As the engine continues to turn, the vibrator breaker points  open, collapsing the L4 magnetic field through T1 primary,  inducing a high-voltage in the secondary of T1 to fire the  spark plug. retard breaker magneto, a single breaker magneto, a starting  vibrator, transformer coils, and a starter and ignition switch.  [Figure 4-31] To operate the system, place the starter switch S3 in the  on position. This energizes starter solenoid L3 and coil  L1, closing relay contacts R1, R2, R3, and R4. With the  magneto switch in the L position, current flows through R1,  the vibrator points, L2, R2, and through the main breaker  points to ground. Current also flows through R3 and the  retard breaker points to ground. Current through L2 builds  up a magnetic field that opens the vibrator points. Then, the  current stops flowing through L2, reclosing the points. These  surges of current flow through both the retard and main  breaker points to ground. When the engine fires, the starter switch is released, de- energizing L1 and L3. This opens the vibrator circuit and  retard breaker points circuit. The ignition switch is then  turned to the both position, permitting the right magneto to  operate in time with the left magneto. The function of the spark plug in an ignition system is to  conduct a short impulse of high-voltage current through  the wall of the combustion chamber. Inside the combustion  chamber, it provides an air gap across which the impulse  can produce an electric spark to ignite the fuel/air charge.  While the aircraft spark plug is simple in construction and  operation, it can be the cause of malfunctions in aircraft  engines. Despite this fact, spark plugs provide a great deal of  trouble-free operation when properly maintained and when  correct engine operating procedures are practiced. Since the starter switch is closed, the engine crankshaft is  turning. When it has turned to the normal advance or running  ignition position, the main breaker points of the magneto  open. However, current still flows to ground through the  closed retard breaker points. As the engine continues to turn,  the retard ignition position is reached, and the retard breaker  points open. Since the main breaker points are still open,  current must flow to ground through coil L4, producing a  magnetic field around the coil L4. Spark Plugs Figure 4-31.  Low-tension retard breaker magneto and starting vibrator circuit. 4-20 S T1 L1 L2 L3 C2 C3 C1 R1 R2 R3 V1 R4 Off S3 On L R Both Off Retard breaker Main  breaker Main breaker Spark plug T2 Spark plug L4 L5 Starter solenoid Starting vibrator Magneto switch Left magneto Right magneto