There's an old saying in the engineering business- - - -one I learned back in the early 1960's when I thought I wanted to be a "train driver". "Constants aren't- - - - - -Variables won't!" Since the current flow in an ignition coil is being interrupted hundreds of times a second by the opening and closing of the points, the rules of Ohm's Law don't apply as they do in a constant current flow situation. There's a pesky little thing known as "inductive reactance" which usually applies to coils in an AC circuit, but it also rears its ugly head where pulsating DC is involved. To over-simplify things, the "resistance" of a winding in an induction coil or a transformer varies as a function of time- - - - -the current starts out at a high rate, and in a tiny fraction of a second, the effective resistance increases as the magnetism created by the current flow induces a "counter'-EMF" which opposes the flow of current. Then the points open and a spike of up to 100 volts or so gets induced into the primary winding. Then the condenser gets involved in the circuit to protect the points from flashing over due to the high induced voltage and the microscopically small gap between the points as they interrupt the current flow.
Simply measuring primary winding resistance might give a ballpark guesstimate about whether a coil is intended for 6V or 12V, but there's a LOT more involved in the calculations than in a simple DC resistive circuit.
Jerry
