Turn the crank to set the copper disk rotating. You will see the rotating copper disk drags the compass needle along.
There must be an external magnetic field Bext pushing on the compass needle, how does it come about?
Electrons which are embedded in the metal of the disk are being dragged along with it with a velocity V.
2) The end of the compass needle produces a vertical magnetic field.
3) The vertical magnetic field Bver produced by the end of the compass needle induces a force F on the moving electrons.
4)The force F moves the electron and produces a radial Current I.
5) The current I flowing in the disk produces the mystery magnetic field Bext.
Original 1918 Museum Exhibit