As with a lot of other automotive electrical system situations, firing a spark plug is not a simple resistive circuit with constant current flow. To initially ionize the gap(s) at the rotor and the spark plus electrode and get the arc flowing, a voltage of somewhere around 7-10 KV is required. Wide plug gaps, rounded electrodes, and loose or corroded connections need more voltage. Once the arc is established, that voltage drops considerably, down to the 3-5 KV range. This current flow continues at the lower voltage until enough of the induced voltage in the coil secondary is lost that the arc cannot be maintained any longer. Then the residual voltage left in the plug wire bounces back and forth from the plug terminal to the distributor cap, losing a few volts at each oscillation. A resistance wire can have up to 1K ohm per inch, which dissipates the residual voltage more quickly, preparing the conductor for the next firing impulse. Since a spark plug fires approximately 60 times per second at 2K RPM, getting rid of that voltage ripple quickly actually makes a resistance wire more efficient than a metal core, plus the current flow is reduced and parts last longer. That "hot blue spark" we used to look for was actually pretty destructive. Resistance wires produce a yellow/orange color spark that lights off the gasoline in the cylinder just as efficiently.
Jerry
