A headlamp bulb, for example, is designed to have a degree of resistance so that it consumes a certain current to glow normally. But there are at least two headlamps in the circuit. If they were connected in series, electric current would have to go through one headlamp to get to the other.
The fuses are usually grouped in a box or on a panel with a cover. Ideally, the box should be fixed in an accessible place — such as the bulkhead under the bonnet.
If the battery voltage drops, less current flows, and eventually there is not enough to make the components work. The extent to which a wire resists the flow of current is called resistance, and is measured in ohms.
Where wires run side-by-side they are bound together in a bundle, in a plastic or fabric sheath, to keep them tidy and less difficult to fit. This bundle of wires stretches over the length of the car, with single wires or small groups of wires emerging where necessary, and is called the wiring loom.