This is one of those, "it never goes away" questions. Hotrod has the practical solution. He always does. If you want the more "technical" solution, then this is for you. The points are a switch. There is a limit to the current that switch can take. Some Engineer figured out how much current those points would take and still live. His or Her math was based on 6 volts. "Cause that was what they worked with at the time. Simple IRV equation. I(current) R(resistance) and, V(voltage). Ohms law. Add the coil, and you get a new set of numbers. Reactance. That is AC resistance. The coil is an Inductor. 6 Volts becomes 20,000 Volts and you get a spark. For those who like math, Voltage equals Current times Resistance. V=I(R). If you measure the coil resistance, multiply that by the Voltage, you will get the current through the points. Kinda. Sorta. Cause the D.C. resistance is close, not correct. So what to do? Relax. This is car and truck stuff. No rockets here. So we then(around 1954) go to 12 Volts. No one is gonna pay an Engineer to recalculate because, that takes money. Same switch, and same coil, add a cheap resistor. Problem solved. That is the "Ballast" resistor. Back to our kinda/sorta problem. Coil D.C. resistance is matched to the ballast resistor. Problem solved. Then sometime around 1967 those sneaky makers hid the "ballast" resistor inside the coil. The true 12 volt coil was invented. Sorta. But, no "ballast" resistor is needed, "cause it is built in. Same switch, same coil, same resistor, new can. Some where the hotrod guys got in the mix. Bigger spark for higher compression. They wanted more power. Use more current. That worked, and burned up points. Change the number of turns inside the coil. That worked, and messed up the current. Burned up the points. So, you now have to match the coil to the resistor. Or, use a "12 volt" coil. Are we having fun yet? Hotrod and his practical solution. Winner.
