So if T coils will run on 6 volt just fine, and they will run on 12 volt but will burn the points just that much faster, than why doesn't running on mag burn the points even faster (assuming the mag puts out some 24-28 volt, I'm guessing)?
AC vs DC
Coil operation is different on the AC supply from the magneto compared to a DC battery supply. Point material tends to migrate from one point to the other on DC, resulting in one point becoming pitted, and the other receiving a build up of point material. Which point loses material, and which gains it depends on polarity.
With the magneto's AC, the polarity constantly changes so the effect cancels out.
Operating characteristics also change in that the average coil current drops as the supply voltage increases - the points open faster, providing a kind of automatic current limiting. This is why absurdly high voltages aren't generated by the coil with 30V AC input.
My observations have been that points run on 6VDC last an extremely long time; mine are up to 30,000km and are still in excellent condition. I can usually tell a set of coils that have been run on 12VDC - the point migration is more obvious and they're usually a bit blackened.
On mag, the points will only cycle 2 or 3 times while the timer is in contact as opposed to maybe 300 times on battery. The transfer (wear) takes place when the point arcs only.
Thank you all. John, guess I was thinking the mag was DC.
I think the mag peaks out to about 18 volts that is why the early lamps (15-16) were 9 volts each, wired in series.
A good Model T magneto puts out 28 - 30 volts at high RPM. You can easily burn out magneto bulbs by operating the engine at too high speed on early cars that do not have the voltage limiter in the circuit.
These are original magneto bulbs operating in my '17 torpedo runabout:
Yep - If you want to see better at night go faster in a T.