The transition to negative ground came around the mid-1950's for American cars, 1967 for English Fords and around 1971 for all other English cars. The preference to use - or + ground didn't matter much. The standardization to negative ground in the Auto industry was only because of the introduction of negative ground electronic gadgets.
If you already converted the car from + to - ground, do not use the original Lucas coil marked CB and SW even though you reversed the two wires on the coil. Purchase a new coil made for negative ground systems. It should read + on one side and - on the other side of the coil.
Here's why:
Primary & Secondary Circuits on a CB / SW positive ground Coil:
When the engine is running, each time the points open, the current stops flowing and the magnetic field collapses around the primary. The secondary has approximately, 20,000 turns of fine wire ( 100 to 1 wire wound ratio secondary to primary ). The collapsing magnetic field will induce a voltage of + 22,000 volts at the core for the spark plugs.
Auto-Transformer action: In a coil the secondary and primary windings are in series. If the wires to CB and SW were reversed then the current will flow in the opposite direction and will defeat the purpose of an auto-transformer effect. The arc-over voltage output of the coil would be approximately 10% less . It is spark current that ignites gasoline. Obviously, if you don't achieve arc-over voltage, no spark current will flow.
In other words, there is approximately 10% reduction of voltage required to jump a spark plug gap. If the car is converted to a negative ground and a negative ground coil isn't installed then the 10% voltage advantage is gone.
