Points are ALWAYS on the ground side of the coil. Makes them easier to build mechanically. Ignition switch is on the hot side. When points are open there will be 12 volts on both coil primary connections. The voltage on the points side will be very slightly lower because of the voltmeter load and the coil primary resistance. For good VOM at 20,000 ohms per volt on the 25 volt scale the voltage drop will be 75 or 100 microvolts, which you won't detect. When the points are closed there will be 12 volts on the battery terminal, and zero on the points wire. If the points don't close that won't happen and there won't be any ignition. If the voltage drops on the battery post of the coil with the points closed, there's external resistance in the battery wire that you don't need with the 12 volt internal resistor coil.

A coil works by storing energy in the core's magnetic field while the points are closed. An inductor like the coil tends to slow the change in current, so the current rises in a few milliseconds from nothing to being limited by the DC resistance. Then then the points open, the coil voltage rises rapidly trying to maintain same current. We electrical engineers call that L* di / dt. The voltage is the inductance times the rate of change of current. Otherwise known as inductive kick. The primary coil induces lots of voltage (tens of thousands of volts) in the many turns of the secondary.

Typically points are closed about half the time as the distributor cam rotates and open about half the time, that ratio is called dwell. So the odds are about 50/50 of finding the points open or closed, modified by where the engine stopped after bouncing off the last compression stroke.

The points have to have the correct gap set on top of the cam to close and open properly. And after setting the gap with a feeler gauge (or in some GM distributors with a dwell meter while running) the timing has to be set so ignition happens at the right piston position, anticipating top dead center sooner the faster the engine is running. That's the job of the centrifugal advance weights. If they haven't been lubricated they will mess up timing and engine running. Timing stuck advanced will nearly stall the starter while cranking. Timing stuck retarded will make for a very smooth running engine with low power.

A bad condenser/capacitor will upset the power in the ignition pulse (good condenser makes for a fat blue spark, a bad condenser makes a thin yellow spark). A shorted condenser will prevent the circuit from opening and so kills spark completely.

Spark is better if the coil primary connection polarity matches the battery polarity.

Besides the points gap, and distributor timing, the cam shaft timing has to be right so the valve open and close at the proper times as well as the distributor gear drive timing has to be right. Then the carburetor mix has to be right because gasoline vapor has to be rich enough but not too rich, its not like hydrogen that will burn from about 2% to 90% in air, gasoline has to be about 15 to 25% to burn.

There are so many things that have to be right for a gasoline engine to run that its sometimes amazing that after working on one that it runs at all. Fuel, air, spark, timing of spark, timing of valves (affected by the cam gears and the valve clearances) all have to be right to run at all or to run good.

Gerald J.