Clamped to the stepladder is a 2.5-meter-long copper tube. (It measures 98.75 inches/250.8 cm long, and is 1.125″ in diameter, with a wall that is about 19/64″ thick and an i.d. of about 0.540″.) With the apparatus is a set of short rods, one non-magnetic steel, one wood, and a cow magnet (sitting on the top step of the stepladder in the photograph). If you drop either the non-magnetic steel rod or the wooden rod into the tube, in the roughly 0.5 seconds it takes for the rod to reach the bottom of the tube it is impossible to climb down and catch it as it comes out. The cow magnet, however, induces eddy currents in the tube as it falls. The downward force on the parts of these currents that flow through the magnetic field (and the resulting upward force on the magnet) slows the magnet’s descent, and in the roughly 11 seconds it takes for the magnet to reach the bottom of the tube, you can leisurely climb down the stepladder to catch it as it comes out. The piece of cardboard between the bottom of the tube and the frame of the stepladder is to prevent the cow magnet from snapping onto the frame as it exits the tube.

You can ask a student to drop either the nonmagnetic steel rod or the wooden rod into the tube and then to climb down and catch it as it comes out at the bottom. This will be impossible to do in the ~0.5 s it takes for the rod to fall through the tube. If the student then drops the cow magnet down the tube, he or she will have plenty of time to climb down and catch it when it exits the bottom of the tube.

Demonstrations 72.09 -- Lenz’s law and 72.24 -- Ring flinger, illustrate the repulsion between the magnetic field due to a current flowing in an aluminum ring, and the changing magnetic flux that induced the EMF that generated the current. In the former, the person performing the demonstration provides the changing flux by alternately pushing a magnet (end on) toward the ring and pulling it away from the ring. In the latter, the changing flux is produced by a solenoid connected to the AC line. In these demonstrations, we consider the current as flowing in a loop, with the rings analogous to loops of wire. In this demonstration, as the magnet falls through the long copper tube, so do the lines of magnetic flux that surround it. As the flux lines that penetrate the tube wall travel down the tube, they induce an EMF in the section of the tube they are passing. If we consider a small part of the circumference of the tube, and say that the moving field lines are emerging from the north end of the magnet (so going through the wall toward the outside of the tube), as the magnetic field passes this region, it induces an EMF that forces positive charges to the right. (Relative to the magnetic field, the charges in the tube are moving upward, and with B pointing outward, they experience a force, F = qv × B, which points to the right.) The regions above and below the area through which the field lines pass, provide return paths for the rightward-flowing charges, and this produces two sets of currents, one above the other, the top set of currents running counterclockwise, and the bottom set running clockwise. Such currents induced in an extended object (as opposed to a ring or a loop of wire) are called eddy currents.

At any given moment, only the charges moving to the right (where the two loops meet) are in the passing magnetic field (or at least that part of the field where the lines are going outward through the wall, essentially perpendicular to it), and they experience a force, F = qv × B, which points downward, opposite the upward motion of the tube relative to the magnet (a drag force). This force thus opposes the motion of the falling magnet, and slows its descent. Since this induction of a rightward-facing EMF occurs around the whole circumference of the tube, the eddy currents form a rightward circulating current (counterclockwise as viewed from the top of the tube) around the tube wall, with leftward (clockwise) currents above and below. These clockwise currents are outside the field that is causing the eddy currents (either off the end of the magnet or toward the middle of the magnet, where the field lines are parallel to the tube), so they do not experience a vertical force. The same thing happens at the other end of the magnet, where the field lines return through the tube wall to the south pole. A device such as this, in which eddy currents slow a moving part, is known as an eddy current brake. Such devices find practical use in certain vehicles, exercise equipment, and in industry. For some examples, see Eddy current brake on Wikipedia, and 5 Applications of Eddy Current Brakes on the head rush technologies web site.

For this apparatus, if we consider the axial change in flux either as the bottom end of the magnet approaches a section of the tube, or as the top end of the magnet recedes from a section of the tube, and use Lenz’s law, we obtain a similar result. For the magnet dropped with its north end down, below the magnet the flux is increasing. The induced EMF must generate a counterclockwise current (as viewed from above) to produce a magnetic field that points upward, opposing that of the magnet, thus repelling it and slowing its descent. Above the magnet, the flux is decreasing, so the induced EMF must generate a clockwise current (again, as viewed from above) to produce a magnetic field that points downward, aligned with that of the magnet, thus attracting it and slowing its descent. This model does not give the counter-rotating currents above and below the ones that provide the upward force on the magnet, but as noted above, these do not contribute to this upward force.

If we reverse the direction of the magnet, this reverses the sign of the induced EMF, so the currents run in opposite directions to those described above, but since both B and v are reversed, F, the force that opposes the motion of the magnet, remains the same.

References:

1) Sears, Francis Weston and Zemansky, Mark W. College Physics, Third Edition (Reading, Massachusetts: Addison-Wesley Publishing Company, 1960) p. 672-3.
2) Halliday, David and Resnick, Robert. Physics, Part Two, Third Edition (New York: John Wiley and Sons, 1977), pp. 787, 791.