TM 10-3930-653-14&P 4-7-8.  IGNITION COIL The  ignition  coil  is  of  an  oil-filled  type.    The  ignition coil case is filled with oil which has good insulating  and heat-radiating characteristics. High  sparking  voltage  is  produced  from  starting  the engine to a high revolution by the external resistor which is used in the ignition coil circuit for 6R-200.  The internal resistor  limits,  to  a  maximum,  safe  flow  of  the  primary current  through  the  coil  and  distributor  contact  points. Thus,  it  protects  the  contact  points  during  slow  speed operating    when    they    are    closed    for    long    intervals. Fig. 4-217.  Construction 4-119