Patience in experimenting

I’ve spend most of today in our new teaching lab, grappling with a piece of experimental equipment. Over the break between our A and B semesters (i.e. now) we’re moving our 2nd and 3rd year undergraduate physics lab out of one room and into another. It’s a small part of a large plan to use the available space to full advantage, and it means lots of things are moving about at the moment. A bit like moving house, with our deadline being two Monday’s time, when semester starts, and everything has to be up and running.

For the most part, we can move a piece of equipment from one room to another, set it up, and have it work without too much difficulty. But there are three or so pieces of equipment that are a location-sensitive. The one I’ve been working with today is a NMR / MRI machine.  It’s just a baby – not the size you find in hospitals – but it is an excellent teaching tool – one of the best bits of kit I think our third years get to play with.

Rather than have a huge coil to generate a magnetic field (what you need for nuclear magnetic resonance), as happens in the hospital, this machine uses the earth’s magnetic field. Convenient, yes, but a bit awkward too, because the field in the lab isn’t exactly nice and uniform. Walk around the room with a compass, and you’ll see the needle drift several degrees – there is, after all, a whole lot of steel in the building, and that is going to influence the magnetic fields in its vicinity.  Now, to use the NMR in the lab, I need to know what size of the magnetic field (because strength of the field controls the resonant frequency). Although the old lab and the new lab are close by, the strength of the earth’s field in the two of them proved surprisingly different, and it took me a while to get any signal on the machine at all.

Setting the machine up in its new location is rather like tuning a radio with a poor quality aerial when you don’t know the frequency of the radio station you want, nor do you quite know the direction of the transmitter, in a room with a lot of electrical noise. It takes a lot of patience, looking at different frequencies, until you pick up a little signal in amongst the noise. (This, being a physics lab, is just loaded with electrical noise – that’s another problem).  It’s an example of how, with noisy data, you can still get a ‘signal’, but you need to average over lots and lots of trials to see it. It’s just basic statistics really; the more averages you take (in my case, the more times I sample the data), the easier it is to see a signal buried in the noise, because the noise slowly ‘averages out’ to zero, while the signal doesn’t. But with a weak signal, it takes some time. 

With a lot of patience, I got there – found my NMR signal. Then I could slowly tune up the apparatus to work with that signal, and finally at the end of the afternoon I have it working tolerably well.  But basically it took a day to do it. 

In research, we often need far longer than a day to tune-up our equipment, and a good experimenter will have lots and lots and lots of patience. ( I do not claim for one moment to be one of these – as an undergraduate I chose my papers very carefully to keep practical work to an absolute minimum – and then went on to do a PhD which just involved pen-and-paper and a computer – ironic that I now teach our experimental physics papers). Patience is certainly a virtue for a physicist. 



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