A shielded aircraft spark plug with a built-in carbon resistor between its terminal end and center electrode. Shielded ignition leads have a metal braid around the insulator, and this braid, with the insulator and conductor, acts as a capacitor and stores electrical energy.

When the spark jumps the gap in the spark plug, the energy stored in the lead capacity causes the spark to continue after the fuel is ignited. This prolonged spark needlessly wears away the spark plug electrodes. The built-in resistor causes enough opposition to this current to reduce the spark duration and increase the life of the spark plug electrodes.