To make a model of a self-exciting dynamo we start with an axis, and let's say there's a disc spinning around this axis like this. It's going to spin this way. ANd we're going to do this inside an externally applied magnetic field which goes from negative to positive, just like the Earth's field.

While this disc spins in this magnetic field, negative charge is building up around the rim of this disc.And the charge is just building up and building up, so if you walked over and touched this -- ouch! zap! you'd get a shock.What we really need here is some way to keep this thing going by itself. Let's start by attaching a little wire to the front here and we'll attach it to this axis.This wire is going to allow a current to flow. Current flows from positive to negative so we'll draw the arrows like this, remember because negative charge was building up here.

So now we have a current flowing and we have a magnetic field, so we're part of the way there. But what we really want is for this thing to go all by itself, so we have to kind of add another step.ANd what we're going to do, instead of just attaching the wire very simply like this, is we're going to take this wire, attach it to the front, and we're going to make a loop with it and loop it around behind the axis and then attach it here again. So now what we've done is we've made a loop and the current is going to flow this way from positive to negative. This looping current flow is going to induce a magnetic field. We can use the right hand rule to see that the magnetic field it induces, given by these faint red lines, is in the same direction as the externally applied field.

What happens is because this current flowing induces the magnetic field, the magnetic field induces more current to flow. In fact, we could now just get rid of the external magnetic field that we put on there in the first place and find that this thing will sustain itself. It will keep spinning, current will keep being produced, and the magnetic field will also keep being produced. This is a pretty good approximation of the way the Earth's magnetic field works. This is a self-exciting dynamo.