Posts Tagged ‘Si’

The Death Knell for SI Base Units?

January 30, 2019

I love the International System of Units – the SI. 

Rooted in humanity’s ubiquitous need to measure things, the SI represents a hugely successful global human enterprise – a triumph of cooperation over competition, and accord over discord.

Day-by-day it enables measurements made around the world to be meaningfully compared with low uncertainty. And by doing this it underpins all of the sciences, every branch of engineering, and trade.

But changes are coming to the SI, and even after having worked on these changes for the last 12 years or so, in my recent reflections I have been surprised at how profound the changes will be.

Let me explain…

The Foundations of the SI

The SI is built upon the concept of ‘base units’. Unit amounts of any quantity are defined in terms of combinations of unit quantities of just a few ‘base units’. For example:

  • The SI unit of speed is the ‘metre per second’, where one metre and one second are the base units of length and time respectively.
    • The ‘metre per second’ is called a derived unit.
  • The SI unit of acceleration is the ‘metre per second per second’
    • Notice how the same base units are combined differently to make this new derived unit.
  • The SI unit of force  is the ‘kilogram metre per second per second’.
    • This is such a complicated phrase that this derived unit is given a special name – newton. But notice that it is still a combination of base units.

And so on. All the SI units required for science and engineering can be derived from just seven base units: the kilogram, metre, second, ampere, kelvin, mole and candela.

So these seven base units in a very real sense form the foundations of the SI.

The seven base units of the SI

The seven base units of the SI

This Hierarchical Structure is Important.

Measurement is the quantitative comparison of a thing against a standard.

So, for example, when we measure a speed, we are comparing the unknown speed against our unit of speed which in the SI is the metre per second.

So a measurement of speed can never be more accurate than our ability to create a standard speed – a known number of ‘metres per second‘ – against which we can compare our unknown speed.

FOR EXAMPLE: Imagine calibrating a speedometer in a car. The only way we can know if it indicates correctly is if we can check the reading of the speedometer when the car is travelling at a known speed – which we would have to verify with measurements of distance (in metres) and time (in seconds).

To create a standard speed, we need to create known distances and known time intervals. So a speed never be more accurately known that our ability to create standard ‘metres‘ and ‘seconds‘.

So the importance of the base units is that the accuracy with which they can be created represents a limit to the accuracy with which we could conceivably measure anything! Or at least anything expressed as in terms of derived unit quantities in the SI

This fact has driven the evolution of the SI. Since its founding in 1960, the definitions of what we mean by ‘one’ of the base units has changed only rarely. And the aim has always been the same – to create definitions which will allow more accurate realisations of the base units. This improved accuracy would then automatically affect all the derived units in SI.

Changes are coming to the SI.

In my earlier articles (e.g. here) I have mentioned that on 20th May 2019 the definition of four of the base units will change. Four base units changing at the same time!? Radical.

Much has been made of the fact that the base units will now be defined in terms of constants of nature. And this is indeed significant.

But in fact I think the re-definitions will lead to a broader change in the structure of the SI.

Eventually, I think they will lead to the abandonment of the concept of a ‘base unit’, and the difference between ‘base‘ units and ‘derived‘ units will slowly disappear.

The ‘New’ SI.

si illustration only defining constants full colour

The seven defining constants of the ‘New’ SI.

In the ‘New’ SI, the values of seven natural constants have been defined to have exact values with no measurement uncertainty.

These are constants of nature that we had previously measured in terms of the SI base units. The choice to give them an exact value is based on the belief – backed up by experiments – that the constants are truly constant!

In fact, some of the constants appear to be the most unchanging features of the universe that we have ever encountered.

Here are four of the constants that will have fixed numerical values in the New SI:

  • the speed of light in a vacuum, conventionally given the symbol c,
  • the frequency of microwaves absorbed by a particular transition in Caesium, atoms conventionally given the symbol ΔνCs, (This funny vee-like symbol ν is the Greek letter ‘n’ pronounced as ‘nu’)
  • the Planck constant, conventionally given the symbol h,
  • the magnitude of the charge on the electron, conventionally given the symbol e.

Electrical Units in the ‘Old’ SI and the ‘New’ SI.

In the Old SI the base unit referring to electrical quantities was the ampere.

If one were to make a measurement of a voltage (in the derived unit volt) or electrical resistance (in the derived unit ohm), then one would have to establish a sequence of comparisons that would eventually refer to combinations of base units. So:

  • one volt was equal to one kg m2 s-3 A-1 (or one watt per ampere)
  • one ohm was equal to one kg m2 s-3 A-2 (or one volt per ampere)

Please don’t be distracted by this odd combination of seconds, metres and kilograms. The important thing is that in the Old SI, volts and ohms were derived units with special names.

To make ‘one volt’ one needed experiments that combined the base units for the ampere, the kilogram, the second and the metre in a clever way to create a voltage known in terms of the base units.

But in the New SI things are different.

  • We can use an experiment to create volts directly in terms of the exactly-known constants ΔνCs×h/e.
  • And similarly we can create resistances directly in terms of the exactly-known constants e2/h

Since h and e and ΔνCs have exact values in the New SI, we can now create volts and ohms without any reference to amperes or any other base units.

This change is not just a detail. In an SI based on physical constants with exactly-known values, the ability to create accurate realisations of units no longer discriminates between base units and derived units – they all have the same status.

It’s not just electrical units

Consider the measurement of speed that I discussed earlier.

In the Old SI we would measure speed in derived units of metres per second i.e. in terms of the base units the metre and the second. And so we could never measure a speed with a lower fractional uncertainty than we could realise the composite base units, the metre or the second.

But in the New SI,

  • one metre can be realised in terms of the exactly-known constants c /ΔνCs
  • one second can be realised in terms of the exactly-known constant ΔνCs

So as a consequence,

  • one metre per second can be realised in terms of the exactly-known constant c

Since these constants are all exactly known, there is no reason why speeds in metres per second cannot be measured with an uncertainty which is lower than or equal to the uncertainty with which we can measure distances (in metres) or times (in seconds).

This doesn’t mean that it is currently technically possible to measure speeds with lower uncertainty than distances or times. What it means is that there is now nothing in the structure of the SI that would stop that being the case at some point in the future.

Is this good or bad?

So in the new SI, any unit – a derived unit or a base unit – can be expressed in terms of  exactly-known constants. So there will no longer be any intrinsic hierarchy of uncertainty in the SI.

On 20th May 2019 as the new system comes into force, nothing will initially change. We will still talk about base units and derived units.

But as measurement science evolves, I expect that – as is already the case for electrical units – the distinction between base units and derived units will slowly disappear.

And although I feel slightly surprised by this conclusion, and slightly shocked, it seems to be only a good thing – making the lowest uncertainty measurements available in the widest possible range of physical quantities.

°C and C are not the same!

October 5, 2016
Sometimes one has to write to the papers!

Sometimes one has to write to the papers!


Sometimes I am unable to stop myself writing to the papers.

Some issues – such as people not using measurement units correctly  – are just too important to let pass.

And people referring to temperature units incorrectly induces apoplexy!

For the record, the degree Celsius is an SI unit for temperature: the degrees C********e and F********t are not.

Their use in everyday language is understandable – many people use the F-word occasionally – and in the correct context, it gives no offence.

But for newspapers and media outlets to do so is outrageous!

And using the abbreviation C instead of °C is just wrong.

As I wrote to The Guardian recently:

Dear Guardian,

The measurement system that underpins all of our physical measurements of the world around us is called the International System of Units, widely referred to as ‘the SI’.

It is a staggering achievement, used daily by hundreds of thousands of scientists and engineers.

It provides a standard way of comparing measurements around the globe and of referring to those measurements. So why has The Guardian invented its own system of units?

To refer to a temperature of 25 degrees Celsius, the standard abbreviation is 25 °C. However The Guardian routinely refers to this as 25C, using the symbol ‘C’ which refers to the SI ‘coulomb’, an amount of electric charge. Why?

You might argue that your meaning is clear in context. And generally it is. But why be wrong when you can be right so easily?


Michael de Podesta

National Physical Laboratory.

P.S. In MS Windows™ systems, the degree symbol is [ALT] + 2 + 4 + 8 on the number keypad and in MacOS the degree symbol is [ALT] + [SHIFT] + 8. In iOS, on numeric keypad use a long press on the zero key to reveal the degree symbol.

P.P.S. There should also be a space between the number and its unit, but I didn’t want to mention that in case you thought I was being pedantic.

More seriously, reporting measurements in the correct units aids clarity of understanding and establishes the basic competence of the author.

Reporting, as The Guardian did this week, that:

“the 2016 temperature is likely to be 1.25C above pre-industrial times, following a warming trend where the world has heated up at a rate of 0.18C per decade.”

merely establishes that the writer knows nothing about measurements.

This is not a matter of style, it’s a matter of just being wrong.


[October 5th 2016: Weight this morning 73.5 kg: Anxiety: Low. I don’t know why, but I just felt OK today :-)]

Summer Science: 155 days to go!

January 26, 2013
Royal Society Selected!

My work has been selected for the Royal Society’s Summer Science Exhibition!

Just before Christmas I heard that I had been successful in bidding for a stand at the prestigious Royal Society Summer Science Exhibition. Competition is stiff with 75% of the proposals being rejected so somebody must think what I am doing is interesting! Wow!

Of course its not just me doing this and its not just about my work. It’s really about NPL and the importance of measurement to the country. And the reason we won a place is largely down to the enthusiasm of my colleague Andrew Hanson.

Today – along with Andrew and the other winners – I visited the Royal Society in London to be briefed on what we need to do. Having visited last year’s exhibition both as a visitor and as a helper at an NPL exhibit on bubbles, I knew that the task ahead would be exhausting. After being briefed on the design of the stand; online video and blogging; press interactions; school visits and official ‘soirees’, I left buzzing with ideas and anxiety.

We are actually combining stands with Terry Quinn and Richard Davies from the BIPM, who will bring along a ‘Do it yourself’ replacement for the kilogram. Terry and his grandson have made a model with wood, lego and a loudspeaker which is accurate to about 10%, but which demonstrates the principles of a much more complex, expensive and accurate device.

Terry Quinn and his nephew have built a simple version of a Watt Balance - a revolutionary way of measuring mass!

Terry Quinn and his nephew have built a simple version of a Watt Balance – a revolutionary way of measuring mass!

The device operates unlike almost every mass measurement made at present. Currently a mass measurement is – ultimately – a determination of the ratio of the unknown mass to the mass of the International Prototype Kilogram – the IPK. Terry’s device – called a ‘Watt Balance’ – measures mass without any reference to the IPK! Despite being very cheap – the mass measurement is made by comparing the force of gravity with an electro-magnetic force. It is – ultimately – a comparison of a mass against electrical standards which we can determine in terms of constants of nature.

Similarly my exhibit is about a thermometer that measures the speed of sound in a gas, which allows us to work out  the average speed of molecules. So we can measure temperature directly in terms of the kinetic energy of molecules. This is unlike any temperature measurement made at the moment. Currently a temperature measurement tells you how much hotter or colder a thing is than the temperature of the triple point of water.

Me and my thermometer. It measures temperature in terms of the kinetic energy of molecules with the copper container.

Me and my thermometer. It measures temperature in terms of the kinetic energy of molecules inside the copper container.

So the theme of the stand will be about measurements in everyday life being traceable to constants of nature rather than human artefacts. I love this idea – the transcendental and profound embedded into the mundane tasks of life. But what should we call the stand? Here are some ideas we are kicking around. Please vote – or suggest a new idea! Anyway, I look forward to seeing you between the 2nd and the 7th July – it’s free!

A Universal Language: the triumph of the SI

October 17, 2011
The World Standards Day Poster.

The World Standards Day Poster.

A Universal Language – a language that would allow genuine communication among all of humanity – has been a dream for as long as languages have existed. For spoken language, I suspect that this will remain a dream, and indeed the diversity of language is probably a cause for celebration. Scientifically, English is undoubtedly the current lingua franca, but scientists in many countries publish their findings in other languages, notably Russian, Chinese, French and German. However, when it comes to scientific measurement, there is an amazing and near universal agreement: the International System of Units – the SI – is the agreed system of measurement units amongst almost all the scientists in almost every country on Earth.

Just like a spoken language, the shared use of the language of measurement enables communication and indicates the existence of shared culture. I am writing this because this is a truly remarkable achievement which is largely uncommented upon. Despite the astonishing diversity of languages and cultures, there is almost universal use of the SI system of units when scientists communicate their results.

To be sure, there are still exceptions. Many subgroups of scientists value their own group culture above the ability to communicate universally. Typically they say they find their familiar units ‘more convenient’ or ‘more natural’. But time is not on their side . Whereas we are all losers when a spoken language is lost, we are all winners when people abandoned the use of angstroms and chose to use nanometres instead.

The SI system may appear to be essentially unchanging, but like all ‘languages’ it evolves with the culture it supports. I once spoke with Richard Davis, at that time the Executive Secretary of CCT, about the difference between the SI and the previous systems of units – the CGS system (centimetre – gram – second) and the MKS system (metre-kilogram-second). ‘Basically‘ he said, ‘the SI is the MKS system, but it has people who care about it‘.

Measurement may be defined as ‘quantitative comparison of an unknown quantity with a standard quantity’, and practically this requires people to agree about the standard quantities and exactly how they are realised – something which changes with time and technology – and how the comparisons are made. So there is an extensive system of committees which discuss points of  detail which affect the minutiae of the measurement system. But the existence of these committees keeps the system of units ‘alive’.

Whereas ‘globalisation’ is a contentious issue when it comes to manufacturing and industry, when considering the language of science, there is no doubt in my mind that it is an unequivocally positive process. And I think it is worth pausing, perhaps just for a moment, to reflect that occasionally people can cooperate in a global scale and achieve great things.

Pause… … …

Bring on Global Warming!

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