The Eddy Current Pendulum
   Eddy currents are induced in conductors in the presence of changing magnetic fields. One of the first observations of eddy currents was made by Bachhoffner of London in 1837 in his design for an induction coil. He found that the coil was more effective when the solid iron core was replaced by a group of iron wires insulated from each other. All subsequent induction coil designs in the 19th century followed this practice.

   Waltenhofen's Pendulum was made by Max Kohl of Chemnitz and cost 50 Marks in the 1900 catalogue. The pendulum blade made of solid copper would quickly come to rest in the magnetic field; the alternate blade, with slots to break up the eddy currents, swings for a longer period of time. This piece of apparatus is at Washington and Jefferson College. 

   Eddy currents produce heat, and this is demonstrated by Foucault's Disk ,  in which a solid copper disk rotated in a magnetic field heats up. 

   I often use a Unipolar Generator  to demonstrate eddy currents. Bringing a large radar magnet close to the freely spinning disk slows it down rapidly. 

   This pair of eddy-current pendula are about 1 m in height, and swing between the poles of a radar magnet. They are in one of the physics lecture hall preparation rooms at the University of Texas in Austin.
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