In uncertain times, its good to know there are some things that never change – such as day follows night, my compass needle always points in the same direction, and England will always underperform at the World Cup. Well, scrub the middle one, actually. In our lab, at any rate, there are some shocking variations in the magnetic field.
I know that because I’ve been doing some more playing with our Earth’s field NMR / MRI equipment, and finding that my resonance frequency is harder to pin down than a politician in an expenses scandal. The system uses the earth’s magnetic field to split the nuclear energy levels of the hydrogen nucleus (proton) in a sample of water; the amount of splitting depends on the strength of the field, and the resonant (Larmor) frequency is proportional to this. When the magnetic field increases, so does the resonant frequency.
In the lab it’s quite common for the frequency to shift by 0.1% (1 part in a thousand) in an hour. If you leave it a couple of weeks between measurements, it may have shifted a whole lot more – in excess of 1%. Some of the drift in resonance will be because the earth’s field changes, for example, there is a diurnal cycle in the strength of the field due to the solar wind interacting with the earth’s field. But I suspect a lot of it is because of the nature of the lab – in a reinforced concrete building (i.e. one with lots of steel in it) surrounded by heaps of other lab equipment. There’s a great deal of potential for the earth’s magnetic field to be changed by the lab environment.
It’s no big hassle, usually, because usually the resonance hasn’t drifted too far from where it was when I last looked, but it can take a bit of time to re-optimize the equipment before we get the students to use it.
And remember, that a 1% shift is trivial when compared to the kind of movements the field makes on a longer timescale – e.g. the drift in position of the magnetic poles, and the reversals of the field at times during the earth’s history. A good overview is found here.