[rotorwrench]

The current only flows one way in a DC electrical system and that is a fact. It flows from positive to negative. The ground path generally has less resistance to flow unless the cars is all crusty and rusty like many are after 90+ years. Same with the copper conductor wires but they are at the end of the circuit path in a positive ground system so the galvanic transfer of materials may have been thought to be an issue. The battery ignition was new territory for Ford in late 1927. They had used the Magneto for many years in the model T which is AC in nature but the battery, distributor, & coil was not something that Henry trusted as much until they started testing it. The thought was that the high tension spark would be more efficient with positive ground. Ford wasn't the only company to use positive ground so there was likely an industry wide consensus on this at the time. The model T had been negative ground so it was a change for Ford.

Another thing that is affected is the starter circuit in these old cars. This is the heaviest current draw in the system and also the most prone to problems in the 6-volt systems. With the body of the system already positively connected through the ground path. The flow only has to travel back to the battery when the starter switch is applied but that is where most of the deterioration would take place if that power cable was attached to the positive terminal. They felt that this would cause more arcing at the switch terminals when the starter was engaged. Maybe they were wrong but this was the early 20th century and folks had a different way of approaching things back then.