Introduction: Eddy Current Swing

When a conducting plate moves trough a magnetic field, it's flux (area of the plate affected by the magnetic field) changes. This induces an Eddy current, this in turn in combination with the magnetic field brings the Lorentz force to life. This force is opposite the direction of the plate and thereby slows it down.

This can be demonstrated using a pendulum swinging a conducting plate. In order to see the effect of the braking it is essential to construct a smooth pendulum. The free swing can be compared to a swing subjected to a magnetic field to demonstrate the effect.


    • Tie-raps
    • paint mixer stick
    • non magnetic metal plate, 5cm x 5cm (1x)
    • Rope
    • Duct-tape
    • Stanley knife
    • Small weights, <10g
    • Retort stand, clamps
    • Marker

    Step 1: Building the Pendulum

    By making use of LEGO Technics parts, such as wheels and drive shafts (see pictures) create a setup where the wheel can freely rotate around its axis. Keeping the wheel fixed on the drive shaft and rotating the shaft also works but this is more prone to friction which will slow down the swing.

    Use hot glue attach the a short wooden plank to the LEGO wheel. Using a plank offers a swing with little deviation so try to avoid using wooden sticks. The length of the plank is a free choice, a longer plank will swing for a longer time but requires a larger set up. A minimum of 15 cm is recommended to notice any significant braking otherwise the swing time will be too short.

    Use duct-tape to attach the aluminium plate to the plank. Do not use hot glue because this step will require some iteration and duct-tape is easy to remove.

    Step 2: Hanging Up the Pendulum

    Using the retort stand in combination with the clamps, put the swing at the right height, so that the plate does not touch the stand when swinging but is low enough to swing between the magnets. Check with a level that the axis on which the wheel rotates is horizontal. Use a piece of rope to support the clamps if they aren't perfectly horizontal. The best way is to sandwich the rope between two tie-raps and then suspend the clamp using the rope. (See picture). Additional pieces of lego can be used to stabilize the pendulum on the stand. The idea is to get a perfect swing with little to zero deviation.

    Step 3: Positioning the Magnets

    Place the magnets in the rest position of the pendulum,on swinging path, in such a way that they do not block the pendulum when it is swinging. The point is that the pendulum can swing freely between the gap the two magnets create. By using small planks and hot glue to create a valley that can hold the magnets. Then use duct-tape to attach the magnets to the "valley'' so that it is easy to reposition the magnets if needed.

    To achieve the maximum effect make sure the distance between the two magnets is minimal but without touching the plate as it swings between them. Any material can be used to achieve the desired distance between the two magnets (Here rubber is used). Be careful when bringing the magnets close together as they are very strong

    Make sure that the right poles of the magnets are placed opposite each other so that they attract each other. If both north and south poles are opposite each other the magnets will repel each other and the magnetic force will be a lot weaker.

    Adjust the height of the pendulum if necessary in order to have the plate covered by the two magnets. This can be done by clamping the pivot point of the pendulum at a different height.

    Step 4: Calibrating the Setup

    To be able to show the effects of the Eddy currents on the aluminium plate, let the pendulum swing freely without subjecting it to the magnetic field. Now bring the magnet valley to the stand and place the pendulum between the magnets and allow it to swing. Hopefully you'll see the swing slow down as it moves trough the magnets, if you used particularly strong magnets it might completely stop during its first pass.