Original subtitles

On the morning of the 14th June, 1940,

several German tank divisions rumbled

through the streets of Paris.

The impossible had happened. Germany had invaded and France had fallen.

But there was one building on the outskirts of Paris that

the Nazis never occupied.

This chateau has the same status as an independent territory.

Its contents are so closely guarded,

I have to hand over my passport to gain access.

Today, an eminent group of scientists have

gathered from all over the world to witness a very special event.

Security is tight, with key holders arriving from three different countries.

The vault holds one of the most important artefacts in our world.

This is a real piece of measurement history. Well, I suppose it's

not really history at all, because this is the kilo.

Under three layers of protective glass is the kilogram master

known as Le Grand K.

It's the weight on which all weights have been based since 1889.

Its importance is so great that neither the Nazis nor

the liberating American forces dared set foot inside here.

And the reason we're here today?

Well, just to check it's still here. But there's a problem.

Tests have revealed that Le Grand K, this scientific celebrity,

is losing weight, creating a crisis in the scientific world.

It's losing approximately 20 billionths of a gram every year.

But why on earth should such a tiny change matter so much?

I'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.

Today we can describe the chaos

and complexity of the universe with just seven fundamental units,

the building blocks of modern science.

And science is obsessed with defining these units with ever-greater precision.

In this series, I want to understand why such extreme

levels of precision are so important,

how we define these units,

and how, through history, each step forward has unleashed

a technological revolution.

In this programme, we'll explore why being able to measure weight is

so important.

And how the race to replace the ageing Grand K

might hold the key to a new way of understanding our world.

This is the story of how we mastered weight.

"How much do I have?" is a question that has driven trade

and commerce since the dawn of civilisation.

And 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.

We tend to believe our eyes rather than trusting

the weight in our hands.

And I've come to London's Borough Market to prove the point.

Excuse me - wonder whether I could get you to take part in a little experiment? Of course, yes.

So, I've got a series of weights here which I've just

put in order of height and what I'd like you to do is to place

the heaviest weight here and the lightest one at your end.

Have a go. See which one you think is the heaviest.

That's....

This little guy, that's the heaviest? OK. What about the next heaviest?

I think this one... that's the lightest.

The lightest of all? I think... OK!

The really surprising thing is that the one you've put at this end,

which you think is the lightest, is in fact the heaviest!

So you thought this one here was the heaviest.

OK, I'm going to give you both of these in your hand - this one is

actually heavier than that one. Do you believe me?

Well, it doesn't feel like that.

No, it doesn't, but let's use the scales.

So I am going to weigh the one that you thought was the lightest,

so that comes out about 424 grams.

OK, let's put your one on.

You think this one is heavier. It's only 345 grams!

Isn't that extraordinary? Because even with that knowledge,

now try and weigh them again. Which one is heavier...

This one. I know! And that's why we need a set of weights

because we're so bad at perception.

Like any good scientist I carried on with the testing.

How's that possible?

And my random shoppers, to a man and a woman,

all chose the same two weights

and they all chose wrong.

OK. Wow!

Seeing if something is big or small massively

skews our perception of how heavy it is.

It is a problem our ancestors

first started grappling with more than 5,000 years ago.

Our earliest evidence comes from the Middle East and was driven

by the emergence of the first cities in Mesopotamia around 3,000 BC.

As populations grew,

a way of fairly trading goods was urgently needed.

People demanded a system of weight that everyone could trust.

Taking their inspiration from nature, they used grain.

Uniform in size and shape, grain was available to all.

The world had its first weights.

Using simple beam balances, which we continue to use today,

we started to trade goods based on their weight in grains.

It wasn't perfect, but with grains varying so little in weight,

the system worked.

It made the movement and sale of goods possible,

enabling humans to live together in bigger cities and allowing the first

economies to grow.

Empires were no longer being built solely by armies.

They were being built by trade.

As commerce developed across the ancient world, a faster

means of weighing produce was needed.

After all, if I wanted to buy something that

weighed 700 grains of barley,

I don't want to have to count out 700 grains each time.

So, gradually, a standardised system of weights began to emerge.

First the Mesopotamians and then the Ancient Egyptians developed

stones and things made out of metals

and brass in order to represent different weights of grain.

It was such an efficient system that it began to be

copied across the civilised world. So here we have standard

weights from China.

These hexagons are standard weights used in Sudan.

And the amazing thing is that despite all of these

different weights and measures, they were all related back to the weight

of a grain, because everyone trusted how much a grain would weigh.

By Roman times, millions of tonnes of produce were being

traded around the world every day.

The ability to compare the weights or masses of two different

kinds of goods so that you can work out how to

exchange between them, that's the key to economic success.

And so it's the demand for economic comparison that drives weight

standardisation throughout history.

By the end of the 13th century, the world had hundreds of different

weights, and nearly all were based on a fixed number of grains.

In England, we'd inherited the pound from the Roman Empire.

It was initially made up of 12 ounces,

which were equivalent to 437 grains of barley.

But the problem all rulers faced was how to keep weight

standardised across a nation.

It was considered such a big issue that even the Magna Carta, the most

celebrated legal document in English history, tried to deal with it.

"Let there be one measure of wine throughout our whole realm,

"and one measure of ale and one measure of corn."

It all sounded great in theory but in practice,

it was virtually impossible to enforce.

Cheating was such a big problem,

regular trials were held to check merchants' weights and measures.

Any found to be wrong were immediately destroyed.

Accurate scales were the only way cheats could be exposed.

Accuracy was power.

Scales were not only a great measuring device.

They also came to symbolise fairness, power,

the very legal system itself.

From Ancient Egypt's Feather of Truth to the paintings

of the great Dutch Masters, scales have featured throughout history.

As it was written in the Bible, "By weight, measure and number,

"God made all things."

Measurement has always been associated in culture with

justice and law and crime. Because what it does is to establish the

equivalence between two things that you otherwise could not compare.

That's what justice means, so it's no coincidence that the

figure of Justice is shown carrying scales, carrying balance pans.

And for centuries, when you made a weight measurement, you had to

show your customers what you were doing, partly to avoid

the possibility of deceit but also to show how just you were -

to be just, was precisely to use balance.

So, with all this moral weightiness flying around, the punishment

for using false measures could be severe.

In 1772 BC, the Code of Hammurabi was introduced into Babylonian law,

which said that any taverner using false weights could be served up with a death penalty.

And in the 18th century, bankers caught cheating

would have to stand in pillory, and brewers... in the dung cart.

But despite the importance we placed on weight

and getting it right, it took one remarkable Englishman

to realise the measurement of weight has a fundamental problem.

It was the great Sir Isaac Newton who first realised that

weight changes depending on where and when you are measuring it.

It was 1665 and Britain was gripped by the Plague,

so Newton decided to flee his college in Cambridge and

he came to the safety of his country retreat here at Woolsthorpe Manor.

And here is the famous apple tree that inspired his observations.

So much has been written about this apple tree, it really has become

a symbol for the turning point in our understanding of the universe.

Newton's eureka moment was witnessed by a friend.

"After dinner, the weather being warm,

"we went into the garden and drank tea, under the shade of some apple trees.

"The notion of gravitation came into his mind.

"Why should that apple always descend perpendicularly to the ground?"

Newton realised there must be a force acting on that apple,

pulling it to the ground, otherwise why wouldn't it just

float in the air, or move sideways or go upwards.

He named that force "gravity" after the Latin word

"gravitas" for heaviness.

Newton's law of gravity was to completely change the way

we think about weight.

We finally understood the subtle but vital difference between weight

and mass and it paved the way for modern measurement.

Now to show how important Newton's discovery was, I've got a piece of metal here

and an incredibly sensitive set of scales.

Now, the scales say that this

piece of metal weighs 368.7025/4...

It's kind of flickering between the two, it's so sensitive.

Now, let's take this piece of metal to the top of this

block of flats and see how much it weighs up there.

Now, up here, the metal weighs 368.69 grams,

so I seem to have lost ten milligrams.

But of course the mass hasn't changed,

what's changed is the gravity.

I've got less gravity up here

than I have got down at the bottom of the block of flats.

If I took this piece of metal another 100,000 metres

up into space then it would weigh hardly anything at all.

Simply put, mass is measuring the amount of stuff there is inside here,

and that doesn't change whether I'm at sea level or out in space.

But the weight does.

In one simple equation,

Newton's genius revolutionised how we thought about weight and mass.

But it would take a real revolution in France, to finally create

the measure of mass that we all use today - the kilogram.

By the middle of the 18th century, weight measurement,

like length, was in a total mess and nobody had it worse than the French.

People were supposed to use the King's measures for pounds and ounces.

But in reality, every village and town had their own system,

all slightly different.

Disputes and arguments were so commonplace that the village took

to chaining the weights and measures to the wall of the local church.

Trade was painfully slow and open to corruption,

and no-one could agree on whose weight was right.

A new international system of measurement was urgently needed.

Letters flew between the powers of Europe.

"Too long have Great Britain and France been at variance

"with each other, for empty honour or guilty interests.

"It is time that the two free nations should unite their exertions

"for the promotion of a discovery that must be useful to mankind."

On the eve of the French Revolution, the great and good

of the French scientific community approached the doomed Louis XVI for

permission to create a new system of length, mass and volume measurement.

The greatest minds of the day gathered here at the

prestigious Academy of Sciences in Paris to brainstorm a solution.

They decided to base their new system on something universal

and unchanging - the Earth.

It was the birth of metrication.

The first unit they fixed was the metre, basing it on one ten-

millionth of the distance between the North Pole and the Equator.

The next was the kilogram, and the task was given to the

father of modern chemistry, Antoine-Laurent Lavoisier.

By day, he was a wealthy tax collector. By night,

he was the greatest chemist in the land.

The French visionaries behind the metric system wanted all the new

measurements to be linked, so they came up with an elegant solution.

The new kilogram was to be equal to the weight of one perfect

cubic decimetre of water...

..a litre.

The idea was very simple.

Anybody with a metre ruler and some water could create their own kilo.

But making a kilo based on the weight of a cubic decimetre

of water turned out to be much more difficult than they thought.

Now, I've got two perfect decimetres of water here.

The trouble is that these don't weigh the same amount.

The colder water weighs 998 grams whilst the hotter water

is 957 grams.

Because the hotter water is, the less dense it is.

And that's the trouble, the weight depends on the temperature.

Not only that, it will depend on what impurities are inside the

water, what the atmospheric pressure is, how far I am above sea level.

There's a real problem with trying to define the kilo

based on the weight of water.

Lavoisier came close to solving the problem of how to accurately

weigh water.

But his brilliant career met an abrupt end

at the hands of the guillotine on the 8th May 1794.

His tax-collecting day job was his downfall.

Next to take up the kilo challenge were scientists

Louis Lef�vre-Gineau and Giovanni Fabbroni.

Four years later, they finally perfected how to measure

a cubit decimetre of distilled water.

A master metal kilogram could finally be cast.

And on the 22nd June, 1799,

they presented their prototype kilogram to the nation.

Called the "kilogramme des Archives", it was made out of the new

wonder metal, platinum.

Soon, kilogram clones, as well as copies of the metre bar,

were being sent to villages and towns across the nation

to bring uniformity to the French Empire.

Their vision was brilliant.

But there was a flaw.

The trouble was that pure platinum, although resistant to air

and water, is actually rather soft and prone to damage.

And that meant bits were easily knocked off,

gradually rendering the hundreds of cloned kilos inaccurate.

The Academy's grand idea was slowly being eroded.

It would take nearly 70 years to realise a new,

more stable master kilo, and then a set of clones would be needed.

London metallurgists Johnson Matthey were given the order to

produce 250 kilograms of platinum mixed with strength-giving iridium.

It was a big order, worth �2.2 million at today's prices.

The man in charge of production, George Matthey,

the world's leading expert in casting platinum, offered to

make the kilos at his state-of-the-art furnaces

at Hatton Garden.

But French pride intervened, insisting it happened here,

at the Conservatoire in Paris.

It was a disaster.

The platinum got contaminated by iron, rendering the whole consignment useless.

It was a huge embarrassment, both for French pride and their pockets.

But it wasn't the death of the kilo or the metric system.

With international trade booming, the benefits of having one

common measurement system were clear for all to see.

And in 1875, diplomats from 17 countries met here in Paris

and agreed to formally adopt the metric system.

With great zeal,

a new kilogram master was commissioned.

The order once again went to Johnson Matthey and this time George Matthey was finally

allowed to cast the most accurate platinum and iridium kilo ever made.

Christened "Le Grand K", it was consigned to a specially-made vault

at a newly established international centre of measurement outside Paris.

And here it is - the Bureau International des Poids et Mesures.

The BIPM. In English, the International Bureau of Weights and Measures.

And this is really international territory.

It's kind of a mark of how important measurement

is to the world that we've created a UN of measurement.

From the beginning, the BIPM's mission was to make sure

measurements were consistent throughout the world.

This is the building that was once home to all the world's master measurements.

Today, most have been retired,

replaced by new definitions based on the universal and unchanging

laws of nature, like the speed of light

and the movement of atoms.

Le Grand K is in fact the only artefact that is still in use.

A measurement dinosaur.

Today, here at the BIPM they're still making clones of that Grand K.

Fabrice here is polishing this until it exactly matches

the mass of the Grand K sitting in the vault downstairs.

Over half the countries in the world have one of these clones.

The next one he's working on is clone number 103 that's going to go

to... well we're actually not allowed to know where it's going to go.

Without Le Grand K, our entire global system of mass

and weight measurement would crumble.

Unfortunately, "crumble" is a little bit of a touchy word

inside this building because that's what's happening to Le Grand K.

I mean, it's not literally crumbling,

but despite the kid-glove treatment

it's received over the last 150 years, it's believed that it

has changed by the equivalent of one grain of sand during its lifetime.

And that's bad news,

because it no longer matches the weight of the world's clones.

A new way to define mass is urgently needed.

Now the race is on to replace the definition of the kilo with

something more fitting for the 21st century,

something based on a universal constant that can be measured

wherever you are in the universe.

We've done it for length - that's now tied to the speed of light,

for time - that's related to the movement of electrons in the atom.

Now we want to do it for the kilo.

It's a head-to-head race between two international teams.

Each one taking a radically different approach to solving

the kilo crisis.

In America, Team Watt Balance are combining the power

of electricity with scales whose principles date back 5,000 years.

Their dream? To redefine the kilo based on energy.

6,000 kilometres away in Germany, Team Silicon Sphere

are trying to count every single atom in a perfect ball of silicon.

It's an immense task - like covering the Earth in sand

and trying to count every single granule.

As the best minds in measurement science fight it out,

Le Grand K's long and illustrious career could soon be over,

but its legacy has been staggering.

From the moment it was adopted, the movement

and sale of goods became much easier and more efficient.

The scientific community jumped on the new "metric" system,

loving its simplicity and the ease they could split or multiply

the metre and the kilogram by ten.

But from the very beginning

of its life in the 18th century, the public remained less convinced.

People were just not interested in revolutionising their everyday

life, what they did when they went shopping, how they exchanged

and bought, in the name of revolutionary purity.

The kilo continues to divide opinion.

In the UK it was only adopted in the 1960s

and its arrival was met with outright hostility.

'All we ask is the freedom of choice to record in the native

'and still legal measures of this country instead of these

'cock-eyed kilograms, which make no sense at all.'

But despite the opposition, today all

but three nations - the United States, Liberia and Myanmar -

have embraced the kilo and the metric system.

While the world was moving towards a unified weight measurement

system, the actual technology of weighing was now lagging behind.

Variations on ancient Mesopotamian

and Egyptian beam balances remained our scales of choice right

up to the 19th century.

The problem was they took so long to use.

In the UK, weighing was made much worse by the Turnpike Act of 1752.

Eager to tax the movement of goods, the government ordered all towns

to "erect a crane machine or engine for the weighing carts and wagons".

At each location, carts had to be unloaded, weighed,

reloaded and weighed once again.

And to the add to the daily misery, every key road demanded tolls, too.

All payable on the weight you were carrying.

With the birth of the Industrial Revolution, things had to change.

Factories to forges now needed raw materials in unprecedented quantities.

And they had to be weighed,

bought and transported with ever-increasing speed and precision.

A faster, more efficient means of weighing was desperately needed.

The solution was the weighbridge.

A technological triumph, the weighbridge,

with its balance scale hidden beneath the floor, would play

a key role in driving our industrial revolution onwards.

Now, loads could be weighed in seconds as they rolled on

and off the bridge.

But it would take electricity to drive the next big

breakthrough in weighing.

Inventor, Charles Wheatstone, championed

the use of electricity in the 1840s.

Experimenting with simple electrical circuits,

he devised a way of measuring electrical resistance.

But it wasn't until a century later that people realised this

very same technology could be used to measure weight.

Today, the need for speedy mass measurement drives our world.

This train is delivering coal to Rugeley Power Station

and as it runs over the track, it's being weighed by load cells,

which are underneath the track.

If we come in here, we can see how much we've weighed so far.

So, hi, Andy. Hi.

So this is the first carriage that's gone over,

so we've got 100 tonnes.

Yeah. So it's much more efficient than weighing it all by hand.

Oh, yeah, very much so.

We can measure at 70 kilometres per hour

so we are talking less than a second per wagon, probably.

Wow, that's extraordinary.

So how's this piece of track actually weighing the train?

Well, underneath the track are several of these.

They're called load cells.

And actually,

it's this little system of wires on the rod which is doing the weighing.

But as soon as something runs over the track,

it compresses the rod and the wires get shorter and fatter.

The resistance goes down

and I get more electrical current running through it.

And suddenly I'm getting a reading.

What's amazing is there's a direct mathematical relationship between

the increase in electrical current and the weight going over the wires.

So we're using the electricity to weigh the train.

In fact, this thing is so sensitive that even if I step on it,

I actually can get how much I weigh.

So let's see.

So how much do I weigh, Andy?

84.

84 kilos?! Yeah.

I don't weigh 84 kilos. Must be the weight of this.

Today, load cells are used the world over.

We've come a long way since the days of the beam balance.

Now everywhere, from roadside weigh stations

to supermarket checkouts use them.

Measuring mass with electricity has changed our world.

We can now weigh, transport

and deliver billions of tonnes-worth of produce with a speed and accuracy

our Victorian forefathers would never have dreamt possible.

Precision mass measurement is key to world commerce.

Now it's the turn of the very small to push the limits of mass measurement.

Here in America, I've come to meet a team who've come up with

a unique approach to measuring some of the smallest living

things on Earth... cells.

Project leader Scott Manalis

is using mass to monitor the growth of cells.

His work could one day revolutionise our fight against cancer.

In his lab, he has built the world's smallest weighing station.

Here, inside a microchip just millimetres in size,

cells are captured and passed over a sensor.

The long, thin section highlighted here,

acts a bit like a diving board.

When a cell passes over it,

it vibrates just like a diving board moves after a diver jumps off it.

The speed of the vibration

is directly linked to the weight of the cell.

So, using simple maths,

Scott can measure the cell with incredible accuracy.

This cell is the equivalent of like a white blood cell,

in terms of its size. OK.

And it weighs 100 picograms.

Picograms, so that's ten... To the minus 12.

All right, OK. So that's a lot of zeros.

So this is incredibly small.

So the cell doesn't weigh very much.

And the precision at which we can weigh it with,

is four orders of magnitude below that.

Wow, that's incredible. So that's ten femtograms...

So a part in a thousand.

One part in 10,000. 10,000!

We care a lot about these things.

We're soon in the domain of extreme numbers,

but what's amazing is Scott's measuring the weight

of a single cell to within a thousand-trillionth of a gram.

His work is revolutionising our understanding of how cells grow.

And by measuring how cells respond to a drug, it could lead to

personalised and far more effective cancer treatment.

It's absolutely amazing, the limits we are now pushing mass measurement.

But scientists are frustrated.

And it's because we're still trying to tie mass back to that

ageing lump of metal in Paris, Le Grand K.

And with Le Grand K's weight unstable, there's a real

urgency to find a new, even more accurate way to define mass.

Now a race is being fought across two continents to retire Le Grand K.

20 miles north of Washington is one of the world's most

accurate set of scales.

This whole area is a car-free zone, and that's because the scales

that are being used here are so sensitive that even the magnetic

field caused by the metal inside the cars can affect the measurements.

Welcome to Team Watt Balance.

Most things in this strange-looking building are made of wood,

and clad in vinyl to minimise the effects of magnetism.

Everything from the power lines to the plumbing

pipes are encased in shielded plastic ducts.

And every single bit of metal that enters the lab, down to this

tiny spare part has to be checked for its levels of magnetism.

Stephan Schlamminger's project is one of the longest-running

metrology experiments in the world.

Its founders have long since retired,

but now the team here are close to fulfilling their dream.

And this is their brainchild. The watt balance.

Inside this cage of pure copper, is a weighing scale whose

principles go back to the very first balances 5,000 years ago.

And it's so sensitive it can measure the kilo to eight decimal places.

So here's our watt balance.

It is a thing of beauty.

It really is.

And you see up here this wheel is like the beam in an old-fashioned beam balance.

That's quite ancient technology, isn't it?

Yeah, it's thousand-year-old technology up on top,

but down here, you will

see the coil that's connected to three rods and this will provide the

counterforce to the gravitational force that this mass is providing.

On one side of the scales,

deep inside the mechanism sits a clone of the Le Grand K.

What's so extraordinary about this device is that on the other side,

instead of a weight, the team are using electrical force

to counterbalance it.

The watt balance defines the kilogram by linking

mechanical power to electrical power.

That's why it's called the watt balance. Right.

Their goal is to measure the amount of electricity needed to

perfectly counterbalance the kilo clone

and redefine the kilogram, based on electrical power.

It sounds straightforward,

but when you are working with one of the most sensitive

scales in the world, everything from car engines to the movement

of the local deer population outside can affect its readings.

Even tiny shifts in gravity, like the phase of the moon

and the level of ground water, need to be measured

and taken into account when this experiment is running.

It seems you're having to keep track of

so many different things in order to pin down that kilo.

That is the art.

That's the art and science of this! Amazing.

So we try to measure this kilo to about four parts per 100 million,

and in order to do so, we need to measure all these

auxiliary qualities like voltage, resistance, gravity, metre,

second, to much better than four parts per hundred million.

Now, after more than 30 years of perfecting the scale's accuracy,

Team Watt Balance are very close to achieving their holy grail -

a new electronic kilogram.

I left the watt balance team realising I was witnessing

a potentially historic moment in the life of the kilogram.

The days of the American kilo making its transatlantic journey to

Paris to be compared against Le Grand K are probably numbered.

But the watt balance team have got a rival.

In Germany, Team Silicon Sphere have got a completely different

approach to redefining the kilo.

And it involves counting the number of atoms in a kilogram

of silicon crystal.

People often talk about counting the number of grains of sand on a beach.

But what Team Silicon Sphere are proposing to do

is in a completely different league.

It's like trying to cover the whole globe in sand

and counting every grain.

But what are these atoms they're trying to count?

It was the Ancient Greeks who first came up with the word "atom"

to define the smallest indivisible particle of matter.

But it took Englishman John Dalton in the 19th century to shed

light on what atoms really are.

At the time, we knew that all matter was made up of different

elements, like carbon and oxygen.

Dalton's brilliance was a radical theory that each element must

consist of atoms of a single unique type and mass.

Dalton would never have dreamt it possible to see or count these atoms.

But now, in a remote lab in Northern Germany,

scientists are attempting to do just that.

What Dalton didn't realise is the sheer number of atoms inside things.

That there are trillion upon trillion inside a single

kilo of silicon.

And it's by counting these atoms,

that the silicon sphere team hope to redefine the kilo.

This is a perfect kilogram sphere of pure silicon.

The culmination of 30 years' work.

It represents one of the most ambitious challenges ever to

be undertaken in measurement history.

Like the watt balance, the silicon sphere project started in the 1970s.

The goal was to measure the atomic distances -

the distance between the atoms in a very perfect crystal.

Silicon was at that time a material, which was used for the semiconductor

industry and was the first very perfect material for that use.

Silicon atoms line up in an extremely rigid

and regular pattern, which in theory makes them easier to count.

The idea was to create a perfect sphere of silicon,

measure its dimensions with extreme precision,

and then calculate the spaces between the atoms,

using a technique called X-ray crystallography.

Then, using simple maths,

they could work out the total number of atoms in the sphere.

The project was supposed to take a couple of years,

but they faced many challenges.

The first, was how to create a perfect sphere.

The levels of perfection the team were seeking

were beyond the capabilities of any machine.

They 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.

And only one man was capable of this.

Australian lens maker Achim Leistner.

He literally used his hands to shape the ball to such an incredible

level of perfection, that if you likened it to the Earth,

the level of its surface would never vary more than a few metres.

Using his extraordinary sense of touch, it's said Achim could

feel silicon's atomic structure with his fingertips.

You need really...

..a feeling how many atoms you have to remove on that side or

on the other side of the sphere, so he had atomic feeling in his hands.

It took months for Achim to perfect his sphere.

Finally, the task of analysing the space between the silicon

atoms could begin.

But on the cusp of realising their dream, disaster struck.

There was a flaw in the very makeup of the silicon.

In its natural state, silicon consists of three different

forms called isotopes.

Now, each different atom has a different mass.

Leistner's sphere contained all three different types of these atoms.

The team needed a pure source of silicon or else the project

was over.

The solution came from an unlikely source.

A nuclear weapons facility.

The Cold War was over and a lot of centrifuge in Russia

were not running for nuclear weapons,

so we were lucky to rent

some of these centrifuge to prepare silicon for our purpose.

A new batch of silicon was sent to Russia

and spun in the same centrifuge that was formerly used to enrich uranium.

This forced out the wayward extra isotopes,

producing pure silicon-28.

Then Leistner had to start the job of polishing all over again.

Finally, after many years, the scientists once again started counting

the space between the atoms.

And trillions of atoms later, they've nearly completed their task.

We hope that in two years, we will have all the information

together for a new definition that means we have a value

with a very small uncertainty -

let us say below two times ten to minus eight.

And that's an accuracy to eight decimal places.

It's the same level of

precision as Team Watt Balance in America are striving for.

At the moment, we are in the pole position to win this race.

Within a few years, Le Grand K could be retired.

But the work here could revolutionise

another of the seven fundamental units we use to describe our world.

Ein Kaffee mit Milch, bitte? Danke.

If the silicon team are successful, then they won't just redefine

the kilo, they could end up redefining the SI unit most

feared by chemistry students across the world - the mole.

It's a word which comes from Latin meaning "massive heap of material".

Now, chemists probably won't like this,

but consider this cup of coffee.

There's a certain ratio of milk to coffee, say one part milk,

to nine parts coffee, which combined,

makes one part perfect milky coffee.

Now the mole does a similar thing for chemists,

but replace the coffee and the milk with atoms and molecules.

Yep, perfect!

All this leads back to our friend, Dalton,

and his work in the 19th century.

When he began his investigation into atoms, he discovered that

atoms from different elements weighed different amounts.

At the centre of every atom is a nucleus containing protons and neutrons.

Different elements have different numbers of these protons

and neutrons, which is why they weigh different amounts.

Throughout the 19th century,

the greatest chemists of the day feverishly tried to work out

the atomic weights of all the known elements.

It led to one of science's greatest-ever achievements,

Dmitri Mendeleev's periodic table.

And if you look at each element on that table,

you'll see their atomic mass written just below them.

It was a huge breakthrough.

Chemists could finally mix

and manipulate elements with new-found precision.

But atoms are far too small to look at and manipulate individually.

What chemists needed was a way of scaling up atomic weight into

something more tangible they could weigh.

And the answer was the mole.

The mole is really just a big number.

A huge number, in fact,

which, when you combine it with the atomic weight of each element,

allows you to work out how many atoms there are inside something.

It's the chemist's way of scaling up the microscopic

world of the atom, to our world of the gram.

It's really the bedrock of modern chemistry, allowing us

to mix things from drugs to fuel with such precision.

But it leaves open one big question.

Exactly how many atoms are there inside a mole?

The number of atoms that we have in a mole is what

we call Avogadro's number.

We can go back to Einstein, for instance, in 1905.

He came up with one of the first estimates of just how big

this number is from looking down microscopes at pollen grains and

from that he was able to get one of our first estimates of the number.

He got the first number right.

He got the six right and he got the 23 zeros right.

While Einstein's groundbreaking work got close to defining

the elusive Avogadro's number, it's the silicon sphere team that

could not only solve the kilo conundrum,

but also solve the centuries-old question of how many atoms

there are in a mole...

..and once and for all, define Avogadro's number.

If this happens,

it will be a remarkable moment in measurement history.

In one astonishing experiment,

two golden units of measurement could be redefined.

We've come a long way since the days of using barley corn weights.

Our quest for ever-greater precision,

has led us into the very fabric of our universe, allowing us to weigh

and analyse things with incredible speed, scale and precision.

In a few years' time, all going well, the BIPM will

decide between atoms or electrical force to redefine the kilo.

The winner is kind of irrelevant.

Both Team Watt Balance

and Silicon Ball have done what seemed impossible,

to redefine the kilo based on the unchanging laws of the universe.

In the pursuit of ever-greater accuracy, these remarkable projects

have brought together thousands of years of scientific endeavour.

But our quest for ever-greater precision doesn't stop here.

The last great measurement frontier will be to

journey inside atoms themselves, to discover what mass really is.

100 metres under the Swiss-French border,

at CERN's particle accelerator, scientists think that

they have discovered a particle that gives things mass - the Higgs boson.

And one day, our human desire for ever-greater precision

may even see mass redefined once more,

and tied to Higgs itself.

If it happens, who knows what the technological impacts will be?

And that's the beauty of measurement.

Every leap in precision leads to new scientific

and technological advances.

Measurement has shaped our history,

and will continue to change our world.

Next, we explore the world of energy.

And how the measurement of light, heat and electricity

have transformed our lives

as I continue my journey into measurement.