‘Selfie’ of the Year

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.


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