Archive for December, 2013

‘Selfie’ of the Year

December 31, 2013
Me reflected in one half of the 'Boltzmann' Sphere

My reflection in one half of the ‘Boltzmann’ Sphere. Click the image for a much larger version.

One of my achievements this year was writing a feature article for Physics World, the magazine for members of the Institute of Physics.

I was pleased with the article, but shocked to find they had put my picture on the front cover. And I mean ‘my picture’ in both senses: it’s a ‘selfie’.

The star of the picture is obviously the beautiful copper hemisphere made by Paul Morantz and his colleagues at Cranfield University. But I took the picture, and that masked, gloved figure is me.

And to round out the year, the nice folk at Physics World chose this as one of their favourite pictures of 2013.

What is it for?

It is one half of a ‘quasi-spherical’ microwave and acoustic resonator at the heart of the most accurate thermometer on Earth!

  • ‘Quasi-spherical’ means that it has been deliberately made to be slightly non-spherical.
  • ‘Resonator’ means that waves whose wavelength just matches the size of the container bounce around thousands of times before decaying.
  • ‘Acoustic’ means that we use sound waves – the device is like a highly-tuned musical instrument
  • ‘Microwave’ means we use also use high-frequency radio waves like a kind of radar to gauge the size of the cavity.

And how does that make a thermometer?

It is complicated.

  • First, the microwave ‘radar’ allows us to work out the size of resonator very accurately. The diameter is about 63 mm (it changes with temperature and pressure) and we can measure it with an uncertainty of just 12 nm i.e. 0.000 012 mm.
    • It is the near perfect manufacture that allows us to use this ‘radar’ technique, and as a byproduct gives the inner surface its mirror finish, and means the photo is a picture of ‘me’ and not ‘it’.
  • Then we find the frequencies of sound wave that resonate. When the sound is at its loudest we know that the wavelength exactly matches the cavity dimensions.
  • Knowing the wavelength and frequency of the sound wave allows us to work out its speed: speed = frequency x wavelength
  • The speed of sound in a gas is directly related to the speed of molecules and fundamentally temperature is a measure of the energy of motion associated with molecular speed.

And so we can work out the temperature! As I write this, it seems unlikely that it could ever work – and looking at the picture below with all the wires attached to the sphere it seems even more so.

The spherical resonator assembled with all its probes.

The spherical resonator assembled and almost ready for action with all its acoustic and microwave probes.

But we have checked our results in every way we know how, and it does seem to be working very well.

Of course this is not the kind of thermometer that you can take somewhere and ‘stick in something’. Instead the idea is that we bring other thermometers to this one and then find out how wrong the other thermometers are.

So I hope you enjoy the picture at the head of this article, because now that the thermometer is working, the inside of the resonator will – in all probability – never be seen again by human eyes.

Happy Christmas 2013

December 20, 2013
Christmasium: 100% pure Christmas: Caution: this element may induce festivactivity with a half-life of 6 days.

Christmasium: 100% pure Christmas.

It’s a little bit early to shut up shop for Christmas, but frankly I have had enough of 2013.

There have been successes, but since 2000, when I escaped the bullying environment of UCL Physics Department, I have never been unhappier at work. I have also never been busier: I can’t recall a single weekend in which I have not been working, preparing for some looming deadline or scrambling to finish stuff which was already late.

So my aim over the break is to stop and try and catch my breath and think how I want to deal with 2014.

However one aspect of my life which I intend to keep up with in 2014 is this blog. I really enjoy the process of trying to distil my thoughts into a few hundred words once or twice a week.

And the thought that you, dear reader, take the time to read these words and occasionally share your comments, makes it seem like the enterprise is not entirely solitary.

So: thank you: I wish you a Happy Christmas and every best wish for 2014.


Protons for Breakfast: Sad news

December 18, 2013
PfB logo

Have you eaten protons for breakfast?

Having just completed the 18th presentation of Protons for Breakfast to the largest and most enthusiastic audience ever, I have reluctantly decided that the time has come to stop.

In fact the course won’t stop just now, but instead there will be two more presentations in the spring and autumn of 2014, and it will stop one year from now.

I confess to feeling sad – heartbroken in fact – but I hope I will feel better in the New Year.

Anyway. I just thought I would let people know.

Cold Candles

December 18, 2013
Do candles which are first chilled in the freezer burn longer than candles at room temperature?

Do candles which are first chilled in the freezer burn longer than candles started at room temperature? No. I lit the candle on the right after taking it straight out of the freezer.

Please forgive this last piece of candle nonsense! After the talk last week the first questioner asked me:

“Why do candles burn for longer after you put them in the freezer?”

Frankly it wasn’t a question I was expecting. I did have the presence of mind to ask if she had compared the cold candles with some she had kept at room temperature.

She said, “No”, but she did say that when she took the candles out of the freezer they had “burned all evening”.

Faced with this experimental ‘fact’ I speculated that perhaps the flame lost some energy melting the colder wax. This seemed to satisfy the questioner, but I was not happy.

So the next night I put a candle in the freezer and after a couple of hours I burned it with a similar candle I had kept at room temperature: I could see no obvious difference.

And so this evening I conducted the definitive experiment. I had previously chilled a candle in the freezer and my (uncalibrated*) thermometer told me its temperature was -13.8 °C. The comparison candle was at room temperature: +18.4 °C. These temperatures should be compared with the melting temperature of wax which is typically 55 °C.

Both candles initially weighed 54 grams and I lit them quickly to maximise any cooling effect before the ‘cold candle’ warmed up.

Making measurements in the usual way  it became apparent that the candles burned at nearly the same rate, and if anything, the ‘cold candle’ burned a little faster!

The three gram difference is only just resolvable with my weighing machine, but it corresponds to nearly 9 mm difference in height and as the photograph at the top of the page shows, the ‘cold candle’ is definitely shorter than the ‘warm candle’.

Graph showing the mass loss versus burning time for a 'Cold Candle' taken straight from the freezer and a 'Warm Candle' taken out of a drawer.

Graph showing the mass loss versus burning time for a ‘Cold Candle’ taken straight from the freezer and a ‘Warm Candle’ taken out of a drawer. Although the difference is not large, the cold candle has burned faster than the warm candle. (Click for larger graph)

So what, I hear you silently wondering, is the point? The point is that I now know the answer to this question! And so do you. Experimental physics has answered one question and – as it frequently does – raised several more. For example: is the difference in burning rates really associated with temperature or was there some other variable I didn’t control. Perhaps another happy evening of experimentation will resolve this!

I love Experimental Physics, and when work is depressing or unsatisfying, the ability to definitively resolve such questions as this – pointless as they are – gives me considerable comfort.


*Sorry: At home I have to operate to much lower standards 😦

Candles at Christmas

December 13, 2013

My colleagues at NPL have just finished making a video of my talk about candles so I thought I would share it with you while it was fresh.

Thanks to Lloyd for the video and thanks to everyone who helped with the talk. Personally I hate watching myself, but I hope you enjoy it.

You can follow up on the topics in the talk with the links below:

Looking on the bright side

December 10, 2013
A parliamentary 'standard candle' - made from spermaceti - a substance found only in the heads of sperm whales.

A parliamentary ‘standard candle’ – made from spermaceti – a substance found only in the heads of sperm whales. Click for a larger view.

I gave a 20-minute talk last Thursday – a Christmas talk about candles. As usual with these things, I started out knowing a little, but the process of preparing the talk involved learning lots of interesting things. And then not mentioning them.

I spoke to colleagues all over NPL to ask for help: one lent me a precision balance to weigh a candle as it burned, and another built me a device to power an electric torch from a candle! I thought that was very cool.

When I asked my colleagues in the optical team about measuring the spectrum of light from a candle their eyes lit up and I could barely stop them talking – they knew so much.

After lending me a spectrometer, they mentioned that they had an old ‘standard candle’ in their office. In the ‘old days’ it was a candle such as the one in the picture at the top that formed humanity’s standard for ‘an amount of light’.

And even though our modern standard  – the candela – is defined quite differently, its magnitude can still be linked back to the amount of light given off by a standard candle.

Gazing at the candle I was astounded. Had a whale really been killed in order to make this candle? The answer was ‘Yes’: I felt like I was holding ivory in my hands.

The idea that we would kill whales in order to extract oil and make candles is now so bonkers that we can hardly credit it. And this made me feel a little better.

It made me realise how desperate people must have been for light, and that for all its faults, our civilisation has now all but solved this problem. And that made me smile – and momentarily reflect on the brighter side of humanity’s adventure with energy.

Why is the Earth the temperature it is?

December 10, 2013
The 'anomaly' of the air temperature above the land surfaces of the Earth. The area highlighted at the top right shows temperature estimates in recent years. The 'zero' corresponds to the average value between 1961 and 1990.

The ‘anomaly’ of the air temperature above the land surfaces of the Earth. The area highlighted at the top right shows temperature estimates in recent years. The ‘zero’ corresponds to the average value between 1961 and 1990.

In all the argumentation about whether the Earth is warming or is undergoing a temperature ‘hiatus’, people often fail to address themselves to some basic questions. And the most basic of these questions is ‘Why is the Earth the temperature it is?’

Recently I addressed this question in a webinar recorded for Chemistry and Industry magazine and this will be broadcast later today (Tuesday December 10th 2013) at 4:00 p.m. UK time.

You can listen in and ask questions by registering here. If you would like the PowerPoint slides you can download them using the link below:

Why is the Earth the temperature it is?

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