Eddy currents are produced when

The core of a transformer is laminated so that

In which of the following devices,the eddy current effect is not used?

$A$ light disc made of aluminium (a nonmagnetic material) is kept horizontally and is free to rotate about its axis as shown in the figure. $A$ strong magnet is held vertically at a point above the disc away from its axis. On revolving the magnet about the axis of the disc,the disc will (figure is schematic and not drawn to scale)-

$A$ magnet is dropped down an infinitely long vertical copper tube.

Which of the following is not an application of eddy currents?