A resistor is one leg of the 3-legged stool that Ohm's Law is all about. Voltage divided by resistance equals current flow in amperes. Let's take a hypothetical situation where a coil primary winding has 4 ohms of resistance. If we apply 12 volts against the winding, grounded by the ignition points, there will be a current flow of 3 amperes- - - -12 divided by 4 = 3. Simple mathematics. Now let's install a lower-resistance coil winding- - - - -2 ohms. With the same 12 volts, we now have 6 amps of current flow, which is too much for a set of points to handle without overheating and burning. If we add a 2 ohm resistor in series with the coil for a total of 4 ohms, we're back to 3 amps of current flow, and the points are happy campers.
OK- - - -I can anticipate the next question- - - -why use a coil that can overheat and burn the points? Simple answer- - - -the battery doesn't always have 12 volts available. When starting a very cold (or very hot) engine, the load of the starter might drop the battery down to 8 or 9 volts. With a 4-ohm coil winding, the current flow in the ignition system suddenly becomes 2 amps or less- - - -not enough to provide a hot spark. The engine fails to start because of a weak spark at the plugs. To assure a hot spark under difficult cranking conditions, let's install a coil capable of firing the plugs properly at very low voltage, and then add a resistor to prevent point burning under low load/high voltage conditions. Just add a circuit to bypass the resistor while cranking. General Motors built several MILLION vehicles with this system, and they worked well. Then some self-appointed experts come along and tells us those design engineers were all wet and they wasted untold millions of dollars of GM's money on an unnecessarily complicated system. They also think the moon is made of green cheese, BTW!
Jerry
