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The National MagLab is funded by the National Science Foundation and the State of Florida.

Magnetic Shunt

Magnetic shunts are often used to adjust the amount of flux in the magnetic circuits found in most electrical motors.

A magnetic circuit is like an electric current in that magnetic flux flows in closed loops through materials of high permeability (such as iron), just as current flows through materials of high conductivity.

Magnetic circuits are found in almost every electrical motor.

Here we llustrate a simple magnetic shunt.


  1. Observe the set up. The shunt is a laminated iron core in the shape of a W. Around the first and second legs of this W are two separate coils of insulated copper wire, both featuring the same number of turns. Above the W is a laminated iron yoke. A light bulb is connected in a circuit with the middle leg.
  2. The wire wrapped around the first leg is connected to an alternating current. This makes an electromagnet and produces a magnetic field around the ring. The magnetic field fluctuates along with the alternating current and induces a current in the second coil via electromagnetic induction. That current is what powers the light bulb.
  3. Now use the slider to move the yoke to the right so that it bridges across the third leg as well. Notice that the bulb dims, because an increasing fraction of the magnetic flux 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.