Posted by David Sherman on Wednesday, April 18, 2001 at 11:10AM :
In Reply to: Pos or Neg Ground posted by Collen on Wednesday, April 18, 2001 at 7:38AM :
I don't know for sure, but I'll speculate. For an old-fashioned electrical system that consists of nothing but battery, starter, generator, mechanical voltage regulator, and lights, everything works equally well on either polarity as you can easily see by swapping polarity on an old vehicle. So I think positive ground early on was just a random choice. It couldn't be a matter of a positive spark being better or worse than a negative one because engines with magnetos generate positive sparks for half the plugs and negative ones for the other half and they work just fine. Also spark polarity could be changed by connecting the spark coil differently regardless of battery polarity.
However, tube-powered electronics really like a negative ground. Vacuum tubes work by emitting electrons (negative charges) from the heated cathode and attracting them to the plate which is several hundred volts positive. A tube radio really "wants" to be negative ground because the cathodes are usually near ground for signal purposes and the amplified signals are on the plates (positive). So electronics engineers have historically preferred to design with a negative ground. With the invention of transistors, available in both polarities (PNP and NPN) this became less important, NPN transistors are slightly cheaper, faster, and stronger than PNPs and circuits using them "prefer" a negative ground like tubes did.
So my guess is that early on it didn't matter and was a random choice. As electronics began to be included in vehicles there was good reason to switch to negative ground. Everything from tube radios to computerised engine controls could be made to work with positive ground, it's just a little bit more complicated and a little more expensive.
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