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If you look at the top of the coil you will see the two terminals the + and -. These are called the primary lead. You first disconnect all wires to the coil. Hook your digital Ohm meter to those two (primary) and if you have the correct scale you should see .5 to 1.0 ohms like Cuz says.

The center of the inside of the coil has a rod like core. There are two wires wrapped around that rod. Once wire is the primary and the other is the secondary. The two wires don’t directly exchange power while wrapped around the core. Touch the center of the coil with the meter and the + terminal to test the secondary. If the meter is set on the right scale you will see the 14k to 20k ohms.

As the electricity flows from the + to the – terminals of the coil trying to seek ground (earth) the electricity induces a current in the secondary wire wrapped around the core.

This electricity builds up in the core and when the points open it cuts off the flow from the + to the – terminal. The built up energy has lost its containment field so to speak and now wants to seek its own ground. The only path it has is to the rotor and the plug wire and fires the plug.

There is oil in the coil too and some folks like to bench test using the ohm Meter and heating up the coil until it is almost too hot to hold. The numbers should not change if the coil is good.

Good Morning Everyone,
What is the best way to heat one up, oven?
The reason I ask, mine is presently out, on da' bench.

Bruce, try heating it in hot but not boiling water.

10/4 Carter, I can do that now that my honey-dew list is punched!

Back to the description of the coil, when the points open there is a large current in the secondary of the coil that has to go somewhere. The current built up by coming through the points. So when the points open it has enough power to jump the points. The condenser helps keep that from happening. The surge heads to the points but then finds it easier to go to the condenser which is a dead end and allows the power to concentrate. By that time the rotor is close enough to the cap electrode for the power to go out that path to the spark plug. Then the points close and the cycle starts all over again.