 The device that I have here is a generator. Let's see what parts it has. I can turn the crank here. When I turn the crank, what it does is turn an armature inside. Now, an armature is a coil of wire that's specially wound, and it turns in a magnetic field. And the purpose, of course, as you know from generators, is to generate electrical current. Now, you can't see the armature inside there, so I'm just telling you about it. What's inside here are a series of gears, and the purpose of the gears are to increase the rate of rotation of the armature compared to the rate of cranking. So I can crank slowly, and I can get a very rapid rotation inside the armature. The output of the generator comes through these two wires, so if I clip this to something external, such as a light bulb, I will get electrical current going through the light bulb. Before I do that, I want to show you another function of this device. This can also act as a motor, which basically acts the reverse as a generator. As a motor, as you know, what we do is we take electrical energy or electrical current, and we convert it to mechanical energy of rotation. So to make it work as a motor, I just take my clips and I connect them to a battery. And when I do, current will pass through the armature inside here. That armature is in a magnetic field, so the magnetic field exerts magnetic force on the current, and that produces a torque which turns the crank. So let's see that happen. All right, so that's the motor. All right. So let's move the battery pack aside, and now we're going to see it work in the reverse as a generator. So this time I will be inputting mechanical energy of rotation, and the result will be electrical energy as an output. So we'll connect this to a light bulb, crank away, and a little bit of cranking. You get some light from the bulb, a lot of cranking. The bulb is very bright. So as I crank faster, I was generating greater current. And as you know, the power dissipated by the bulb is the square of the current times the resistance of the bulb. So greater current means greater power dissipation, hence more light. Now there's something else interesting going on with this that you didn't, you couldn't see, but I could feel it and I want to talk to you about that. When I turn this, I feel a mechanical resistance to my turning. Something is pushing back on my hand. And that depends on what I have connected to the circuit. So if, for example, there is nothing connected to the circuit, this is very easy to turn. If I have the light bulb, it's moderately easy. And if I connect it together, it's very hard to turn. The difference in these three situations is the resistance of the external circuit. Here the external circuit has essentially zero resistance. It's only the wires. Here we had infinite resistance. And here we had maybe about 10 ohms of resistance for that light bulb. So as you know, the amount of resistance is going to influence the amount of current in the circuit. So we had more current when there was less resistance. Now let's examine the cause of this mechanical resistance to turning from the viewpoint of something you know about induced electromagnetic fields, and that is Lenz's law. Lenz's law tells us that an induced EMF gives rise to a current that opposes the original change in flux. So let's see how that applies to our generator right here. As I turn this, I'm producing a current, and that current is passing through the armature. So we have a current that is being moved in a wire through this magnetic field. The result of that is an induced EMF. That induced EMF will generate a current which actually opposes the current that I'm producing by turning it. And the result of that opposition is to create a force on the armature which opposes the force which I'm applying to the crank. That appears in the form of a torque which is called a countertorque. So there's a countertorque opposing the torque or the forward torque of my turning, alright? And that's going to be greater when there's more current in the external circuit. So that's greatest when there's no resistance in the circuit or essentially zero resistance in the circuit so that there's the greatest amount of current. And that resistance, that countertorque is least when the resistance is infinite and there's no current in the outer circuit.