How is electrical current transmitted from the revolving part of the crane to its lower structure?

The transmission of electrical current from the revolving part of the crane to its lower structure is done through a collector ring system. This system consists of rings that are mounted on the rotating part of the crane, allowing for a continuous electrical connection as the crane rotates. The collector rings work in conjunction with brushes that maintain contact with the rings, facilitating the flow of electricity regardless of the angle of rotation. This design is essential for ensuring that the crane's electric systems, such as those used for controls and operations, function reliably without interruption.

The other options do not serve the purpose of transmitting electrical current in this specific context. Electrical panels and circuit boards are primarily involved in distributing and managing electrical power within systems rather than providing a direct transmission mechanism from a rotating to a stationary part of a crane. Transistors are electronic components used for amplification or switching but are not used for the physical transmission of electrical current in this scenario.