Some components must be connected in series. For example, the sender in the fuel tank varies its resistance according to the amount of fuel in the tank, and sends a small electrical current to the fuel gauge.
However, a component with a high current consumption must not be connected using wires which are too thin, or the wires will overheat, blow a fuse, or burn out.
The starter motor has its own heavy cable, direct from the battery. The ignition circuit furnishes the high-tension impulses to the sparkplugs; and the charging system includes the generator, which recharges the battery. All the other circuits are called ancillary (subsidiary) circuits.
The two components are connected in series so that the varying resistance in the sender will affect the position of the needle on the gauge.