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Back in the hayday of electric trolleys, the side grounded affected underground metal water and gas pipes. Electrolysis would eat holes in some pipes. So the polarity that added metal to the pipe was used. Trouble is the pipes at a distance needed the opposite polarity of pipes close to the power plant. Whichever pipe owner yelled the loudest got the preferred polarity. In vehicular batteries, there has been the postive negative argument that lasted from the first ones until a mixture of alternators and solid state radios came out, and even for a while solid state radios (both AM/FM and two way) were made with a switchable polarity to work with either side of the DC system grounded. Before that tube type two way radios had polarity switch provisions for decades. Eventually, SAE, I think, decided the various arguments were poorer than the consistency of one universal ground polarity and so decreed negative ground. Fact is, either polarity works and always has. And if the wiring design is going to use the frame and major castings for one side of the circuit, there just has to be that choice of polarity made. In the 30s automotive engineers argued at length for one or the other, some makes were positive ground, some were negative ground. I have forgotten their arguments, but I think the main one was about corrosion of connections at the frame. Besides which, when we learned current was electron flow and by convention electrons have a negative charge, all our polarities are backwards anyway. We can blame that inversion on Ben Franklin. He had two choices and guessed wrong. In another way, positive and negative are simply mathematical concepts arbitrarily applied to electric polarity. Either works, we just have to agree on which one to use so we can swap measurements. When we develop the freedom to work with either polarity, we are no longer bothered about one or the other and I had to develop that freedom going from designing vacuum tube circuits to germanium transistor circuits, then switch back for silicon transistor circuits. Now I don't care which way is up, the numbers all come out and the circuits work so long as the power and the circuit polarity match. I used to have a book by B.G. Lamme with a collection of articles and I think it may have had one on the automotive polarity arguments. There were many more options on the choice of frequency for AC systems and settling on only a couple frequencie world wide (neglecting trolleys) took half a century of discussion. Gerald J.
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