A spark plug is an electrical device found in the cylinder head of some internal combustion engines that carries electrical energy and turn fuel into working energy. To enable the spark plug to generate energy, it relies heavily on the spark plug wires or ignition wires.
The spark plug wires on automobiles are responsible for harnessing tens of thousands of volts and delivering that energy to the spark plugs every time the engine is started. Spark plug wires or ignition wires are connected to the spark plugs and distributor or ignition coils. Compared to typical wires, plug wires are built to have a lot of resistance. This resistance greatly reduces the radio static being generated by the ignition system. A standard plug wire has a resistance of about 10,000 to 15,000 ohms per foot of length--if it's measurably higher, the wire will probably malfunction.
However, the longer the spark plug wires goes in service, the more brittle its insulation gets and the core also develops cracks. The resulting breakdown in the spark plug wire means the electricity will be redirected elsewhere, causing engine misfiring, fouled spark plugs, and rough running. Unfortunately, spark plug wire problems can be hard to detect since spark plug wire insulation and core problems are hidden below the spark plug wire surface.
It's worth nothing that spark plugs do not actually create heat but only remove it. Working as a heat exchanger by pulling unwanted thermal energy away from the combustion chamber, spark plugs transfer the heat to the engine's cooling system. The spark plug's firing end temperature must be kept at optimum levels to prevent pre-ignition and fouling.