Access screw Wire core Cover To battery negative To booster terminal of magneto Contact spring To starter side of start switch Internal grounding strip Primary winding Condenser Vibrator moving contact Vibrator stationary contact External ground  through mounting bolt Figure 4-24.  Booster coil. Figure 4-25.  Booster coil schematic.  4-16 acts as an electromagnet only when current flows in the  primary coil, it loses its magnetism as soon as the primary  coil circuit is broken. This permits the action of the spring  to close the contact points and again complete the primary coil circuit. This remagnetizes the coil core, and again  attracts the movable contact point, which again opens the  primary coil circuit. This action causes the movable contact  point to vibrate rapidly, as long as the start switch is held  in the closed, or on, position. The result of this action is a  continuously expanding and collapsing magnetic field that  links the secondary coil of the booster coil. With several  Figure 4-25  illustrates, in schematic form, the booster coil  components shown in  Figure 4-24 . In operation, battery  voltage is applied to the positive (+) terminal of the booster  coil through the start switch. This causes current to flow  through the closed contact points to the primary coil and  ground.  [Figure 4-25]  Current flow through the primary coil  sets up a magnetic field about the coil that magnetizes the  coil core. As the core is magnetized, it attracts the movable  contact point, which is normally held against the stationary  contact point by a spring. As the movable contact point is pulled toward the iron  core, the primary circuit is broken, collapsing the magnetic  field that extended about the coil core. Since the coil core  Since the regular distributor terminal is grounded through  the primary or secondary coil of a high-tension magneto,  the high-voltage furnished by the booster coil must be  distributed by a separate circuit in the distributor rotor. This  is accomplished by using two electrodes in one distributor  rotor. The main electrode, or finger, carries the magneto  output voltage; the auxiliary electrode or trailing finger,  distributes only the output of the booster coil. The auxiliary  electrode is always located so that it trails the main electrode,  thus retarding the spark during the starting period.  other terminated at a high-tension terminal. The high-tension  terminal is connected to an electrode in the distributor by an  ignition cable. Coil core Primary winding Stationary  contact point Moving  contact point + Secondary winding Internal grounding strip Movable contact spring Condenser