Until the early 1970s, most British sports cars featured a standard non-ballasted ignition system with a 12V coil that produced a full power spark at that voltage. The later ballasted ignition system used a modified coil that delivered a full power spark which was reduced by installing a ballast resistor or resistor wire in the power feed to the coil.
These coils have a second feed running from the starter solenoid that allows 12V to be generated only at startup and a reduced level for ordinary operation to extend the life of the contact breaker points.