At the National Physical Laboratory we aim to make the best measurements in the world, and I am currently engaged in building the most accurate thermometer ever made. Without going into all the details, it works by measuring the speed of sound in a rather perfectly made spherical resonator. The speed of sound varies with the temperature of the gas in the resonator, and by monitoring the resonant frequency of the gas in the resonator, we can determine the temperature. The experiment is a bit more complicated, but that’s the gist of it. But sometimes the extent of the precision required is frightening.
Our resonator is made of two hemispheres of copper, 120 mm in diameter with a wall thickness of approximately 12 mm. The flanges that comes together to make the inner spherical cavity were machined to be especially flat. Our measurements indicate that they are flat to within one thousandth of a millimetre – barely more than a wavelength of light. And yet when we assembled the two hemispheres, we found out that this was not good enough! We were able to detect even this tiny imperfection. And our results would not make sense until we corrected for it. A colleague from France recently assembled some similar hemispheres and found that the flanges were not flat by 20 thousandths of a millimetre – 0.02 millimetres – and considered this to be a disaster. It is sometimes frightening when what is considered in nearly all other circumstances to be ‘good enough’, is just not acceptable.
And today I was struggling over the argon gas that we use to fill the resonator. We use ultra pure gas, but in order to work out what speed of sound to expect, we need to measure the isotopic composition of the argon we used. To do this, colleagues in Scotland have made special measurements for us – but I just can’t understand the results. It is one more area in which I am struggling to become an expert very quickly. Maybe tomorrow it will make sense…
And so what I mean by the title of this blog is that sometimes, as I worry about the n‘th degree of perfection of a surface, or the isotopic composition of the gas, I find that I enter worlds – like Alice in Wonderland – where what other people think are small things, appear to me enormously large and signficant – and I find myself wishing (albeit temporarily) that I were trying to achieve something just a little bit easier.