Most contact breaker systems incorporate a ballast resistor in the primary circuit. The ballast resistor is inserted in series in the primary circuit between the ignition switch and the positive terminal of the ignition coil.
When the engine is running, its resistance provides a voltage drop in the primary circuit, which lowers the voltage applied to the coil. The coil is designed to operate at this lower voltage level and still provide the step-up transformer action needed, for secondary circuit operation.
However, during cranking, a parallel connection from the starting circuit by-passes the ballast resistor. The voltage available at the battery terminals at this time, will now be applied to the positive terminal of the ignition coil. This voltage will give a boost to the current flowing in the primary circuit.
The increased current flow strengthens the magnetic field to increase the voltage induced in the primary winding, and consequently increase the voltage in the secondary winding. So even under adverse conditions, sufficient ignition energy is available to ignite the air-fuel mixture and start the engine.
