particles that contain hydrocarbon materials. The water in the  hydrocarbon material is evaporated during flashover, leaving  carbon to form a conducting path for current. When moisture  is no longer present, the spark continues to follow the carbon  track to the ground. This prevents the spark from getting to  the spark plug, so the cylinder does not fire.  Magnetos cannot be hermetically sealed to prevent moisture  from entering a unit, because the magneto is subject to  pressure and temperature changes in altitude. Thus, adequate  drains and proper ventilation reduce the tendency of flashover  and carbon tracking. Good magneto circulation also ensures  that corrosive gases produced by normal arcing across the  distributor air gap, such as ozone, are carried away. In some  installations, pressurization of the internal components of  the magnetos and other various parts of the ignition system  is essential to maintain a higher absolute pressure inside the  magneto and to eliminate flashover due to high altitude flight.  This type of magneto is used with turbocharged engines  that operate at higher altitudes. Flashover becomes more  likely at high altitudes because of the lower air pressure,  which makes it easier for the electricity to jump air gaps.  By pressurizing the interior of the magneto, the normal air  pressure is maintained and the electricity or the spark is  held within the proper areas of the magneto even though the  ambient pressure is very low.  Even in a pressurized magneto, the air is allowed to flow  through and out of the magneto housing. By providing  more air and allowing small amounts of air to bleed out for  ventilation, the magneto remains pressurized. Regardless of  the method of venting employed, the vent bleeds or valves  must be kept free of obstructions. Further, the air circulating  through the components of the ignition system must be free of  oil since even minute amounts of oil on ignition parts result  in flashover and carbon tracking. The ignition lead directs the electrical energy from the  magneto to the spark plug. The ignition harness contains an  insulated wire for each cylinder that the magneto serves in the  engine.  [Figure 4-11]  One end of each wire is connected to  the magneto distributor block and the other end is connected  to the proper spark plug. The ignition harness leads serve a  dual purpose. It provides the conductor path for the high- tension voltage to the spark plug. It also serves as a shield  for stray magnetic fields that surround the wires as they  momentarily carry high-voltage current. By conducting these  magnetic lines of force to the ground, the ignition harness  cuts down electrical interference with the aircraft radio and  other electrically sensitive equipment.  Ignition lead capacitance increases the electrical energy  required to provide a spark across the plug gap. More magneto  primary current is needed to fire the plug with the shielded  lead. This capacitance energy is discharged as fire across the  plug gap after each firing of the plug. Reversing the polarity  during servicing by rotating the plugs to new locations, the  plug wear is equalized across the electrodes. The very center  of the ignition lead is the high-voltage carrier surrounded by  a silicone insulator material that is surrounded by a metal  mesh, or shielding, covered with a thin silicone rubber coating  that prevents damage by engine heat, vibration, or weather.  Capacitance is the ability to store an electrostatic charge  between two conducting plates separated by a dielectric. Lead  insulation is called a dielectric, meaning it can store electrical  energy as an electrostatic charge. An example of electrostatic  energy storage in a dielectric is the static electricity stored  in a plastic hair comb. When shielding is placed around the  ignition lead, capacitance increases by bringing the two  plates closer together. Electrically, the ignition lead acts as  a capacitor and has the ability to absorb and store electrical  energy. The magneto must produce enough energy to charge  the capacitance caused by the ignition lead and have enough  energy left over to fire the plug.  A magneto is a high frequency radiation emanating (radio  wave) device during its operation. The wave oscillations  produced in the magneto are uncontrolled and cover a wide  range of frequencies and must be shielded. If the magneto and  ignition leads were not shielded, they would form antennas  and pick up the random frequencies from the ignition system.  The lead shielding is a metal mesh braid that surrounds the  entire length of the lead. The lead shielding prevents the  radiation of the energy into the surrounding area.  Figure 4-11.  A high-tension ignition harness.  Ignition Harness 4-8