Dwell is the old expression for closed points, ie when current flows thru the coil. And like pointed out earlier, for Kettering type (points) ignition the dwell was a compromise and max current was sometimes limited by a resistor in series with the +12 V feed to the coil.
During the time when current flows in the coil , the magnetic circuit is charged, and thats where the spark energy comes from. At a fast interruption of the charge current there will be a collapse in the magnetic field which is very rapid and the "flyback" voltage will be transformed by the HT winding in the coil to several kiloVolts , which is what causes the spark plug to flash over. ( Lentz law, again!)
The current in the primary winding of the ignition coil will rise at a rate which is governed by the relationship between the coil reactance and resistance ( tau). This is shown very nicely in the scope pic above.
After 1 tau the coil has stored abt 63%of the maximum posible energy in the iron core . After this a lot of the charging current just produces heat in the primary LV winding .
The tau (X/R )of most ignition coils are designed so that the coil is charged to
a sufficient level for a good spark in just a few milliseconds , in order to cope with high rpms.
And with electronically controlled ignition it is standard practice NOT to charge the coil longer than those few ms , even at low rpms .
So , measuring the dwell( coil charging degrees for one cylinder duty cycle) in degrees will give just a few degrees at low rpms , but maybe up towards 50 degrees ( or even more ) at higher rpms .
But the actual coil charging TIME is the same .
This is why we set "DWELL " in ms.
Nice way to always have maximum spark energy available without cooking he ignition coil or inserting a esistor in series which cuts down on hi-RPM erformance.
Normal dwell time will be in the region of 2-5 ms and the discharge time is very short, much shorter than 0,1 ms.
OT maybe , but had to get it off of my chest