Ignition Leads or High Tension cables or spark plug wires or spark plug cables are the wires that connect a distributor, ignition coil, or magneto to each of the spark plugs in some types of internal combustion engine. "High tension lead" or"cable" is also used for any electrical cable carrying a high voltage in any context. Tension in this instance is a synonym for voltage. High tension leads, like many engine components, wear out over time. Each lead contains only one wire, as the current does not return through the same lead, but through the earthed/grounded engine which is connected to the opposite battery terminal (negative terminal on modern engines). They have an outer insulation several times thicker than the conductor, made of a very flexible and heat-resistant material such as silicone or EPDM rubber. The thick insulation prevents arcing from the cable to an earthed engine component. A rubber"boot" covers each terminal. Dielectric grease can be used to improve insulation; a small amount can be applied in the inside of the rubber boot at each end of each wire to help seal out moisture. The wire from each spark plug is just long enough to reach the distributor, without excess. Each end of a spark plug wire has a metal terminal that clips onto the spark plug and distributor, coil, or magneto. There are dedicated spark plug wire pliers, tools designed for removing the terminal from a spark plug without damaging it.