A magnetic circuit is like an electric current in that magnetic flux flows in closed loops through materials of high permeability (such as iron or ferrite), just as current flows through materials of high conductivity. The terms mmf (magnetomotive force), flux and reluctance in a magnetic circuit correspond respectively to emf (electromotive force), current and resistance in an electrical circuit.
Magnetic circuits are found in almost every electrical motor. Magnetic shunts are often used to adjust the amount of flux in these circuits. The above tutorial illustrates a simple magnetic shunt.
The shunt consists of a laminated Iron Core in the shape of a “W.” Around the first and second legs of this core are two separate coils of insulated Copper Wire, both featuring the same number of turns. Positioned across these two legs is a laminated Iron Yoke. The wire wrapped around the first leg is connected to a source of alternating current, making this ring an electromagnet and producing a magnetic field (not visible in the tutorial) around the ring. Because that current is constantly alternating, the magnetic field fluctuates along with it, and induces a current in the second coil via electromagnetic induction. (This is the same process that makes transformers work.) That current is what powers the light bulb that glows to the right of the shunt.
Use the Yoke Position slider to move the yoke to the right so that it bridges across all three legs. Notice that the bulb dims considerably when the yoke makes contact with the third leg, which serves as the magnetic shunt. Moving the yoke farther onto the third leg further dims the bulb until it is effectively turned off.
The bulb dims because an increasing fraction of the magnetic flux (the total of which remains constant) passes through that third leg. Consequently, less flux passes through the second leg. That means less electricity is generated in the circuit connected to the bulb, which dims as a result.
You may also move the yoke position further to the left so that the yoke is in contact only with the first leg. Notice that the bulb extinguishes under this scenario, because the ring has been broken, so there is no longer an electromagnet.