 All right, so nuclear magnetic resonance describes this phenomenon where there's a difference in energy levels for the two spin states of a nucleus, a proton, and a hydrogen atom, for example. And you might think that the simple application of that would be to, since that energy difference depends on both the magnetic field experienced at the nucleus and determines the frequency of light you could use to make a transition between those two levels, the straightforward application would be to apply a magnetic field, stick a proton, stick a hydrogen atom near a large magnet, for example, which generates a magnetic field. And then you can predict the frequency that you would need to use to make a nucleus transfer from one of these nuclear spin states to the other. That turns out to be a little too simplistic because of an effect called shielding. And that relates to the fact that if here's our hydrogen nucleus, the proton at the center of a hydrogen atom, of course that atom contains not just a nucleus, but it also contains an electron. So there's an electron that's not just at one point in space, but it's got a wave function that's delocalized, it's spread out in some wave functions surrounding the hydrogen atom. So there's an electron and at the center of the atom there's the nucleus of the proton. The electron has an opposite charge from the proton if we apply an electric field. So exert an electric field, I'll call that field B naught, that's the electric, sorry magnetic field, exert a magnetic field in the vicinity of this hydrogen atom. You might remember from a physics class that what a magnetic field does to a moving electrical charge like the charge on this electron is it exerts a force on that charge, a force called the Lorenz force. So a moving electrical charge like the electron experiences some force. You can use the right hand rule to determine how that force behaves. I won't use the right hand rule here because my right and left get mirrored so that would I think confuse things more than it would help, but if there's a nucleus, there's an electron in the vicinity of that nucleus, the magnetic field is going to cause that electron to circulate in a circular motion around the nucleus of the electron. And then the other thing to remember from a physics second semester physics class is that in turn a moving electrical charge generates its own magnetic field and in this case what happens is the induced magnetic field induced by the motion of that electrical charge is in the opposite direction to the magnetic field that was there in the first place. So what that means is the total magnetic field experienced at the nucleus is not the external magnetic field generated by the magnet, but it's a little bit smaller. In fact the total magnetic field experienced at the nucleus is, if we think of that as the external magnetic field, add in this induced magnetic field which points in the opposite direction, we can think of that as since the induced magnetic field is proportional to the strength of the applied external magnetic field, we can think of that as a reduction in the total magnetic field. The magnetic field experienced at the nucleus is the external magnetic field shielded or reduced by some amount that we're calling sigma and that sigma is the shielding constant that describes the extent to which the magnetic field is shielded or reduced by the electron. So again just to summarize what's going on, the nucleus doesn't feel the full strength of this magnetic field because the presence of the electron with an opposite charge around it shields it from the full strength of that field. So that shielding constant is relatively small but it's quite important and that's going to lead to some of the most important uses of nuclear magnetic resonance in particular because that shielding constant depends on the chemical environment of the protons in the molecule itself so that's what we'll talk about in a little more detail coming up next.