LED and the white traces: the + path, lit when the loop is closed.Steel plate: the way back. Gold pads and magnets: where blocks meet.Three blocks, 40 mm square, on the steel plate. Each live side has a sprung gold pad between two magnets, so the LED block clicks on from any side. Close the switch and about 21 mA flows out through the blocks and home through the plate under them. That is why three blocks are enough.