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The ammeter used by Ford in the 6-volt positive ground era was designed to work with that polarity. If polarity is changed then the ammeter would have to follow suit. DC ammeters read the flow of current in or out from the battery so they are polarity sensitive.
The 6-volt converted alternators, usually the GM 10si, have an internal jumper to make them positive ground units. If a person goes 12-volt negative ground then the ammeter wires have to be reversed.
Later Fords with the inductive meters will require that the main power lead be reversed through the loop if polarity is changed.