Posted by KEB1 on May 06, 2008 at 07:26:16 from (192.31.106.34):
In Reply to: battery question posted by farm M on May 05, 2008 at 19:51:31:
I'm an electrical engineer with 30+ years experience, and I have never been able to find a definitive answer to the positive/negative ground question. Technically, the term "ground" is a misnomer; there is no connection to physical earth. However, in common usage, an electrical reference point such as a tractor or car chassis is commonly referred to as "ground".
Contrary to what Phil says below, the polarity of the high voltage spark has nothing to do with whether the positive or negative side of the battery is connected to the reference point. The polarity of the high voltage spark is determined by the relationship of the primary and secondary windings in the coil. In fact, the high voltage pulse is always negative with respect to the reference point, i.e., "positive ground". That's why the primary connectors on ignition coils are marked '+' and '-'.
The center electrode on a spark plug is always hotter than the ground electrode. Just like the hot filament in a vacuum tube, the hotter surface emits electrons easier. Therefore, by having the center electrode negative, it is easier to initiate a spark from the hotter surface.
I've heard a number of opinions for the positive/negative ground debate in old vehicles, including electrolytic effects on chassis corrosion, but have never found a defnitive engineering reason for one or the other.
Until electroncis started appearing in cars it really didn't make any difference as near as I can tell. Lights, starter motors, etc., don't care which direction the electrons move, only that current is flowing. When electronics started to become common, there was a marketing reason for standardizing so that manufacturers didn't have to build both six and 12 volt and positive and negative ground hardware.
The major reason for going to higher voltages is based on on the necessary wire size, which is driven by current. For example, to light a 50 watt headlamp takes about 8 amps at six volts, 4 amps at 12 volts, and 2 amps at 24 volts. Higher voltage = lower current flow for a given load = smaller wire size. Lower current also equals less losses for a given wire size, making higher voltage systems more efficient.
If I ever find a defnitive explanation for why one polarity was used over another, I'll post it. I suspect it was just convention, i.e., one manufacturer started out with a particular voltage/polarity, and others followed suit.
If any out there knows of a definitive refernce, I'd love to see it. I have a book from the 1920's which describes automotive electrical system operation, and it is silent on the issue of polarity.
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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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