On connecting the apparatus to a pile, the rod rotates around the magnet owing to the interaction between the current flowing through it and the magnetic field generated by the cylindrical magnet.
If on the other hand we introduce the U-shaped magnet and hold it in place with the screw for that purpose, so that the rod falls between the two arms, on one side or the other of the cylindrical magnet, this, which would tend to make it rotate, is ineffective; the U-shaped magnet, however, depending on the direction of the current, would attract it or repel it. If the direction of the current is such that the rod is pushed away from the centre, at a certain point it detaches itself from the mercury and the current is interrupted; however, lacking the interaction with the magnet, it moves back to the centre, contact is re-established and it is once again repelled, and oscillates again. The oscillations continue as long as contact with the pile is maintained.