Beest I've found says it was company custom that chose the ground. Some makers were positive ground, some were negative ground in the 6 volt days.
Some of the arguments were based on corrosion from experiences with water and sewer pipes along electric RR, trouble with that argument is, the corrosion or erosion problem depends which end of the circuit it is. The direction of current is opposite from the pipe far from the power house as it is close to the power house.
Fact is either polarity worked. But confused people and with solid state electronics it was more bother to make dual polarity radios but I've owned some so it wasn't impossible. Two way radios were made first and longest with reversible polarities both tube and solid state.
I think eventually SAE set a standard and the auto and tractor makers followed the standard which simplified training of mechanics.
But then there were the JD diesels with a +12 and a -12 pair of load circuits to allow 24 volt charging and starting.
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Today's Featured Article - Third Brush Generators - by Chris Pratt. While I love straightening sheet metal, cleaning, and painting old tractors, I use every excuse to avoid working on the on the electrics. I find the whole process sheer mystery. I have picked up and attempted to read every auto and farm electrics book with no improvement in the situation. They all seem to start with a chapter entitled "Theory of Electricity". After a few paragraphs I usually close the book and go back to banging out dents. A good friend and I were recently discussing our tractor electrical systems when he stated "I figure it all comes back to applying Ohms Law". At this point
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