With the P-lead ground open (mag ON, engine running), an o-scope attached to the P-lead should show a significant variation in voltage with opening and closing of the points. Voltage will be low with the points closed and the rotor magnet sweeping the primary coil. When the points open the coil's magnetic field will collapse and induce as much as 300 volts in the primary winding.
Voltage (and amperage) with points closed is a function of magnetic flux strength and the speed at which it sweeps the primary coil windings. Flux is fixed but speed varies with RPM. Voltage peak when the points open is a function of amperage created during that sweep, capacitance, and the condition of the points.