Mechanical points can carry a maximum of about 5 amps without burning. The resistance of the coil primary winding is the main control of primary current, but the relationship is not a straight function of Ohm's Law. As current flows through a coil of any sort, there is an initial surge of amperage that's almost a "dead short" situation, followed by a rapid buildup of "inductive reactance"- - - - -an opposition to current flow created by the buildup of magnetic induction in the windings of the coil. Simply measuring the resistance of a coil winding will not give an accurate estimate of the current it flows, since at 2K RPM, for instance, the coil is being turned on and off approximately 100 times per second. Lots of "higher mathematics" are involved in engineering an ignition system, but it can all be made a moot point by simply installing the wrong coil. Most Ford systems do not use a ballast resistor- - - -they simply accept the lower voltage during starting. Old Henry Ford didn't trust his engineers very much!
Jerry
