It's the current flowing through the primary windings of the coil that creates a magnetic field. When the points open (or the ecu switches the primarie's off) this magnetic field collapses and a potential is created in the secondary windings that is equivalent to the ratio of turns of wire around the core in the primary windings (very few) to the turns of wire in the secondary (very many) times the voltage on the primaries. It's this turns ratio that allows the coil to generate 20K-30K volts at the plug for an OEM syle coil or 40K or more on a performance coil.
In a properly running engine it never takes the full potential of the coil to create a spark. So at idle it may only take 10K volts for the spark to occur but as rpm increases the voltage to create a spark increases.
The magnetic field that makes the coil work takes a specific amount of time to build up for a specific amount of current flow. However, the need for sparks in the engine is based on rotation and the time available for the current to flow and generate the magnetic field to generate the spark decreases with increasing rpm. You wind up with a coil that has less potential to create a spark as just as the engine requires more potential for the spark.
The ballast resistor comes into play here. The coil is designed so that it can create an adequate spark at max rpm. However at low rpm and especially with points ignition with the key on, engine off points closed there is too much current flow. This excess current flow generates heat which over time can damge the coil and cause failure.
The ballast resistor is basically a variable resistor that increases resistance with heat. Under the low rpm conditions the ballast gets hot resulting in a increase in resistance that limits the current flow through coil protecting it from over heating. As the rpm's increase the average time the current is flowing decreases allwoing the ballast to cool off resulting in more current flow to the coil allowing it to produce a higher potential when it's needed.
5 volts sounds low, typically you expect to see 8. If you were to bypass the ballast the engine will still run but you run the risk of damaging the coil over time. You can purcahse coils that are designed to operate without a ballast or you could go with Capacitive discharge system like a MSD which doesn't require one.
However, I woul check the health of your wiring first. Measure your battery voltage. Then check the voltage at the upstream side of the ballast it should not be any more than 1/2 a volt lower. Then measure the down stream side of the ballast then measure at the + of the coil, again no more than 1/2 v difference. If you have more then there are some bum connections in your wiring.
