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00:00on the morning of the 14th of June 1940 several German tank divisions rumbled
00:08through the streets of Paris the impossible had happened Germany had
00:14invaded and France had fallen but there was one building on the outskirts of
00:28Paris that the Nazis never occupied this chateau has the same status as an
00:37independent territory its contents are so closely guarded I have to hand over my
00:44passport to gain access today an eminent group of scientists have gathered from
00:53all over the world to witness a very special event
01:03security is tight with key holders arriving from three different countries
01:08the vault holds one of the most important artifacts in our world this is a real piece of measurement
01:20history well not really history at all this is the kilo under three layers of protective glass is the
01:30kilogram master known as Le Grand Cay it's the weight on which all weights have been based since 1889 its
01:42importance is so great that neither the Nazis nor the liberating American forces dared set foot inside here
01:49and the reason we're here today well just to check it's still here but there's a problem tests have
02:00revealed that Le Grand Cay this scientific celebrity is losing weight creating a crisis in the scientific
02:09world it's losing approximately 20 billionths of a gram every year but why on earth should such a tiny change matter so much
02:23I'm on a journey to investigate the world of measurement and to see how our drive for precision has really changed the course of history
02:35today we can describe the chaos and complexity of the universe with just seven fundamental units the
02:46building blocks of modern science and science is obsessed with defining these units with ever greater
02:53precision in this series I want to understand why such extreme levels of precision are so important how we
03:02define these units and how through history each step forward has unleashed a technological revolution
03:13in this program we'll explore why being able to measure weight is so important
03:20and how the race to replace the aging Grand Cay might hold the key to a new way of understanding our world
03:32this is the story of how we mastered weight
03:39how much do I have is a question that has driven trade and commerce since the dawn of civilization
03:57and today weights are still central to all our lives the reason we're so reliant on weights and scales is in part down to our own inability to accurately gauge weight
04:04we tend to believe our eyes rather than trusting the weight in our hands
04:11we tend to believe our eyes rather than trusting the weight in our hands
04:19gauge weight we tend to believe our eyes rather than trusting the weight in our hands
04:29and i've come to london's borough market to prove the point
04:35excuse me wonder whether i could get you to take part a little experiment yeah so i've got a
04:41series of weights here which i've just uh put in order of height what i'd like you to do is to
04:46place the heaviest weight here and the lightest one at your end have a go see which one you think is
04:51the heaviest that's this uh the little guy here okay that's the heaviest okay what about the next
05:03heaviest uh i think this one yeah it's the lightest the lightest of all i think okay now the really
05:13surprising thing is that the one that you put at this end which you think is the lightest
05:18is in fact the heaviest and this one here so you thought this is the heaviest okay i'm going to
05:22give you the both these in your hand um this one is actually heavier than that one do you believe me
05:29well it doesn't feel like no it doesn't let's use the scales so i'm going to weigh the one that you
05:34thought was the lightest um so that comes out about 424 grams okay let's put your one on you think this
05:42one's heavier it's only 345 grams that's extraordinary because even with that knowledge
05:48now try and weigh them again did you say which one's heavier i know absolutely and that's why we
05:55need a set of weights because we're so bad at the perception like any good scientist i carried on
06:03with the testing and my random shoppers to a man and a woman all chose the same two weights and they
06:12all chose wrong okay wow seeing if something is big or small massively skews our perception of how heavy it
06:22is it is a problem our ancestors first started grappling with more than 5 000 years ago
06:33our earliest evidence comes from the middle east and was driven by the emergence of the first cities
06:40in mesopotamia around 3000 bc
06:47as populations grew a way of fairly trading goods was urgently needed
06:53people demanded a system of weight that everyone could trust
06:57taking their inspiration from nature they used grain
07:07uniform in size and shape grain was available to all
07:13the world had its first weights
07:18using simple beam balances which we continue to use today
07:23we started to trade goods based on their weight in grains
07:29it wasn't perfect but with grains varying so little in weight the system worked
07:37it made the movement and sale of goods possible
07:41enabling humans to live together in bigger cities and allowing the first economies to grow
07:47empires were no longer being built solely by armies
07:55they were being built by trade
08:01as commerce developed across the ancient world a faster means of weighing produce is needed
08:06after all if i wanted to buy something that weighed 700 grains of barley i don't want to have
08:12to count out 700 grains each time so gradually a standardized system of weights began to emerge
08:20first the mesopotamians and the ancient egyptians developed stones and things made out of metals and
08:26brass in order to represent different weights of grain
08:32it was such an efficient system that it began to be copied across the civilized world
08:37so here here we have standard weights from china
08:43these hexagons are standard weights used in sudan
08:48and the amazing thing is that despite all of these different weights and measures they were
08:53all related back to the weight of a grain because everyone trusted how much a grain would weigh
08:59by roman times millions of tons of produce were being traded around the world every day
09:12the ability to compare the weights or masses of two different kinds of goods so that you can work
09:19out how to exchange between them that's the key to economic success and so it's the demand for
09:26economic comparison that drives weight standardization throughout history
09:35by the end of the 13th century the world had hundreds of different weights
09:41and nearly all were based on a fixed number of grains
09:47in england we'd inherited the pound from the roman empire
09:50it was initially made up of 12 ounces which were equivalent to 437 grains of barley
10:03but the problem all rulers faced was how to keep weight standardized across a nation
10:11it was considered such a big issue that even the magna carta the most celebrated legal document in english
10:18history tried to deal with it
10:23let there be one measure of wine throughout our whole realm and one measure of ale and one measure of corn
10:33it all sounded great in theory but in practice it was virtually impossible to enforce
10:39cheating was such a big problem regular trials were held to check merchants weights and measures
10:50any found to be wrong were immediately destroyed
10:55accurate scales were the only way cheats could be exposed accuracy was power
11:02scales were not only a great measuring device they also came to symbolize fairness power the very legal system itself
11:33from ancient egypt's feather of truth to the paintings of the great dutch masters scales have featured throughout history
11:47as it was written in the bible by weight measure and number god made all things
12:02in the bible by weight measure measurement has always been associated in culture with justice and law
12:09and crime because what it does is to establish the equivalence between two things that you otherwise
12:18could not compare that's what justice means so it's no coincidence that the figure of justice is shown
12:26carrying scales carrying balance pan
12:31and for centuries when you made a weight measurement you had to show your customers what you were doing
12:38partly to avoid the possibility of deceit but also to show how just you were to be just was precisely to use balance
12:48so with all this moral weightiness flying around the punishment for using false measures could be severe
13:05in 1772 bc the code of hammurabi was introduced into babylonian law
13:10which said that any taverner using false weights could be served up with a death penalty
13:15and in the 18th century bankers caught cheating would have to stand in pillory
13:27and brewers in the dunk cart
13:33but despite the importance we placed on weight and getting it right
13:38it took one remarkable english man to realize the measurement of weight has a fundamental problem
13:45it was the great sir isaac newton who first realized that weight changes depending on where and when you are measuring it
14:05it was 1665 and britain was gripped by the plague so newton decided to flee his college in cambridge
14:12and he came to the safety of his country retreat here at walsthorpe manor
14:17and here is the famous apple tree that inspired his observations
14:25so much has been written about this apple tree it really has become
14:28a symbol for the turning point in our understanding of the universe
14:36newton's eureka moment was witnessed by a friend
14:42after dinner the weather being warm we went into the garden and drank tea
14:47under the shade of some apple trees
14:50the notion of gravitation came into his mind
14:56why should that apple always descend perpendicularly to the ground
15:03newton realized that there must be a force acting on that apple pulling it to the ground
15:08otherwise why wouldn't it just float in the air or move sideways or go upwards
15:13he named that force gravity after the latin word gravitas for heaviness
15:19newton's law of gravity was to completely change the way we think about weight
15:27we finally understood the subtle but vital difference between weight and mass
15:33and it paved the way for modern measurement
15:39now to show how important newton's discovery was i've got a piece of metal here an incredibly
15:45sensitive set of scales now the scales say that this piece of metal weighs 368.70254
15:54kind of flickering between the two it's so sensitive
15:57now let's take this piece of metal to the top of this block of flats see how much it weighs up there
16:03now up here the metal weighs 368.69 grams so i seem to have lost 10 milligrams but of course the mass
16:21hasn't changed what's changed is the gravity i've got less gravity up here than i have got down at the
16:27bottom of the block of flats and if i took this piece of metal another hundred thousand meters up into
16:32space then it would weigh hardly anything at all simply put mass is measuring the amount of stuff
16:39there is inside here and that doesn't change whether i'm at sea level or out in space but the weight does
16:49in one simple equation newton's genius revolutionized how we thought about weight and mass
16:55of mass but it would take a real revolution in france to finally create the measure of mass that
17:05we all use today the kilogram
17:13by the middle of the 18th century weight measurement light length was in a total mess
17:18nobody had it worse than the french people were supposed to use the king's measures for pounds
17:23announces but in reality every village and town had their own system all slightly different
17:30disputes and arguments were so commonplace that the village took to chaining the weights and measures
17:35to the wall of the local church
17:42trade was painfully slow and open to corruption
17:45and no one could agree on whose weight was right a new international system of measurement was urgently needed
17:58letters flew between the powers of europe
18:04too long have great britain and france been at variance with each other
18:07for empty honor or guilty interests it is time that the two free nations should unite
18:13their exertions for the promotion of a discovery that must be useful to mankind
18:21on the eve of the french revolution the great and good of the french scientific community
18:26approached the doomed louis the 16th for permission to create a new system of length mass and volume measurement
18:34the greatest minds of the day gathered here at the prestigious academy of sciences in paris
18:43to brainstorm a solution
18:52they decided to base their new system on something universal and unchanging the earth
18:58the first unit they fixed was the meter basing it on 110 millionth of the distance between the north pole and the equator
19:17the next was the kilogram and the task was given to the father of modern chemistry
19:23antoine laurent lavoisier by day he was a wealthy tax collector by night he was the greatest chemist in the land
19:35the french visionaries behind the metric system wanted all the new measurements to be linked
19:40so they came up with an elegant solution
19:43the new kilogram was to be equal to the weight of one perfect cubic decimeter of water a liter
20:01the idea was very simple anybody with a meter ruler and some water could create their own kilo
20:08but making a kilo based on the weight of a cubic decimeter of water turned out to be much more
20:16difficult than they thought
20:21now i've got two perfect decimeters of water here the trouble is that these don't weigh the same amount
20:28the colder water weighs 998 grams
20:33whilst the hotter water 957 grams because the hotter water is the less dense it is
20:43and that's the trouble the weight depends on the temperature not only that it'll depend on what
20:47impurities are inside the water what the atmospheric pressure is how far i am above sea level there's a
20:54real problem with trying to define the kilo based on the weight of water
21:02lavoisier came close to solving the problem of how to accurately weigh water but his brilliant career
21:09met an abrupt end at the hands of the guillotine on the 8th of may 1794
21:15his tax collecting day job was his downfall
21:26next to take up the kilo challenge were scientists louis lefebvre jeanot and giovanni frebroni
21:35four years later they finally perfected how to measure a cubic decimeter of distilled water
21:41a master metal kilogram could finally be cast
21:49and on the 22nd of june 1799 they presented their prototype kilogram to the nation called the
21:56kilogram des archives it was made out of the new wonder metal platinum soon kilogram clones as well as
22:03copies of the meter bar were being sent to villages and towns across the nation to bring uniformity to the
22:09french empire
22:15their vision was brilliant
22:19but there was a flaw
22:24the trouble was that pure platinum although resistant to air and water is actually rather
22:29soft and prone to damage and that meant bits were easily knocked off
22:35gradually rendering the hundreds of cloned kilos inaccurate
22:42the academy's grand idea was slowly being eroded
22:46it would take nearly 70 years to realize a new more stable master kilo
22:52and then a set of clones would be needed london metallurgists johnson mathie were given
22:58the order to produce 250 kilograms of platinum mixed with strength giving iridium
23:08it was a big order worth 2.2 million pounds at today's prices
23:12the man in charge of production george mathie the world's leading expert in casting platinum
23:21offered to make the kilos at his state-of-the-art furnaces at hatton garden
23:28but french pride intervened insisting it happened here at the conservatoire in paris
23:34it was a disaster the platinum got contaminated by iron rendering the whole consignment useless
23:43it was a huge embarrassment both the french pride and their pockets
23:47but it wasn't the death the kilo or the metric system with international trade booming the
24:01benefits of having one common measurement system were clear for all to see and in 1875 diplomats
24:10from 17 countries met here in paris and agreed to formally adopt the metric system
24:19with great zeal a new kilogram master was commissioned
24:25the order once again went to johnson mathie and this time george mathie was finally allowed to cast
24:32the most accurate platinum and iridium kilo ever made
24:36christened le grand quai it was consigned to a specially made vault at a newly established
24:45international center of measurement outside paris
24:55and here it is the bureau international des poids et mesures the bipm or in english the international
25:02bureau of weights and measures and this is really international territory it's kind of a mark of
25:07how important measurement is to the world that we've created a un of measurements
25:17from the beginning the bipm's mission was to make sure measurements were consistent throughout the world
25:24this is the building that was once home to all the world's master measurements
25:36today most have been retired replaced by new definitions based on the universal and unchanging
25:44laws of nature like the speed of light and the movement of atoms
25:50the grand quai is in fact the only artifact that is still in use a measurement dinosaur
26:12today here at the bipm they're still making clones of that grand quai fabrice here is polishing this
26:19until it exactly matches the mass the grand quai sitting in the vault downstairs
26:26over half the countries in the world have one of these clones
26:30the next one he's working on is clone number 103 that's going to go to well we're actually not
26:35allowed to know where it's going to go without le grand quai our entire global system of mass and
26:42weight measurement would crumble
26:44unfortunately crumble is a little bit of a touchy word inside this building because that's what's
26:51happening to le grand quai i mean it's not literally crumbling but despite the kick-glove
26:56treatment it's received over the last 150 years it's believed that it's changed by the equivalent of one
27:02grain of sand during its lifetime and that's bad news because it no longer matches the weight of the
27:11world's clones a new way to define mass is urgently needed now the race is on to replace the definition
27:21of the kilo with something more fitting for the 21st century something based on a universal constant
27:27that can be measured wherever you are in the universe
27:29we've done it for length that's now tied to the speed of light for time that's related to the
27:39movement of electrons in the atom now we want to do it for the kilo
27:48it's a head-to-head race between two international teams each one taking a radically different approach
27:56to solving the kilo crisis in america team watt balance are combining the power of electricity
28:05with scales whose principles date back 5 000 years their dream to redefine the kilo based on energy
28:176 000 kilometers away in germany team silicon sphere are trying to count every single atom in a perfect ball
28:26of silicon
28:31it's an immense task like covering the earth in sand and trying to count every single granule
28:40as the best minds in measurement science fight it out
28:44legron k's long and illustrious career could soon be over but its legacy has been staggering
28:56from the moment it was adopted the movement and sale of goods became much easier and more efficient
29:09the scientific community jumped on the new metric system loving its simplicity
29:14and the ease they could split or multiply the meter and the kilogram by 10
29:26but from the very beginning of its life in the 18th century the public remained less convinced
29:31people were just not interested in revolutionizing their everyday life what they did when they went shopping
29:42how they exchanged and bought in the name of revolutionary purity
29:46the kilo continues to divide opinion
29:54in the uk it was only adopted in the 1960s and its arrival was met with outright hostility
30:02all we ask is the freedom of choice to record in the native and still legal measures of this country
30:10instead of these cockeyed kilograms which make no sense at all
30:14but despite the opposition today all but three nations the united states liberia and mayanmar have
30:21embraced the kilo and the metric system
30:24the world was moving towards a unified weight measurement system
30:39the actual technology of weighing was now lagging behind
30:45variations on ancient mesopotamian and egyptian beam balances remained our scales of choice right up to
30:52the 19th century
30:56the problem was they took so long to use
31:02in the uk weighing was made much worse by the turnpike act of 1752
31:10eager to tax the movement of goods the government ordered all towns to
31:15erect a crane machine or engine for the weighing carts and wagons
31:19at each location carts had to be unloaded weighed reloaded and weighed once again
31:30and to add to the daily misery every key road demanded tolls too
31:35all payable on the weight you were carrying
31:41with the birth of the industrial revolution things had to change
31:44factories to forges now needed raw materials in unprecedented quantities and they had to be weighed
31:53bought and transported with ever-increasing speed and precision
31:57a faster more efficient means of weighing was desperately needed
32:10the solution was the way bridge
32:15a technological triumph the way bridge with its balance scale hidden beneath the floor would play a key
32:21role in driving our industrial revolution onwards
32:28now loads could be weighed in seconds as they rolled on and off the bridge
32:35but it would take electricity to drive the next big breakthrough in weighing
32:39inventor charles wheatstone championed the use of electricity in the 1840s
32:52experimenting with simple electrical circuits he devised a way of measuring electrical resistance
32:59but it wasn't until a century later that people realized this very same technology could be used to measure weight
33:09the use of electricity in the 1840s
33:14today the need for speedy mass measurement drives our world
33:23this train is delivering coal to Rugeley power station and as it runs over the track
33:28it's being weighed by load cells which are underneath the track
33:33if we come in here you can see how much we've weighed so far
33:39so hi andy hi so this is the first uh carriage that's gone over so we've got 100 tons
33:48yeah they're much more efficient than sort of weighing it all by hand oh yeah very much so yeah
33:53and we can measure at 70 kilometers an hour so you're talking less than a second per wagon probably
33:59wow that's extraordinary yeah
34:00so how's this piece of track actually weighing the train well underneath the track are several of these
34:11they're called load cells and actually it's this little system of wires on the rod which is doing the weighing
34:18but as soon as something runs over the track it compresses the rod and the wires get shorter and fatter
34:24the resistance goes down and i get more electrical current running through it
34:29and suddenly i'm getting a reading what's amazing is that there's a direct mathematical relationship
34:34between the increase in electrical current and the weights going over the wires so we're using the
34:40electricity to weigh the train in fact this thing is so sensitive that even if i step on it i actually
34:46can get how much you can get how much i weigh so let's see
34:51so how much do i weigh andy 84 84 kilos yeah don't weigh 84 kilos must be the weight of this
35:05today load cells are used the world over
35:08we've come a long way since the days of the beam balance now everywhere from roadside weigh stations
35:20to supermarket checkouts use them measuring mass with electricity has changed our world
35:28we can now weigh transport and deliver billions of tons worth of produce with a speed and accuracy
35:34our victorian forefathers would never have dreamt possible precision mass measurement is key to world
35:42commerce now it's the turn of the very small to push the limits of mass measurement
35:55here in america i've come to meet a team who've come up with a unique approach
36:00to measuring some of the smallest living things on earth
36:05cells
36:14project leader scott manalis is using mass to monitor the growth of cells
36:21his work could one day revolutionize our fight against cancer
36:25in his lab he has built the world's smallest weighing station
36:34here inside a microchip just millimeters in size cells are captured and passed over a sensor
36:43the long thin section highlighted here acts a bit like a diving board when a cell passes over it
36:51it vibrates just like a diving board moves after a diver jumps off it the speed of the vibration
36:58is directly linked to the weight of the cell so using simple maths scott can measure the cell
37:05with incredible accuracy this cell is equivalent to like a white blood cell in terms of its size okay
37:12and it weighs 100 picograms picograms so that's uh 10 to the minus 12. all right okay so it's an
37:20incredibly small thing so the cell doesn't weigh very much yeah and the precision at which we can weigh
37:25it with uh is four orders of magnitude below that wow that's incredible so that's 10 femtograms so femto
37:31now takes us from 12 to 15. right one part in 10 000. 10 000. yeah yeah yeah we care a lot about this
37:41we're soon in the domain of extreme numbers but what's amazing is scott's measuring the weight of a
37:48single cell to within a thousand trillionth of a gram his work is revolutionizing our understanding of how
37:56cells grow and by measuring how cells respond to a drug it could lead to personalized and far more
38:04effective cancer treatment
38:10it's absolutely amazing the limits we are now pushing mass measurement but scientists are frustrated
38:18and it's because we're still trying to tie mass back to that aging lump of metal in paris
38:25the grand k and with the grand k's weight unstable there's a real urgency to find a new even more
38:32accurate way to define mass now a race is being fought across two continents to retire the grand k
38:54drake
38:5820 miles north of washington is one of the world's most accurate set of scales
39:06this whole area is a car free zone and that's because the scales that are being used here are so
39:09sensitive that even the magnetic field caused by the metal inside the cars can affect the measurements
39:15Welcome to Team Watt Balance.
39:20Most things in this strange-looking building are made of wood and clad in vinyl
39:25to minimise the effects of magnetism.
39:30Everything from the power lines to the plumbing pipes
39:33are encased in shielded plastic ducts.
39:37And every single bit of metal that enters the lab,
39:40down to this tiny spare part, has to be checked for its levels of magnetism.
39:59Stefan Schlaminger's project is one of the longest-running metrology experiments in the world.
40:05Its founders have long since retired,
40:07but now the team here are close to fulfilling their dream.
40:12And this is their brainchild, the Watt Balance.
40:17Inside this cage of pure copper is a weighing scale
40:36whose principles go back to the very first balances 5,000 years ago.
40:41And it's so sensitive, it can measure the kilo to eight decimal places.
40:50So here's our Watt Balance.
40:52It is a thing of beauty. Wow.
40:54It really is.
40:55And you see up here, this reel is like the beam in an old-fashioned beam balance.
41:00That's quite ancient technology, isn't it?
41:01Yeah, it's 1,000-year-old technology up on top.
41:03But down here you will see the coil that's connected to these three rods.
41:07And this will provide the counterforce to the gravitational force that this mass is providing.
41:12On one side of the scales, deep inside the mechanism, sits a clone of the LeGrand K.
41:18What's so extraordinary about this device is that on the other side, instead of a weight,
41:25the team are using electrical force to counterbalance it.
41:30The Watt Balance defines the kilogram by linking mechanical power to electric power.
41:35That's why it's called the Watt Balance.
41:36Right.
41:37Their goal is to measure the amount of electricity needed
41:40to perfectly counterbalance the kilo clone
41:43and redefine the kilogram based on electrical power.
41:48It sounds straightforward.
41:53But when you are working with one of the most sensitive scales in the world,
41:58everything from car engines to the movement of the local deer population outside
42:02can affect its readings.
42:06Even tiny shifts in gravity, like the phase of the moon and the level of groundwater,
42:11need to be measured and taken into account when this experiment is running.
42:18It seems you're having to keep track of so many different things in order to pin down that kilo.
42:24That is the art.
42:25That's the art and the science of this.
42:28That's amazing.
42:29So we try to measure this kilo to about four parts per hundred million.
42:35And in order to do so, we need to measure all these auxiliary quantities like voltage, resistance, gravity, meter, second,
42:44to much better than four parts per hundred million.
42:46Now, after more than 30 years of perfecting the scale's accuracy,
42:54Team Watt Balance are very close to achieving their holy grail.
42:58A new electronic kilogram.
43:00I left the Watt Balance team realising I was witnessing a potentially historic moment in the life of the kilogram.
43:18The days of the American kilo making its transatlantic journey to Paris
43:33to be compared against Le Grand Cay are probably numbered.
43:38But the Watt Balance team have got a rival.
43:41In Germany, Team Silicon Sphere have got a completely different approach to redefining the kilo.
43:46And it involves counting the number of atoms in a kilogram of silicon crystal.
43:55People often talk about counting the number of grains of sand on a beach.
43:59But what Team Silicon Sphere are proposing to do is in a completely different league.
44:04It's like trying to cover the whole globe in sand and counting every grain.
44:15But what are these atoms?
44:16They're trying to count.
44:19It was the ancient Greeks who first came up with the word atom to define the smallest indivisible particle of matter.
44:28But it took Englishman John Dalton in the 19th century to shed light on what atoms really are.
44:36At the time, we knew that all matter was made up of different elements, like carbon and oxygen.
44:42Dalton's brilliance was a radical theory that each element must consist of atoms of a single, unique type and mass.
44:55Dalton would never have dreamt it possible to see or count these atoms.
44:59But now, in a remote lab in northern Germany, scientists are attempting to do just that.
45:10What Dalton didn't realise is the sheer number of atoms inside things.
45:23That there are trillion upon trillion inside a single kilo of silicon.
45:27And it's by counting these atoms that the silicon sphere team hope to redefine the kilo.
45:35This is a perfect kilogram sphere of pure silicon.
45:45The culmination of 30 years' work.
45:51It represents one of the most ambitious challenges ever to be undertaken in measurement history.
46:00Like the watt balance, the silicon sphere project started in the 1970s.
46:05The goal was to measure the atomic distances, the distance between the atoms in a very perfect crystal.
46:18Silicon was, at that time, a material which was used for the semiconductor industry and was the first very perfect material for that use.
46:28Silicon atoms line up in an extremely rigid and regular pattern, which in theory makes them easier to count.
46:43The idea was to create a perfect sphere of silicon, measure its dimensions with extreme precision,
46:51and then calculate the spaces between the atoms using a technique called X-ray crystallography.
47:00Then, using simple maths, they could work out the total number of atoms in the sphere.
47:09The project was supposed to take a couple of years, but they faced many challenges.
47:15The first was how to create a perfect sphere.
47:23The levels of perfection the team was seeking were beyond the capabilities of any machine.
47:31They scoured the globe and found the only way to create a sphere to the level of perfection they needed was to do it by hand.
47:40And only one man was capable of this, Australian lens maker, Akim Loisner.
47:52He literally used his hands to shape the ball to such an incredible level of perfection
47:58that if you likened it to the earth, the level of its surface would never vary more than a few metres.
48:05Using his extraordinary sense of touch, it's said, Akim could feel silicon's atomic structure with his fingertips.
48:16You need really a feeling how many atoms you have to remove on that side or on the other side of the sphere.
48:23So it had an atomic feeling in his hands.
48:28It took months for Akim to perfect his sphere.
48:32Finally, the task of analysing the space between the silicon atoms could begin.
48:41But on the cusp of realising their dream, disaster struck.
48:46There was a flaw in the very make-up of the silicon.
48:53In its natural state, silicon consists of three different forms called isotopes.
48:59Now, each different atom has a different mass.
49:03Leisner's sphere contained all three different types of these atoms.
49:08The team needed a pure source of silicon or else the project was over.
49:14The solution came from an unlikely source.
49:17A nuclear weapons facility.
49:26The Cold War was over and a lot of centrifuge in Russia were not running for nuclear weapons.
49:36So we were lucky to rent some of this centrifuge to prepare silicon for our purpose.
49:45A new batch of silicon was sent to Russia and spun in the same centrifuge that was formerly used to enrich uranium.
49:56This forced out the wayward extra isotopes, producing pure silicon-28.
50:02Then Leisner had to start the job of polishing all over again.
50:12Finally, after many years, the scientists once again started counting the space between the atoms.
50:18And trillions of atoms later, they've nearly completed their task.
50:28We hope that in two years, we will have all the information together for a new definition.
50:38That means we have a value with a very small uncertainty, let us say below 2 times 10 to minus 8.
50:46And that's an accuracy to 8 decimal places.
50:51It's the same level of precision as Teamwork Balance in America are striving for.
50:56At the moment, we are in the pool position to win this race.
51:07Within a few years, Le Grand Quay could be retired.
51:12But the work here could revolutionise another of the seven fundamental units we use to describe our world.
51:19If the silicon team is successful, then they won't just redefine the kilo.
51:41They could end up redefining the SI unit most feared by chemistry students across the world, the mole.
51:49It's a word which comes from Latin, meaning massive heap of material.
51:58Now, chemists probably won't like this, but consider this cup of coffee.
52:02There's a certain ratio of milk to coffee, say one part milk to nine parts coffee,
52:09which combined makes one part perfect milky coffee.
52:14Now, the mole does a similar thing for chemists, but replace the coffee and the milk with atoms and molecules.
52:22Yeah, perfect.
52:23All this leads back to our friend Dalton and his work in the 19th century.
52:32When he began his investigation into atoms, he discovered that atoms from different elements weighed different amounts.
52:39At the centre of every atom is a nucleus containing protons and neutrons.
52:44Different elements have different numbers of these protons and neutrons,
52:48which is why they weigh different amounts.
52:57Throughout the 19th century, the greatest chemists of the day feverishly tried to work out the atomic weights of all the known elements.
53:06It led to one of science's greatest ever achievements, Dmitri Mendeleev's periodic table.
53:13And if you look at each element on that table, you'll see their atomic mass written just below them.
53:25It was a huge breakthrough.
53:28Chemists could finally mix and manipulate elements with newfound precision.
53:32But atoms are far too small to look at and manipulate individually.
53:41What chemists needed was a way of scaling up atomic weight into something more tangible they could weigh.
53:48And the answer was the mole.
53:50The mole is really just a big number.
53:56A huge number, in fact, which, when you combine it with the atomic weight of each element,
54:00allows you to work out how many atoms there are inside something.
54:05It's the chemists' way of scaling up the microscopic world of the atom to our world of the gram.
54:11It's really the bedrock of modern chemistry,
54:14allowing us to mix things from drugs to fuel with such precision.
54:18But it leaves open one big question.
54:22Exactly how many atoms are there inside a mole?
54:28The number of atoms that we have in a mole is what we call Avogadro's number.
54:34We can go back to Einstein, for instance, in 1905.
54:37He came up with one of the first estimates of just how big this number is.
54:41From looking down microscopes at pollen grains,
54:44and from that he was able to get one of our first estimates of the number.
54:48He got the first number right, he got the six right,
54:51and he got the 23 zeros right.
54:56While Einstein's groundbreaking work got close to defining the elusive Avogadro's number,
55:01it's the silicon sphere team that could not only solve the Kilo conundrum,
55:08but also solve the centuries-old question of how many atoms there are in a mole.
55:16And once and for all, define Avogadro's number.
55:20If this happens, it will be a remarkable moment in measurement history.
55:26In one astonishing experiment, two golden units of measurement could be redefined.
55:32We've come a long way since the days of using barleycorn weights.
55:36Our quest for ever greater precision has led us into the very fabric of our universe,
55:41allowing us to weigh and analyse things with incredible speed, scale and precision.
55:46In a few years' time, all going well, the BIPM will decide between atoms or electrical force
55:52to redefine the Kilo.
55:54The winner is kind of irrelevant.
55:57Both Team Watt Balance and Silicon Ball have done what seemed impossible
56:01to redefine the Kilo based on the unchanging laws of the universe.
56:07In the pursuit of ever greater accuracy,
56:10these remarkable projects have brought together thousands of years of scientific endeavour.
56:18But our quest for ever greater precision doesn't stop here.
56:27The last great measurement frontier will be to journey inside atoms themselves,
56:34to discover what mass really is.
56:37100 metres under the Swiss-French border of CERN's particle accelerator,
56:54scientists think that they've discovered a particle that gives things mass,
56:58the Higgs boson.
57:00And one day, our human desire for ever greater precision
57:04may even see mass redefined once more.
57:07And tied to Higgs itself.
57:13If it happens, who knows what the technological impacts will be?
57:18And that's the beauty of measurement.
57:20Every leap in precision leads to new scientific and technological advances.
57:25Measurement has shaped our history and will continue to change our world.
57:29Next, we explore the world of energy.
57:50And how the measurement of light, heat and electricity have transformed our lives.
57:55As I continue my journey into measurement.
58:00We've a passion for porcelain here on BBC4 tomorrow night.
58:11As our Beautiful Things season kicks off at nine.
58:15There's a little taster of that coming up in just a moment.
58:17Next tonight, though, brand new Storyville from Israel with the law in these parts.
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