Summer Science

May 26, 2018

Video Capture 2

For some months now I have been preparing for the Royal Society Summer Science Exhibition.

We have been working with the fabulous team at Science Projects on developing seven demonstration experiments – one for each of the seven SI base units.

Being so distracted, the deadline for submitting a video almost passed me by. In fact my colleague Andrew Hanson and I remembered with just one day to go!

So after a necessarily short planning phase, Andrew and I shot the video below on Andrew’s iPhone.

The background noise on some of the sections was problematic and Andrew had to do a great deal of filtering to get anything close to audible.

But given that everything was shot in’one take’, we were pretty happy with it, even if it came out a bit long (5’20”)

The end of the film was forced on us because my colleagues from the ‘length team’ were both absent when the end of the film was shot at about 7:30 p.m.!

After feedback from the team at the Royal Society we were asked to shorten the video and we took that opportunity to re-shoot the start and end of the movie with a proper microphone.

And here is the final shortened version (2’34”) which should be on the Royal Society site next week.

I hope you enjoy it.


Thanks to everyone who helped: Andrew Hanson, Brian Madzima, Rachel Godun, Stuart Davidson, Robin Underwood, Teresa Goodman, Lucy Culleton, Masaya Kataoka and Jonathan Fletcher


The Last Artifact

May 20, 2018

Handling a kilogram (but not THE kilogram). Picture taken from The Last Artifact Web Site

Don’t pack away your Royal Wedding party gear just yet! Today (Sunday 20th May) is World Metrology Day 2018!

And that means there are just 5 months and 26 days until the commencement of the 26th General Conference on Weights and Measures (CGPM).

At this governmental level gathering, it will hopefully be decided to go ahead with the redefinition of four of the base units of the International System of Units, the SI.

And if matters proceed as planned, in one year’s time – World Metrology Day 2019 – we will finally make the change.

It’s all about the kilogram

All the unit redefinitions – of the kilogram, the ampere, the kelvin and the mole – are important.

But the redefinition of the kilogram has been the hardest and is considered an event of such significance that someone is making a high-end film about it.

I was fortunate enough to meet the co-director Ed Watkins and his crew when they swung by NPL last year to film.

The film will be released on World Metrology Day 2019, but the trailer (below) certainly looks intriguing.

How mass measurement will change.

At the moment, when we weigh something we:

  • compare the force of gravity on that object with the force of gravity on a standard object.
  • and the force of gravity on that standard object is known by comparison against the force of gravity on a more special standard object
  • Add so we proceed in many steps until eventually, we encounter a weighing against the International Prototype of the Kilogram (the IPK). This single unique ‘artifact’ currently defines what we mean by ‘one kilogram’.

This kind of repeated comparison against standards until we reach a defining artefact is completely normal in traditional metrology.

In future, when we weigh something we will:

  • compare the force of gravity on that object with the force of gravity on a standard object.
  • and the force of gravity on that standard object is known by comparison against the force of gravity on a more special standard object
  • Add so we proceed in many steps until eventually, we encounter a weighing on a Kibble Balance or a weighing against a specially-made silicon sphere.

It is these two new options that represent the change.

  • When we weigh an object on a Kibble Balance, we compare the gravitational force on an object with an electromagnetic force which can be calculated in terms of volts and amperes and related to fundamental physical constants.
  • Alternatively, the special silicon spheres have their mass calculated in terms of their physical properties: size, density etc.

In either case, the final definition of what we mean by one kilogram is determined by the basic physical measurements, and is no longer simply a comparison against an arbitrary physical artifact.

That’s it. It’s a small change, but as I am sure the film will make clear, a profound one.

The James Webb Space Telescope

May 10, 2018

Last week I was on holiday in Southern California. Lucky me.

Lucky me indeed. During my visit I had – by extreme good fortune – the opportunity to meet with Jon Arenberg – former engineering director of the James Webb Space Telescope (JWST).

And by even more extreme good fortune I had the opportunity to speak with him while overlooking the JWST itself – held upright in a clean room at the Northrop Grumman campus in Redondo Beach, California.

[Sadly, photography was not allowed, so I will have to paint you a picture in words and use some stock images.]


In case you don’t know, the JWST will be the successor to the Hubble Space Telescope (HST), and has been designed to exceed the operational performance of the HST in two key areas.

  • Firstly, it is designed to gather more light than the HST. This will allow the JWST to see very faint objects.
  • Secondly, it is designed to work better with infrared light than the HST. This will allow the JWST to see objects whose light has been extremely red-shifted from the visible.

A full-size model of the JWST is shown below and it is clear that the design is extraordinary, and at first sight, rather odd-looking. But the structure – and much else besides – is driven by these two requirements.

JWST and people

Requirement#1: Gather more light.

To gather more light, the main light-gathering mirror in the JWST is 6.5 metres across rather than just 2.5 metres in the HST. That means it gathers around 7 times more light than the HST and so can see fainter objects and produce sharper images.


Image courtesy of Wikipedia

But in order to launch a mirror this size from Earth on a rocket, it is necessary to use a  mirror which can be folded for launch. This is why the mirror is made in hexagonal segments.

To cope with the alignment requirements of a folding mirror, the mirror segments have actuators to enable fine-tuning of the shape of the mirror.

To reduce the weight of such a large mirror it had to be made of beryllium – a highly toxic metal which is difficult to machine. It is however 30% less dense than aluminium and also has a much lower coefficient of thermal expansion.

The ‘deployment’ or ‘unfolding’ sequence of the JWST is shown below.

Requirement#2: Improved imaging of infrared light.

The wavelength of visible light varies from roughly 0.000 4 mm for light which elicits the sensation we call violet, to 0.000 7 mm for light which elicits the sensation we call red.

Light with a wavelength longer than 0.000 7 mm does not elicit any visible sensation in humans and is called ‘infrared’ light.

Imaging so-called ‘near’ infrared light (with wavelengths from 0.000 7 mm to 0.005 mm) is relatively easy.

Hubble can ‘see’ at wavelengths as long as 0.002 5 mm. To achieve this, the detector in HST was cooled. But to work at longer wavelengths the entire telescope needs to be cold.

This is because every object emits infrared light and the amount of infrared light it emits is related to its temperature. So a warm telescope ‘glows’ and offers no chance to image dim infrared light from the edge of the universe!

The JWST is designed ‘see’ at wavelengths as long as 0.029 mm – 10 times longer wavelengths than the HST – and that means that typically the telescope needs to be on the order of 10 times colder.

To cool the entire telescope requires a breathtaking – but logical – design. There were two parts to the solution.

  • The first part involved the design of the satellite itself.
  • The second part involved the positioning the satellite.

Cooling the telescope part#1: design

The telescope and detectors were separated from the rest of the satellite that contains elements such as the thrusters, cryo-coolers, data transmission equipment and solar cells. These parts need to be warm to operate correctly.

The telescope is separated from the ‘operational’ part of the satellite with a sun-shield roughly the size of tennis court. When shielded from the Sun, the telescope is exposed to the chilly universe, and cooled gas from the cryo-coolers cools some of the detectors to just a few degrees above absolute zero.

Cooling the telescope part#2: location

The HST is only 300 miles or so from Earth, and orbits every 97 minutes. It travels in-to and out-of full sunshine on each orbit. This type of orbit is not compatible with keeping a gigantic telescope cold.

So the second part of the cooling strategy is to position the JWST approximately 1 million miles from Earth at a location known as the second Lagrange point L2.

At L2 the gravitational attraction of the Sun is approximately 30 times greater than the gravitational attraction of the Earth and Moon.

At L2 the satellite orbits the Sun in a period of one year – and so stays in the same position relative to the Earth.

  • The advantage of orbiting at L2 is that the satellite can maintain the same orientation with respect to the Sun for long periods. And so the sun-shade can shield the telescope very effectively, allowing it to stay cool.
  • The disadvantage of orbiting at L2 is that it is beyond the orbit of the moon and no manned space-craft has ever travelled so far from Earth. So once launched, there is absolutely no possibility of a rescue mission.

The most expensive object on Earth?

I love the concept of the JWST. At an estimated cost of $8 billion, if this is not the most expensive single object on Earth, then I would be interested to know what is.

But it has not been created to make money or as an act of aggression.

Instead, it has been created to answer the simple question

I wonder what we would see if we looked into deep space at infrared wavelengths.”. 

Ultimately, we just don’t know until we look.

In a year or two, engineers will place the JWST on top of an Ariane rocket and fire it into space. And the most expensive object on Earth will then – hopefully – become the most expensive object in space.

Personally I find the mere existence of such an enterprise a bastion of hope in a world full of worry.


Many thanks to Jon Arenberg  and Stephanie Sandor-Leahy for the opportunity to see this apogee of science and engineering.


Breathtaking photographs are available in galleries linked to from this page


Error Bar: Update

April 17, 2018

Error Bar

Friends – I have found the author of the above image that I mentioned in my previous post

And I knew them all along! What are the chances of that!

The author is John Kennedy from the Meteorological Office who blogs under the pseudonym ‘diagram monkey’.

The story of the image’s creation can be found here.

It is based on a real similarly-named barthe Aero Bar in San Diego.

The Aero Club Bar

The Aero Club Bar, San Diego

The World of Diagram Monkey

John’s blog contains some wonderful resources.

Do check out his post to find out how he came close to a humiliating early death at the hands of an orange.



Error Bar

April 15, 2018

Error Bar

This picture arrived in my in box through the medium of Twitter.

The Bar

It shows the Error Bar, with 20 ± 2 beers on tap, and a neon sign in which two glasses conspire to make an uncertainty indication.

It must surely be run by a burned-out metrologist who couldn’t take the heat of cutting-edge metrology.

The modern day equivalent of Graham Greene’s ‘whisky priest’, they retired to a town with barely a single calibration laboratory.

Here, they run the Error Bar and (unheeded) give advice on uncertainty estimation to random passers by while dispensing precise doses of tequila, with amounts of ethanol traceable to the SI base unit mole .

The awning of the bar sports the logo of the BIPM – the International Bureau of Weights and Measures – where they were seconded for a summer.

Error Bar detail 2

However it was here that their true love slipped away while they worked on an impossible uncertainty budget. And they never recovered.

In memory of their lost love,  they commissioned the local blacksmith to create a railing on the disabled access ramp which reflects the uncertainty that life always entails.

Error Bar detail 1

The Restaurant

And after a drinking a glass or two of tequila, one can retire to the restaurant next door – Measurands (literally meaning “the things which are measured”).

Error Bar detail 3

Is this place real?

I doubt it. 

But the picture has been created with great care by a metrologist and (if they can ever confess to creating this picture)  I would love to shake their hand…

…and perhaps buy them a drink in this little out-of-the-way bar I have heard of…



April 7, 2018


Back in December 2010 I wrote about Hans Rosling with a post titled:

Hans Rosling: You’re my hero

Sadly, Hans Rosling died in February 2017.

But aware of his imminent death, he worked with his son and daughter-in-law to write a book which captures some key elements of his world view, which he summarises as…


The book has two strands that run through all the chapters.

  • The first strand is that we are tremendously ignorant about the world. Repeatedly he expresses his surprise at our levels of ignorance. And given our access to facts, he asks why we are not just randomly ignorant, but have views which are  systematically wrong. He suggests it is a kind of cognitive bias.
  • The second strand is that we can stop being ignorant if we want to. And with this aim, he and his son invented delightful ways to view developmental data.

I won’t try to precis the book, but as someone who experiences intense anxiety in everyday situations, I found the section “Statistics as Therapy” particularly affecting.

As one specific example I went to the download page of Gapminder and downloaded a Powerpoint file with a graph showing how extreme poverty in the world has changed over time.

Extreme Poverty

It is clear that collectively humanity has made truly astonishing progress in reducing the awfulness of crushing poverty. [The Powerpoint file and the web site includes links so you can chase the data sources and check them.]

Why do we not celebrate this fantastic achievement?

This graph tells a good news story compared with which any news story we have experienced in the last 10 years is irrelevant.

This is a story of an epochal and positive change in the world of which most people – including myself – are largely ignorant.

The idea that the world – while acknowledging all its faults and injustices – is dramatically better than it is has ever been, feels like a balm against the ‘news-ification’ of reality that we experience.

Hans Rosling’s Death

I leave you with a video which I find intensely moving.

It shows Hans, his son,and his daughter-in-law explaining why they wrote the book.

Somehow knowing that he is no longer with us feels like a very intense – and despite the fact that I never knew him, personal – loss.



The fact that I feel Hans Rosling’s death personally is probably a cognitive bias caused by his many engaging video appearances.

This page on the Gapminder website contains links to many of his best videos. I cannot recommend them strongly enough.

Air Temperature

April 1, 2018

Recently, two disparate strands of my work produced publications within a week of each other.

Curiously they both concerned one of the commonest measurements made on Earth today – the measurement of air temperature.

  • One of the papers was the result of a humbling discovery I made last year concerning a common source of error in air temperature measurements. (Link to open access paper)
  • On the other  paper I was just one amongst 17 authors calling for the establishment of global reference network to monitor the climate. My guess is that most people imagine such a network already exists – but it doesn’t! (Link to open access paper)

I am writing this article because I was struck by the contrasting styles of these papers: one describing an arcane experimental detail; and the other proposing a global inter-governmental initiative.

And yet the aim of both papers was identical: to improve measurement so that we can more clearly see what is happening in the world.

Paper 1

In the middle of 2018 I was experimenting with a new device for measuring air temperature by measuring the speed of sound in air.

It’s an ingenious device, but it obviously needed to be checked. We had previously carried out tests inside environmental chambers, but the temperature stability and uniformity inside the chambers was not as good as we had hoped for.

So we decided to test the device in one of NPL’s dimensional laboratories. In these laboratories, there is a gentle, uniform flow of air from ceiling to floor, and the temperature is stable to within a hundredth of a degree Celsius (0.01 °C) indefinitely.

However, when I tried to measure the temperature of the air using conventional temperature sensors I got widely differing answers – varying by a quarter of a degree depending on where I placed the thermometer. I felt utterly depressed and humiliated.

Eventually I realised what the problem was. This involved stopping. Thinking carefully. And talking with colleagues. It was a classic case of eliminating the impossible leaving only the improbable.

After believing I understood the effect, I devised a simple experiment to test my understanding – a photograph of the apparatus is shown below.


The apparatus consisted of a set of stainless steel tubes held in a clamp stand. It was almost certainly the cheapest experiment I have ever conducted.

I placed the tubes in the laboratory, exposed to the downward air flow, and  left them for several hours to equilibrate with air.

Prior to this experience, I would have bet serious amounts of money on the ‘fact’ that all these tubes would be at the same temperature. My insight had led me to question this assumption.

And my insight was correct. Every one of the tubes was at a different temperature and none of them were at the temperature of the air! The temperature of the tubes depended on:

  • the brightness of the lights in the room – which was understandable but a larger effect than I expected, and
  • the diameter of the tubes – which was the truly surprising result.

Results 1

I was shocked. But although the reason for this is not obvious, it is also not complicated to understand.

When air flows air around a cylindrical (or spherical) sensor only a very small amount of air actually makes contact with the sensor.

Air reaching the sensor first is stopped (it ‘stagnates’ to use the jargon). At this point heat exchange is very effective. But this same air is then forced to flow around the sensor in a ‘boundary layer’ which effectively insulates the sensor from the rest of the air.

Air flow

For small sensors, the sensor acquires a temperature close to that of the air. But the air is surprisingly ineffective at changing the temperature of larger sensors.

The effect matters in two quite distinct realms.


In metrology – the science of measurement – it transpires that knowledge of the temperature of the air is important for the most accurate length measurements.

This is because we measure the dimensions of objects in terms of the wavelength of light, and this wavelength is slightly affected by the temperature of the air through which the light passes.

In a dimensional laboratory such as the one illustrated below, the thermometer will indicate a temperature which is:

  • different from the temperature of artefacts placed in the room, and
  • different from the temperature of the air.


Unless the effect is accounted for – which it generally isn’t – then length measurements will be slightly incorrect.


The effect is also important in climatology. If a sensor is changed in a meteorological station people check that the sensor is calibrated, but they rarely record its diameter.

If a calibrated sensor is replaced by another calibrated sensor with a different diameter, then there will be a systematic effect on the temperatures recorded by the station. Such effects won’t matter for weather forecasting, but they will matter for people using the stations for a climate record.

And that brings me to Paper 2

Paper 2

Hadcrut4 Global Temperature

When we see graphs of ‘global temperatures’ over time, many people assume that the data is derived from satellites or some ‘high-tech’ network of sensors. Not so.

The ‘surface’ temperature of the Earth is generally estimated in two quite distinct parts – sea surface temperature and land surface temperature. But both these terms are slight misnomers.

Considering just the land measurements, the actual temperature measured is the air temperature above the land surface. In the jargon, the measurement is called LSAT – the Land Surface Air Temperature.

LSAT is the temperature which human beings experience and satellites can’t measure it.

LSAT data is extracted from temperature measurements made in thousands of meteorological stations around the world. We have data records from some stations extending back for 150 years.

However, it is well known that data is less than ideal: it is biased and unrepresentative in many ways.

The effect described in Paper 1 is just one of many such biases which have been extensively studied. And scientists have devised many ways to check that the overall trend they have extracted – what we now call global warming – is real.

Nonetheless. It is slightly shocking that a global network of stations designed specifically with the aim of climate monitoring does not exist.

And that is what we were calling for in Paper 2. Such a climate network would consist of less than 200 stations world-wide and cost less than a modest satellite launch. But it would add confidence to the measurements extracted from meteorological stations.

Perhaps the most important reason for creating such a network is that we don’t know how meteorological technology will evolve over the coming century.

Over the last century, the technology has remained reasonably stable. But it is quite possible that the nature of data acquisition for meteorological applications will change  in ways we cannot anticipate.

It seems prudent to me that we establish a global climate reference network as soon as possible.


Paper 1

Air temperature sensors: dependence of radiative errors on sensor diameter in precision metrology and meteorology
Michael de Podesta, Stephanie Bell and Robin Underwood

Published 28 February 2018
Metrologia, Volume 55, Number 2

Paper 2

Towards a global land surface climate fiducial reference measurements network
P. W. Thorne, H. J. Diamond, B. Goodison , S. Harrigan , Z. Hausfather , N. B. Ingleby , P. D. Jones ,J. H. Lawrimore , D. H. Lister , A. Merlone , T. Oakley , M. Palecki , T. C. Peterson , M. de Podesta , C. Tassone ,  V. Venema, K. M. Willett

Published: 1 March 2018
Int. J. Climatol 2018;1–15.

Obesity Policy

March 6, 2018

BBC Story Extract

Today the BBC are reporting that:

Britain needs to go on a diet, says top health official

The article states that people should allow:

  • 400 kilo-calories for breakfast
  • 600 kilo-calories for lunch
  • 600 kilo-calories for dinner

which adds up to 1600 kilo-calories a day. With this dietary intake, most adults in sedentary occupations will lose weight or maintain a healthy weight.

However, the article then goes on to say:

It is recommended that women should eat no more than 2,000 kilo-calories a day, while men should limit their intake to 2,500 kilo-calories.

No! As I pointed out previously, this is just too many calories for both men and women with sedentary lifestyles.

Any government campaign based on these figures is bound to fail.

Calories versus Age

For someone of my height and weight, the government’s recommended dietary intake is about 30% too high.

Is weight homeostasis possible?

February 28, 2018

I am slightly obsessed with my weight. Forgive me: I am 58 and have spent many decades repeatedly putting on weight slowly, and then losing it rapidly.

For many years I have wondered why can’t I just eat modestly and trust my body to “sort itself out!”

My recent discovery of the Mifflin St Joer equations (link) has allowed me to  simulate my weight over time, and my calculations are allowing me to understanding my own experience.

But my calculations have also raised a profound question:

  • Is homeostasis of weight even possible?


Homeostasis (or Homoeostasis) is the term given to physiological systems which conspire to keep something constant.

For example, we have systems that maintain our body temperature without any conscious effort. I don’t have to berate myself for being too hot and promise myself that in the future I will try to be cooler.

No. Our bodies sort out their internal temperature. I understand the system consists of temperature sensitive cells and nervous system reflexes that control blood flow, sweat glands, shiver reflexes, and our desire to undertake activity.


And I have generally imagined that in a more perfect world, a similar kind of system would underpin my desire to eat.

In this ideal world, I would naturally maintain my weight without any obvious effort on my part – stopping eating when I had eaten ‘enough’.

I had thought such a system actually existed. One part of the system is supposed to arise from the competing actions of hormones such as ghrelin – which makes us experience hunger – and leptin – which makes us feel satiated.

Together, ghrelin and leptin are supposed to act as part of a system of energy homeostasis.

However, having run many simulations of my own weight versus time (see below) and reflected on this, I am sceptical.

“But I know a bloke who…”

We all know people who seem to be able to eat at their ease and not put on weight.

I have no explanation for that, but then I have never experienced that myself.

My experience is that my weight either increases or decreases over time. What I have never observed it to do in all my 58 years on Earth is to stay the same! (I have written about this before: story 1 or story 2.)

What’s the problem?

I programmed the Mifflin St Joer equations into a spreadsheet to see the predicted effect on my weight of various dietary and exercise choices.

You can download the spreadsheet here and perform calculations about yourself in the privacy of your own computer. 

I entered my current age (58.2 years) and weight (74 kg), and I used the MSJ equations to predict what would happen to my weight if I ate 1800 kiloCalories (kCal) a day.

The results are shown below together with the effect of eating 50 kCal/day more or less

Weight versus Age Projection

  • The red line suggests that if I eat 1800 kCal/day then my weight will gradually decline over the next couple of years stabilising at about 71 kg. That would be dandy.
  • However, the dotted green lines show what would happen if I got my calorific intake wrong by ± 50 kCal per day. This is plus or minus half of a small glass of wine, or a half a biscuit either eaten, or not eaten.

These ‘alternate realities’ predict that my weight in three years time might be anywhere between 64 kg and 77 kg – a range of 13 kg!

To be within a kilogram of the predicted weight, my average energy intake would need to match 1800 kCal/day within 10 kCal a day. That is less than a single mouthful of food!

I don’t believe that any autonomic system can achieve that level of control. 

Weight versus Age Projection 2

So what?

Reflecting on these simulations, I don’t believe that the systems within our bodies that mediate ‘energy homoeostasis’ operate well over many years.

At least they don’t operate well in an environment where calories are so easy to obtain.

So I think my experience of slow weight gain over time is not a fault with my autonomic nervous system, or a moral failing on my part. It is just the way things are.

Asking the thinerati

Asking several slim individuals around the coffee machine this morning confirmed my view. They all were either (a) young (b) self-conscious about fitting into clothes or (c) weighed themselves regularly.

Personally I have resolved to keep weighing myself and using this to provide manual feedback.

How is my weight doing? Thank you for asking. It’s been just about stable since Christmas and I intend to keep it that way!

SI at the RI

February 23, 2018


MdeP at the RI

On Monday 16th October 2017 I gave a talk at about the International System of Units (the SI) at the Royal Institution (the RI) in London.

It wasn’t a great talk, but it was at the RI. And I stood where Michael Faraday stood!

The RI have now processed the video and produced an edited version: enjoy 🙂

The RI have tended to retain the video of me talking rather than showing the animated PowerPoint slides. If you would like the full multimedia experience, you can download the presentation using the link below.

On the day

I was nervous and arrived ridiculously early with a couple of glass Dewars containing triple point of water cells.

I waited outside the lecture theatre for Martin Davies from the RI to arrive.

When he arrived, he noticed the Dewars and without hesitation he turned to the wall and pointed out the painting above where I was standing, and said:

“What a coincidence: your standing by a picture of Sir James Dewar lecturing in this theatre!”

Henry Dewar at the RI

It’s hard to convey the historical significance of Royal Institution without sounding trite. So I won’t try.

But it is a special place for chemists and physicists alike, and I feel honoured to have even had the chance to stand on that spot.


Thanks to Chris Brookes and Martin Davies for a memorable day.

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