Barlow's Wheel

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A magnetic field can be used to produce continuous rotation using the apparatus below, known as Barlow's wheel.

Barlow's wheel consists of a piece of copper in the shape of a star. The tips of the copper wheel pass between the poles of a magnet and dip into a mercury bath. The apparatus is connected to a battery. A wire from one terminal of the battery is set to dip into the mercury bath and the other terminal is connected to the stand as shown above. Current flows through the wheel in the direction shown and the wheel experiences a force given by Fleming's left hand rule. This force is experienced until the circuit is broken by the tip of the wheel leaving the mercury, but the wheel continues to rotate and when the next tip of the wheel enters the wheel, contact is remade, the circuit is complete and current can flow again, producing a force in the wheel in the same direction as before.