Switching Poles Automatically
The rotating electromagnet will have to be connected to the 2 wires through which we pass the current. Since it's rotating on a center shaft, we can't have a solid connection. Instead we have to extend the wires from the coil out along the shaft and let the electric contact be made with brushes which touch the wires along the shaft.
Figure 4. A rotating electromagnet changes poles as contacts are made first one way, then the other.
This is a simple way to reverse the current in the coil of the electromagnet.
Increasing Efficiency
Instead of using only one pole of a stationary magnet, we can use both. This is done by shaping the stationary magnet around the path of the rotating electromagnet. This way we have the benefit of the attracting and repelling forces from both poles. The effect is doubled.
We can also wrap wires around this circular iron and make an electromagnet of it. But when we wire this magnet we use no brushes because we want the current to flow in one direction only.
The stationary electromagnet is called the field. The rotating electromagnet is the armature.