On Mon, 11 Feb 2019 00:43:10 -0600, Fox's Mercantile
wrote:

On 2/11/19 12:32 AM, wrote:
So, how can the coil deliver a spark
during the time the points are open, since the secondary is relying on
the primary (-) to complete the circuit, which is also being switched on
and off by the points. .

Sigh. When you open the points,the magnetic field collapses
which creates a HUGE spike, not related to the turns ratio.
The "return" is through the primary back to the + terminal
on the battery.

This is proof that not all the videos on youtube are accurate. I watched
one and the guy insisted that the (-) terminal was the return path for
the secondary. Another video said that EITHER primary connector can be
used to test a coil for a spark.

It does seem kind of bizarre that the (+) terminal is used for the high
voltage return, (knowing that spark plugs complete the circuit by being
connected to the engine block, which is connected to the battery
GROUND).

I guess this is why I think the circuits are weird. They do not seem to
match general electronic knowledge. But they work and have worked for
well over a half century, so I am not going to dispute it. Yet, it still
seems odd to me.... Especially when I see the high voltage is actually
grounding THRU the 12 volt battery.... (without damaging the battery).

Then too, I recall the time I connected a 12v battery charger to a
battery connected to a 12VDC electric fencer. It took about 3 seconds
for me to see a bright flash inside the battery charger, then smoke.
Every diode in that battery charger was instantly destroyed. I still do
not understand why the high voltage came thru the "primary" of that
fencer, but I now know to never connect a charger to a battery until the
fencers is turned off.

And on that same note, I know someone else who connected one of those
fencers to the battery in his car for a temporary livestock fence. Not
only did he get a nasty shock when he touched his car, but his
alternator was destroyed.

https://auto.howstuffworks.com/ignition-system3.htm