 Now we look at the magnetic field due to a long straight wire. Well, here's the equation we come up with from that. And this comes from an integration of the Biosavart law. We're not going to show the detailed integration that gave us this equation. We're just going to work with it now that we have it. So in this equation, again, our mu naught is our magnetic constant, the permeability of free space. I is our current. And A here is the distance to the wire. So if you can imagine I've got some sort of long wire carrying current, and I want to find the magnetic field at some point P, then A is going to be the distance straight down from that point to the wire.