The reason ignition systems run a resistor in the primary circuit is for current control. Mechanical points can switch approximately 5 amps of current with a reasonable amount of reliability, so the current flow in the primary windings needs to remain below that threshold. Higher current flow results in very short point life. The problem is, that current flow isn't constant- - - - -at 2000 RPM, the points of a 6 cylinder engine are opening and closing 100 times per second- - - - -and the primary current is starting and stopping at the same rate- - - - -going from no current flow at all to maximum, and back to zero- - - - -repeatedly. The resistance of the primary coil winding is one factor to consider, plus another little detail caused by current flow in the primary windings called "inductive reactance" caused by the coil windings, and also voltage spikes in the primary caused by the magnetic field inducing up to 100 volts where there used to be anywhere from 4.5 to 7.5 volts, which is also happening 100 times per second. The engineering involved in even a simple point-switched ignition circuit is pretty amazing!
Jerry
