This whole area is a car-free zone, and that's because the scales
0:39:01 > 0:39:05that are being used here are so sensitive that even the magnetic
0:39:05 > 0:39:09field caused by the metal inside the cars can affect the measurements.
0:39:09 > 0:39:11Welcome to Team Watt Balance.
0:39:14 > 0:39:18'Most things in this strange-looking building are made of wood,
0:39:18 > 0:39:24'and clad in vinyl to minimise the effects of magnetism.
0:39:24 > 0:39:28'Everything from the power lines to the plumbing pipes
0:39:28 > 0:39:31'are encased in shielded plastic ducts.
0:39:31 > 0:39:35'And every single bit of metal that enters the lab, down to this
0:39:35 > 0:39:40'tiny spare part, has to be checked for its levels of magnetism.
0:39:52 > 0:39:56'Stephan Schlamminger's project is one of the longest-running
0:39:56 > 0:39:59'metrology experiments in the world.
0:39:59 > 0:40:02'Its founders have long since retired,
0:40:02 > 0:40:05'but now the team here are close to fulfilling their dream.'
0:40:07 > 0:40:11And this is their brainchild. The watt balance.
0:40:25 > 0:40:30Inside this cage of pure copper is a weighing scale whose
0:40:30 > 0:40:35principles go back to the very first balances 5,000 years ago.
0:40:37 > 0:40:40And it's so sensitive it can measure the kilo
0:40:40 > 0:40:42to eight decimal places.
0:40:44 > 0:40:46So here's our watt balance.
0:40:46 > 0:40:48It is a thing of beauty.
0:40:48 > 0:40:49It really is.
0:40:49 > 0:40:53And you see up here this wheel is like the old-fashioned beam balance.
0:40:53 > 0:40:55That's quite ancient technology, isn't it?
0:40:55 > 0:40:58Yeah, it's thousand-year-old technology up on top,
0:40:58 > 0:41:00but down here you will see the coil that's connected
0:41:00 > 0:41:03to three rods, and this will provide the counterforce
0:41:03 > 0:41:06to the gravitational force that this mass is providing.
0:41:06 > 0:41:08'On one side of the scales,
0:41:08 > 0:41:13'deep inside the mechanism, sits a clone of the Le Grand K.
0:41:13 > 0:41:17'What's so extraordinary about this device is that on the other side,
0:41:17 > 0:41:21'instead of a weight, the team are using electrical force
0:41:21 > 0:41:24'to counterbalance it.'
0:41:24 > 0:41:27The watt balance defines the kilogram by linking
0:41:27 > 0:41:29mechanical power to electrical power.
0:41:29 > 0:41:31- That's why it's called the watt balance.- Right.
0:41:31 > 0:41:34'Their goal is to measure the amount of electricity needed to
0:41:34 > 0:41:38'perfectly counterbalance the kilo clone
0:41:38 > 0:41:42'and redefine the kilogram, based on electrical power.'
0:41:45 > 0:41:47It sounds straightforward,
0:41:47 > 0:41:50but when you are working with one of the most sensitive
0:41:50 > 0:41:54scales in the world, everything, from car engines to the movement
0:41:54 > 0:41:59of the local deer population outside, can affect its readings.
0:42:00 > 0:42:03Even tiny shifts in gravity, like the phase of the moon
0:42:03 > 0:42:07and the level of ground water, need to be measured
0:42:07 > 0:42:10and taken into account when this experiment is running.
0:42:13 > 0:42:17It seems you're having to keep track of so many different things in order
0:42:17 > 0:42:19- to pin down that kilo. - That is the art.
0:42:19 > 0:42:23That's the art and science of this! Amazing.
0:42:23 > 0:42:28So we try to measure this kilo to about four parts per 100 million,
0:42:28 > 0:42:31and, in order to do so, we need to measure all these
0:42:31 > 0:42:36auxiliary qualities like voltage, resistance, gravity, metre,
0:42:36 > 0:42:41second, to much better than four parts per hundred million.
0:42:42 > 0:42:47Now, after more than 30 years of perfecting the scale's accuracy,
0:42:47 > 0:42:51Team Watt Balance are very close to achieving their holy grail -
0:42:51 > 0:42:54a new electronic kilogram.
0:43:06 > 0:43:09'I left the watt balance team realising I was witnessing
0:43:09 > 0:43:13'a potentially historic moment in the life of the kilogram.'
0:43:23 > 0:43:27The days of the American kilo making its transatlantic journey
0:43:27 > 0:43:32to Paris to be compared against Le Grand K are probably numbered.
0:43:32 > 0:43:35But the watt balance team have got a rival.
0:43:35 > 0:43:39In Germany, Team Silicon Sphere have got a completely different
0:43:39 > 0:43:41approach to redefining the kilo.
0:43:41 > 0:43:44And it involves counting the number of atoms in a kilogram
0:43:44 > 0:43:46of silicon crystal.
0:43:49 > 0:43:53People often talk about counting the number of grains of sand on a beach.
0:43:53 > 0:43:56But what Team Silicon Sphere are proposing to do
0:43:56 > 0:43:58is in completely different league.
0:43:58 > 0:44:02It's like trying to cover the whole globe in sand
0:44:02 > 0:44:04and counting every grain.
0:44:08 > 0:44:12'But what are these atoms they're trying to count?'
0:44:14 > 0:44:17It was the Ancient Greeks who first came up with the word "atom"
0:44:17 > 0:44:21to define the smallest indivisible particle of matter.
0:44:22 > 0:44:27But it took Englishman John Dalton in the 19th century to shed
0:44:27 > 0:44:29light on what atoms really are.
0:44:31 > 0:44:34At the time, we knew that all matter was made up of different
0:44:34 > 0:44:37elements like carbon and oxygen.
0:44:37 > 0:44:42Dalton's brilliance was a radical theory that each element must
0:44:42 > 0:44:46consist of atoms of a single unique type and mass.
0:44:49 > 0:44:52Dalton would never have dreamt it possible to see
0:44:52 > 0:44:54or count these atoms...
0:44:57 > 0:45:00..but now, in a remote lab in Northern Germany,
0:45:00 > 0:45:04scientists are attempting to do just that.
0:45:11 > 0:45:16'What Dalton didn't realise is the sheer number of atoms inside things.
0:45:16 > 0:45:19'That there are trillion upon trillion inside a single
0:45:19 > 0:45:22'kilo of silicon.
0:45:23 > 0:45:26'And it's by counting these atoms
0:45:26 > 0:45:29'that the silicon sphere team hope to redefine the kilo.
0:45:35 > 0:45:40'This is a perfect kilogram sphere of pure silicon.
0:45:42 > 0:45:44'The culmination of 30 years' work.
0:45:44 > 0:45:48'It represents one of the most ambitious challenges ever to
0:45:48 > 0:45:51'be undertaken in measurement history.
0:45:54 > 0:45:58'Like the watt balance, the silicon sphere project
0:45:58 > 0:45:59'started in the 1970s.'
0:46:03 > 0:46:07The goal was to measure the atomic distances -
0:46:07 > 0:46:12the distance between the atoms in a very perfect crystal.
0:46:12 > 0:46:17Silicon was at that time a material which was used for the semiconductor
0:46:17 > 0:46:23industry, and was the first very perfect material for that use.
0:46:26 > 0:46:29Silicon atoms line up in an extremely rigid
0:46:29 > 0:46:34and regular pattern, which in theory makes them easier to count.
0:46:37 > 0:46:42The idea was to create a perfect sphere of silicon,
0:46:42 > 0:46:45measure its dimensions with extreme precision,
0:46:45 > 0:46:49and then calculate the spaces between the atoms
0:46:49 > 0:46:53using a technique called X-ray crystallography.
0:46:54 > 0:46:57'Then, using simple maths,
0:46:57 > 0:47:01'they could work out the total number of atoms in the sphere.'
0:47:03 > 0:47:07The project was supposed to take a couple of years,
0:47:07 > 0:47:10but they faced many challenges.
0:47:12 > 0:47:16The first was how to create a perfect sphere.
0:47:17 > 0:47:20The levels of perfection the team were seeking
0:47:20 > 0:47:23were beyond the capabilities of any machine.
0:47:25 > 0:47:30They scoured the globe and found the only way to create a sphere
0:47:30 > 0:47:34to the level of perfection they needed was to do it by hand.
0:47:37 > 0:47:40And only one man was capable of this.
0:47:40 > 0:47:43Australian lens maker Achim Leisner.
0:47:46 > 0:47:50He literally used his hands to shape the ball to such an incredible
0:47:50 > 0:47:54level of perfection that, if you likened it to the Earth,
0:47:54 > 0:48:00the level of its surface would never vary more than a few metres.
0:48:00 > 0:48:05Using his extraordinary sense of touch, it's said Achim could
0:48:05 > 0:48:09feel silicon's atomic structure with his fingertips.
0:48:09 > 0:48:12You need really...
0:48:12 > 0:48:15a feeling how many atoms you have to remove on that side or
0:48:15 > 0:48:20on the other side of the sphere, so he had atomic feeling in his hands.
0:48:22 > 0:48:26It took months for Achim to perfect his sphere.
0:48:28 > 0:48:31Finally, the task of analysing
0:48:31 > 0:48:34the space between the silicon atoms could begin.
0:48:35 > 0:48:39But, on the cusp of realising their dream, disaster struck.
0:48:41 > 0:48:44There was a flaw in the very make up of the silicon.
0:48:47 > 0:48:51In its natural state, silicon consists of three different
0:48:51 > 0:48:53forms called isotopes.
0:48:53 > 0:48:56Now, each different atom has a different mass.
0:48:56 > 0:49:01Leisner's sphere contained all three different types of these atoms.
0:49:02 > 0:49:05The team needed a pure source of silicon,
0:49:05 > 0:49:08or else the project was over.
0:49:08 > 0:49:12The solution came from an unlikely source.
0:49:15 > 0:49:18A nuclear weapons facility.
0:49:20 > 0:49:25The Cold War was over and a lot of centrifuge in Russia
0:49:25 > 0:49:30were not running for nuclear weapons,
0:49:30 > 0:49:33so we were lucky to rent
0:49:33 > 0:49:39some of this centrifuge to prepare silicon for our purpose.
0:49:42 > 0:49:44A new batch of silicon was sent to Russia
0:49:44 > 0:49:49and spun in the same centrifuge that was formerly used to enrich uranium.
0:49:50 > 0:49:54This forced out the wayward extra isotopes,
0:49:54 > 0:49:58producing pure silicon-28.
0:50:00 > 0:50:04Then Leisner had to start the job of polishing all over again.
0:50:06 > 0:50:08Finally, after many years,
0:50:08 > 0:50:11the scientists once again started counting
0:50:11 > 0:50:13the space between the atoms.
0:50:17 > 0:50:22And, trillions of atoms later, they've nearly completed their task.
0:50:25 > 0:50:29We hope that in two years, we will have all the information
0:50:29 > 0:50:34together for a new definition that means we have a value
0:50:34 > 0:50:36with a very small uncertainty -
0:50:36 > 0:50:40let us say below two times ten to minus eight.
0:50:40 > 0:50:44And that's an accuracy to eight decimal places.
0:50:46 > 0:50:48It's the same level of precision as Team Watt Balance
0:50:48 > 0:50:51in America are striving for.
0:50:54 > 0:51:01At the moment, we are in the pole position to win this race.
0:51:02 > 0:51:05Within a few years, Le Grand K could be retired.
0:51:05 > 0:51:08But the work here could revolutionise
0:51:08 > 0:51:13another of the seven fundamental units we use to describe our world.
0:51:27 > 0:51:30Ein kaffee mit milch, bitte. Danke.
0:51:31 > 0:51:36If the silicon team are successful, then they won't just redefine
0:51:36 > 0:51:39the kilo, they could end up redefining the SI unit most
0:51:39 > 0:51:43feared by chemistry students across the world - the mole.
0:51:44 > 0:51:49'It's a word which comes from Latin meaning "massive heap of material."'
0:51:52 > 0:51:54Now, chemists probably won't like this,
0:51:54 > 0:51:56but consider this cup of coffee.
0:51:56 > 0:52:01There's a certain ratio of milk to coffee, say one part milk,
0:52:01 > 0:52:04to nine parts coffee, which, combined,
0:52:04 > 0:52:08makes one part perfect milky coffee.
0:52:08 > 0:52:11Now the mole does a similar thing for chemists,
0:52:11 > 0:52:15but replace the coffee and the milk with atoms and molecules.
0:52:17 > 0:52:18Yep, perfect!
0:52:20 > 0:52:23All this leads back to our friend Dalton,
0:52:23 > 0:52:25and his work in the 19th century.
0:52:25 > 0:52:29When he began his investigation into atoms, he discovered that
0:52:29 > 0:52:33atoms from different elements weighed different amounts.
0:52:33 > 0:52:35At the centre of every atom
0:52:35 > 0:52:38is a nucleus containing protons and neutrons.
0:52:38 > 0:52:42Different elements have different numbers of these protons
0:52:42 > 0:52:46and neutrons, which is why they weigh different amounts.
0:52:51 > 0:52:54Throughout the 19th century,
0:52:54 > 0:52:57the greatest chemists of the day feverishly tried to work out
0:52:57 > 0:53:00the atomic weights of all the known elements.
0:53:00 > 0:53:03It led to one of science's greatest ever achievements -
0:53:03 > 0:53:08Dmitri Menedeleev's periodic table.
0:53:11 > 0:53:13And if you look at each element on that table,
0:53:13 > 0:53:17you'll see their atomic mass written just below them.
0:53:19 > 0:53:21It was a huge breakthrough.
0:53:21 > 0:53:23Chemists could finally mix
0:53:23 > 0:53:26and manipulate elements with new-found precision.
0:53:28 > 0:53:33But atoms are far too small to look at and manipulate individually.
0:53:35 > 0:53:40What chemists needed was a way of scaling up atomic weight into
0:53:40 > 0:53:42something more tangible they could weigh.
0:53:42 > 0:53:45And the answer was the mole.
0:53:47 > 0:53:50The mole is really just a big number.
0:53:50 > 0:53:51A huge number, in fact,
0:53:51 > 0:53:54which, when you combine it with the atomic weight of each element,
0:53:54 > 0:53:58allows you to work out how many atoms there are inside something.
0:53:58 > 0:54:02It's the chemist's way of scaling up the microscopic
0:54:02 > 0:54:05world of the atom to our world of the gram.
0:54:05 > 0:54:09It's really the bedrock of modern chemistry, allowing us
0:54:09 > 0:54:13to mix things from drugs to fuel with such precision.
0:54:13 > 0:54:15But it leaves open one big question.
0:54:15 > 0:54:19Exactly how many atoms are there inside a mole?
0:54:23 > 0:54:26The number of atoms that we have in a mole is what
0:54:26 > 0:54:28we call Avogadro's number.
0:54:28 > 0:54:31We can go back to Einstein, for instance, in 1905.
0:54:31 > 0:54:34He came up with one of the first estimates of just how big
0:54:34 > 0:54:38this number is from looking down microscopes at pollen grains and
0:54:38 > 0:54:42from that he was able to get one of our first estimates of the number.
0:54:42 > 0:54:44He got the first number right.
0:54:44 > 0:54:47He got the six right, and he got the 23 zeros right.
0:54:50 > 0:54:53While Einstein's groundbreaking work got close to defining
0:54:53 > 0:54:59the elusive Avogadro's number, it's the silicon sphere team that
0:54:59 > 0:55:02could not only solve the kilo conundrum,
0:55:02 > 0:55:05but also solve the centuries-old question of how many atoms
0:55:05 > 0:55:07there are in a mole...
0:55:10 > 0:55:14..and once and for all define Avogadro's number.
0:55:14 > 0:55:16If this happens,
0:55:16 > 0:55:20it will be a remarkable moment in measurement history.
0:55:20 > 0:55:22In one astonishing experiment,
0:55:22 > 0:55:26two golden units of measurement could be redefined.
0:55:26 > 0:55:30We've come a long way since the days of using barley corn weights.
0:55:30 > 0:55:32Our quest for ever greater precision
0:55:32 > 0:55:36has led us into the very fabric of our universe, allowing us to weigh
0:55:36 > 0:55:40and analyse things with incredible speed, scale and precision.
0:55:40 > 0:55:44In a few years' time, all going well, the BIPM will
0:55:44 > 0:55:48decide between atoms or electrical force to redefine the kilo.
0:55:48 > 0:55:51The winner is kind of irrelevant.
0:55:51 > 0:55:53Both Team Watt Balance
0:55:53 > 0:55:55and Silicon Ball have done what seemed impossible -
0:55:55 > 0:56:00to redefine the kilo based on the unchanging laws of the universe.
0:56:01 > 0:56:06In the pursuit of ever-greater accuracy, these remarkable projects
0:56:06 > 0:56:10have brought together thousands of years of scientific endeavour.
0:56:12 > 0:56:17But our quest for ever greater precision doesn't stop here.
0:56:22 > 0:56:25The last great measurement frontier will be to journey
0:56:25 > 0:56:31inside atoms themselves, to discover what mass really is.
0:56:42 > 0:56:45100 metres under the Swiss-French border,
0:56:45 > 0:56:48at CERN's particle accelerator, scientists think
0:56:48 > 0:56:52they have discovered a particle that gives things mass -
0:56:52 > 0:56:53the Higgs boson.
0:56:53 > 0:56:58And one day, our human desire for ever greater precision
0:56:58 > 0:57:01may even see mass redefined once more,
0:57:01 > 0:57:05and tied to Higgs itself.
0:57:07 > 0:57:11If it happens, who knows what the technological impacts will be?
0:57:11 > 0:57:13And that's the beauty of measurement.
0:57:13 > 0:57:17Every leap in precision leads to new scientific
0:57:17 > 0:57:19and technological advances.
0:57:19 > 0:57:21Measurement has shaped our history,
0:57:21 > 0:57:24and will continue to change our world.
0:57:39 > 0:57:42'Next, we explore the world of energy.
0:57:44 > 0:57:48'And how the measurement of light, heat and electricity
0:57:48 > 0:57:50'have transformed our lives
0:57:50 > 0:57:54'as I continue my journey into measurement.'