Hunstanton Sand

I’ve just started reading a book called The Spirit of Enquiry by Susannah Gibson, celebrating the 200th anniversary of the Cambridge Philosophical Society, an interesting society of which I was once a committee member (as well as a prize-winner). I am struck by the fact that the building where my GP’s surgery now hangs out, was actually purpose-built for the Society, something I had not appreciated before. The room where I’ve sat around waiting for Covid vaccinations was once their Reading Room, at a time when that was quite a novel concept (the College Libraries were only available to current members, and not MA’s still resident in the city, for instance). Having found this a fascinating book, written by someone attached to this University’s History and Philosophy of Science Department, I am pleased to have been invited to the book launch of Gibson’s next book. Bluestockings: The First Women’s Movement is due out at the end of this month.

The Spirit of Enquiry starts off describing what motivated Victorian natural philosophers in Cambridge, led by Adam Sedgwick and John Henslow, to feel such a society was necessary. The latter was a botanist. Indeed, he was Professor of Botany although, late in life, apparently a very delinquent one. The former was Professor of Geology, and spent time walking the cliffs at Hunstanton examining the strata. These are fine cliffs, tending to erosion like so much of the East coast, cliffs I have visited just for the pleasure of visiting the seaside, but also (in my much earlier life) for ornithological ventures in the cold of winter. My most recent interactions with Hunstanton’s beaches are, however, more closely allied to my Physics: Hunstanton sand.

The last lecture course I taught before I retired from the Physics Department was the first year Waves and Quantum Waves. It was an unsatisfactory course in many ways, as I was required to include a great deal of classical optics (stuff such as the Lensmaker’s equation, for instance), when the students wanted to be let loose on the quantum material, which consequently got very squeezed. The syllabus was not of my making. However, in an earlier incarnation of this course, when the classical waves part was taught at a more advanced level (and without the optics material) and there was more time to think deeply about implications of some of the topics, I had a lecture demonstration I loved involving Hunstanton sand. And I know it was Hunstanton sand because it came in the sort of shaker good cooks use to spread flour on the worktop to stop pastry sticking, to which an ancient luggage label was attached reading ‘Hunstanton sand’. Although I doubt this went back to Sedgwick’s time, indeed the Cavendish Laboratory only opened in 1874, it certainly gave the impression of being very venerable. It came along with a heavy brass plate about 30cm wide (probably in reality it was a foot square), an example of a Chladni’s plate.

If you look on the web for what the point of a Chladni’s plate is, you will find all kinds of neat videos demonstrating how it can be used to show a pattern of standing wave nodes by plugging a sand-covered plate into a frequency generator: at appropriate frequencies, when the wavelength is some suitable fraction of the length of the side of the plate, standing waves are set up. It is indeed a beautiful way of revealing complex patterns, building on the mathematics of standing waves in two dimensions (which is what I was teaching). But the demonstration I gave was more arresting and memorable, I think, even if also more risky. With a device to generate a wide range of frequencies, it is easy to dial up the exact frequency you know will give the desired pattern. No risk there at all. But perhaps students remember things that don’t go according to plan rather more than something they can find easily on YouTube. That was at least my motivation in doing things the hard way.

The third item of this ancient lecture demonstration consisted of a bow. It was an utterly appalling bow, if you were a string player, with no tension in the hair remaining after all these years, and no way of increasing it except by manually holding it taut. I suspect it once had been a double bass bow as it was quite short. (As an ex-viola player, upon occasion I took in my own bow to make life easier, given that mine was in rather better shape.) Instead of electrically generating different frequencies to set up the standing waves, the original demonstration design relied on ‘playing’ the plate with the bow.

There were some marks scribed on the plate to indicate where the bow should be placed to get the appropriate resonance, but they were pretty approximate. Consequently, in my experience, it was necessary to move the location of the bow back and forth a little to find the place where the plate ‘sang’ – which it would most pleasingly when I got it right. A beautiful harmonic would be forthcoming, echoing round the lecture theatre (large: I used to lecture to well over one hundred students). More than once I got a spontaneous round of applause when this happened. Every year (at least five I think) bar one, I managed to find the sweet spot. Sometimes, I even risked finding a higher harmonic to show how the sand bounced around until it found the new pattern of nodal lines. It was immensely satisfying – apart from that one year when, try as I might, I never quite got it and the standing wave pattern on the plate was blurred, the true note transformed into a messy noise.

All in all, it was far more satisfactory, for me and, I hope, for the students, than simply playing a video of someone else’s experiment uploaded onto YouTube. Every year, at the end of the lecture, I would try to return my Hunstanton sand to the flour shaker. This was a messy enterprise, but I felt the sanctity of this particular ancient sand in its luggage-labelled container. Who knows who’d made the trip out to Hunstanton to collect it? After the end of the lecture, the kit would be replaced in some wooden cabinet in the Cavendish Museum. I wonder if any of this will survive the move to  the third incarnation of the Cavendish in the soon-to-be-finished (but who knows quite when, building work being what it is) Ray Dolby Centre, otherwise known as Cavendish III. I lectured in the so-called New Cavendish, its second home; the equipment no doubt was first used in the original Cavendish on Free School Lane (a brief history can be found here).

My days of undergraduate lecturing are over. I’m sure, just as I participated in the translation of delivery style from blackboard and chalk, to writing on an overhead projector, to prepared overheads, to powerpoint which may, for all I know, be superseded by a further electronic transformation, I fear too many demonstrations will be called up from the web. I loved my old-fashioned experiment, even as I also used more modern approaches too. The latter is certainly more likely to be fail-safe. So, happy memories of Hunstanton sand.

 

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