:)Reverse the battery connections and also those to the ammeter. Then check that when you turn the headlight on, the ammeter shows a discharge.
Next disconnect the D and F connections at the dynamo. Find an old piece of wire. Connect one end of this to the battery supply line - now the negative side, of course. Flick the other end of the piece of wire on the end of the F terminal of the dynamo momentarily and that's it. You've jolted the magnetic field to the right way round.
Now re-connect the dynamo and run the engine to check that the system is charging. Your ammeter will confirm this fact.
This was copied from, I think, the Motor Cycle magazine of about 1955. Some of these bits and pieces I have put on disk as memory fails me these days.
Regards, Geoffus