In a 6 volt system when the points close current flows through the points, coil, and switch, to the battery. The issue is amount of current flow. The circuit (points and coil) is designed for a maximum of 3 to 4 amps of current.
When battery is increased to 12 volts the 6 volts is doubled and so is the current, exceeding the design of coil and points. Excess current will soon destroy both. A ballast resistor approximately equal to the resistance of the coil primary can be added in series between coil and battery. This drops 6 volts across the coil and 6 volts across the resistor maintaining the 3 to 4 amps of current.
Or with 12 volts, a true 12 volt coil (primary of coil wound for 12 volts rather than 6 such as NAPA IC14SB)can be used without the need for a resistor. It's internal resistance limits the 12 volt system to 3 to 4 amps.
With current held within design limits the points and condenser will work with either 6 or 12 volt supply.
The current flowing in the circuit is governed by voltage and resistance, the dwell time is irrelevant.
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