Final answer:
Battery current flows in the primary circuit of an ignition coil when a complete circuit is formed by the closing of a switch, allowing current to travel from the battery's positive terminal, through the coil, and back to the negative terminal. When this current is interrupted, an arc can be produced.
Step-by-step explanation:
Battery current flows through the primary circuit of a battery ignition coil when the circuit is complete and the switch is closed. This action allows charges to flow from the positive terminal of the battery, through the switch, then through components like the ignition coil and ultimately back to the negative terminal of the battery. The direction of conventional current is from positive to negative. In the context of a 12-V battery in an automobile, the ignition circuit utilizes the battery's power to generate large voltages needed for the spark plugs. In addition, when the current through a large inductor such as an ignition coil is interrupted with a switch, an arc is produced across the open terminals of the switch. This phenomenon occurs due to the inductor's tendency to resist changes in current flow, which induces a large voltage across the open switch capable of jumping through the air in the form of an arc.