Try our new info resource - "Aladdin's Cave" (Main menu)Just added a separate link to Ash's Dropbox thread (shortcut)
"One clarification/ question - when the points are closed, is the current just earthing through the points backplate"Yes, the primary circuit is completed by earthing through that route. With the points closed the primary coil winding induces energy to be stored in the secondary "HT" windings that are wrapped around the primary, but not connected to them. In concept like the current contactless charging systems for phones etc.When the secondary coil holds enough energy, the primary earth is opened with the points. At this point the energy wants to escape from it's stored state and lacking that primary route will seek any earth, which is supplied by the leads and spark plugs ultimately to give a ground route out, jumping that plug gap then gives us the spark.There's a system backlash too, just as the plug starts to spark it comes back through the entire system to induce an escape route across the points gap but prevented from doing so by providing a buffer in the form of the condenser to absorb the shock sufficient that it's unwilling to jump the points gap. If the condenser fails, then the spark jumps the points gap and doing so terminates the one at the plug. It's really a little bit of a magical balancing act reliant on helpful potential faults and useful characteristics being arranged in balance over the whole system.