How alternator light works, a more detailed description Options

[Tom]

After reading many accounts of how this circuit works, I felt compelled to investigate further as I did not understand how two positives would cause a light to operate. They won't. One must be somewhat negative to complete the circuit. Internet searches turned up the same basic explanation, still was not buying it. I think it was being oversimplified.
This is how I think the alt light works:
When the key is on and engine not running, there is 12 volts + at the alt light power side coming from the fused side of fuse #9. The other side goes to a junction on the relay board with D+. With the key to off and a meter connected between D+ and ground at the relay board, the reading is 12 ohms. As soon as the key is turned to on, the reading jumps to 12.5 meg ohms and the light comes on. If the wire for D+ to the alt is removed, the reading stays the same and the light stays on. Removing the VR caused the reading to jump to infinity and the light goes out.
For the light to work, there has to be power to one side of the light and some resistance reading to ground for the other. Looking at the wiring diagram, one can follow the blue wire to the junction at the relay board at D+, then up thru the VR to a set of relay contacts, then down thru a ( resistor ?, not sure) and then down to the DF connection and on to the rotor where the current will produce a magnetic field. After the rotor, it goes to ground. When the alt spins enough RPM's, a voltage is produced and fed back to the VR, causing the relay to open and removes the ground path for the alt light.
I could be entirely wrong here, but this is what I see and my readings more or less confirm it. If you see an error in my thinking, please post and let me know.
Thanks,
Tom

[toolguy]

Light comes on when the alternator out voltage on the red lead is lower than the battery voltage. . reverse polarity flow. . .

Red white is switched battery 12.5 or so. . that is what lights the bulb at idle. . the bulb is connected to potential lower than 12 .5 volts thru the diodes in the alternator. . with the alternator turning, the voltage out of the alternator on the Blue wire to the regulator reaches the same voltage as the red [B+] out to the battery so there is no differentiation in voltage so no flow anymore thru the light and the light dims. both side now being equal voltage. .
.
and yes, the alternator needs voltage to excite the fields to begin the charging cycle.
spinning an alternator without hooking up a batter will not produce voltage. . generators did it by
having magnets in the outside fields. .

Really simplified version. .left out the technical stuff. It's only confusing