401 Dynamo

[peterg]

Not an auto electrician either, but I have a little understanding.
The dynamo output goes through the control box which has two cut outs.
The dynamo uses an electro magnet and the first cut out is also controlled by electro magnatism.
When dynamo output is low the first cut out breaks, stopping output going to the car. At that point power flows from the battery giving a negative charge on the ammeter.
With the cut out open power from the dynamo is fed back down the field cable to the electro magnet, energising it and causing the dynamo to produce more output. When it reaches about 12 volts it is producing enough power to energise the cut out and close it, allowing power to enter the system. Once it exceeds the draw you will get a positive charge through the ammeter.
Finally when the voltage gets to about 14.5 volts the other cut out opens, stopping power to the field and reducing dynamo output to stop the dynamo over charging the battery.
The important question is, at what engine speed will the dynamo produce a positive charge with all the lights on?