The battery provides electrical power when the scraped parts of the turn are in contact with the bearing. Thus we have an electric circuit through which a current passes through the magnetic field associated with this current, transforming it into a small (unnatural) magnet.
The magnet fixed on the stack has one of its poles facing the turn; And when it becomes a magnet, there is an interaction between them. When the loop has the same type of pole to which it is attached, we will have a repulsive force that will move the loop. This movement often depends on an initial push.
An important detail: when the loop has the opposite pole to that of the magnet to which it is attached, the force that will exist will be of attraction and the movement of the loop will be damped and may even result in the end of its movement.
To solve this problem and prevent the electric motor from stopping, we use one end of the fully scraped loop where the chain can always pass through and the other half scraped so that the chain will only pass through that end when the scraped part is in Contact with the rod.