When this topic first appeared, the comment about auto transformer effect set me a pondering. When the points break a high voltage appears across the pri; if + earth this is – at the points end, if –ve earth, + at the points end. The secondary if connected in the same direction experiences an induced voltage of the same polarity. I have drawn every conceivable combination of direction of connection of secondary and leads etc but I can only get a –ve spark in series additive with the pri and battery (and hence earth) with + earth battery. (Points in effect tapped off part way up the coil)
Maybe I have got it wrong somewhere (again, according several) but perhaps this was the reason for adoption of + earth, not reduced corrosion, the usual somewhat curious “explanation”. It is a few hundred volts cf several thousand, so not vital. I did not consider the lesser earth path thru the condenser.

Anyone who enjoys puzzles can try diagrams for themselves.

It is all related to Lenzs law which, as Tony notes above, the internet seems to have made incredibly complex since it was taught to my father on a trades course in the 3rd form, somewhat before. You would think someone could provide Wikipedia with understandable examples?

In my Seven when in everyday use I could detect reduced harshness with smaller plug gaps. I reckon a smaller spark effectively retards the ignition process slightly.

Location: Auckland, NZ