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the inductance of the primary winding allows the current to increase faster, but would lead to a higher maximum current that will shorten the life of the points and increase heating of the coil. The ballast resistor placed in series with the primary winding creates a voltage drop proportional to the current. When the points initially close, current is low so voltage drop across the resistor is low and most of the battery voltage acts across the coil. Once current builds up, voltage drop across the resistor increases, leaving less battery voltage

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