[rotorwrench]

The coil doesn't store any energy. It's just an inductor. Induction coils generally don't work on direct current (DC) power but they will work for alternating current (AC) power and are used a lot in electronics to step voltages up. They work for an automotive ignition due to the pulsating direct current provided by the breaker points since it is half wave alternating current.

A hotter spark is a product of more modern technology. The systems only need to create a spark hot enough to light off the flame front in the cylinder. If a designer wanted a hotter or higher voltage spark back in the day, it would cost them in lower reliability of the system components. After the advent of modern electronic ignitions, there was a lot less worry about reliability with breaker points and condensers since they no longer had them. Coils were designed with materials that can take a lot more heat so reliability was no longer a problem with them and they could add a lot more windings to increase voltage step up. They can generate a spark that can actually be heard snapping while the engines running on some designs. I could always hear the spark on the GM HEI set ups while the engine was idling. They had to have a lot wider gaps for the spark plugs as well. A DC points ignition would never be able to do this for very long.

The model T uses four trembler coils for its ignition (one for each cylinder). They put out a shower of sparks instead of just one spark per ignition event. That makes them hotter than any 6-volt coils but they were powered off a magneto set up on the flywheel.

The model A went to a can type coil that had an oil inside for an insulator so it could take some heat and not need a ballast to control current.

When the Ford V8 came out, they changed the coil to a different design that evolved with several different configurations but they had to have a ballast to control the current (heat) or they would lose reliability. They would put out just as hot a spark as the model A but they only used about half the voltage of a 6-volt system to do it.

When Ford went back to the can type coils for the 8BA in 1948, they did away with the ballast resistor and it didn't come back till they changed over to 12-volt systems. The ballast resistor is all about reliability in the old systems.