On initial starting, a storage battery is connected through the ignition switch (called "Contactor" in the figure above).
This is timed so the points are closed for the majority of the engine cycle, allowing current to flow through the ignition coil, and are opened momentarily when a spark is desired.
The primary winding has a small number of turns and by Faraday's law of induction has a voltage spike develop across it of the order of 250 volts.
This occurs simultaneously with the points opening and the coil delivering a high voltage to the center electrode.
The inductance of the primary winding limits the speed at which the current through it can increase to the necessary level to provide enough energy to create a spark.
The ballast resistor placed in series with the primary winding creates a voltage drop proportional to the current.
This in turn introduces resistance at the point contacts that reduces primary current and resulting spark intensity.
A second problem involves the mechanical cam-follower block that rides on the distributor cam and opens the points.
Tune-ups for older vehicles usually involve replacing the points and condenser and setting the gap to factory specifications.
These components can develop conductive "sneak paths" on their surfaces (also called 'tracking') across which the coil's secondary voltage produces a current, often in the form of an arc, that bypasses the spark plug.