[rotorwrench]

When Mallory was helping Ford's engineers design their V8 systems, Henry's insistence on using the distributor mounted coils required coil designs that were outside the normal ignition coil specs for the time. The Model A & B coil equired no resistor. The first V8 coils used a cylinder type coil but Ford needed more room down front for clearance. The divers helmet type coils and the later 21A types all required a bit more control of the available amperage in the power line to them in order to lessen heat build up. The little coils only needed 4 amps or so to do a good reliable job so Ford used a 0.8 OHM ballast to control the current draw. Yes, it also dropped the voltage but that's just the way OHM's law works. The current is what caused the heat build up so the resistor brought the heat level to reliable specs for those types of small coils.