Ballast Ignition Systems

I wish I had a pound for every time I’ve been asked about these and why they are necessary. Folks seem to think some kind of wizardry is involved.
The Ballast Resistor Ignition system, if my memory serves me right, was a cunning invention of the Ford Motor Company when they were experiencing poor starting on a certain prodigiously used 4 cylinder engine.
To get round the massive current draw from the starter motor engaging, which left precious little for the ignition coil to produce a healthy spark. This situation is aggravated even more when the weather is colder in the winter causing the starter to draw even more from the battery, and if the starter is worn. The problem then was to find enough current for the ignition coil while the starter was doing its job.
The ingenious answer was to use a 6 volt coil and wack 12 volts through it which produced a massive high discharge for the spark plugs, up to 30% more. This is all very well but it's obvious the 6 volt coil is not going to put up with double the voltage for long. In fact it's only needed while the starter is engaged and pulling its high discharge on the battery.
Therefore the 12 voltage boost can be removed once the starter has disengaged and the engine has started. In normal running the ignition feed to the coil is via a separate feed through a Ballast Resistor dropping the coil voltage to 6 volts.
I said I thought this was a Ford invention, but I can assure you a lot of other manufacturers caught on to this ingenious system and copied it, some using a 9 volt coil. So a word of warning is to always check the coil voltage if replacing it.  Also a lot of manufactures hide the Resistor as an in line one buried in the wiring harness. If the coil is not clearly marked with its voltage or your workshop manual is at all vague an AVO / Multimeter is the order of the day to check the coil feed to ascertain the running voltage.