Okay, this is becoming a moot point with distributorless ignitions, but why did they switch the primary sides of the coils on distributor-type ignitions? Surely the coil wouldn’t burn out, since even at a 750RPM idle, a 4-banger would discharge 25X/second. I assume they had a reason not to leave them hot and only switch the secondary side with the distributor. But I’d like to know what the reason was/is. Help a curious SOB understand
The primary side is switched because a spark occurs when the magnetic field built up by energizing the primary side of the coil collapses and induces the energy in the secondary windings of the coil. This induction occurs when the primary side of the coil is switched off so that the magnetic field collapses. The secondary windings then take the energy of the collapsed field and discharge a high voltage spark. The primary coil is then switched on to produce a magnetic field again.
There would be no output (spark) from the coil if you didn’t switch it on and off. The spark happens when you switch it off.
As others have mentioned, the spark is generated by pulsing the current through the coil. When the magnetic field collapses when the coil is switched off, the coil discharges the high voltage pulse. If cars had alternating current electrical systems instead of DC, you could possibly get away with not switching the coil on and off.