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RE: reversing positive ground

To: <mg-t@Autox.Team.Net>
Subject: RE: reversing positive ground
From: "James Fischer" <>
Date: Mon, 3 Jan 2005 07:51:02 -0500
I mostly just read and learn from what is posted, but I am
fascinated by this discussion, even though I would no sooner
convert an MG to negative ground than I would replace the
Smith's instruments with VDO gauges.

(Disclaimer, I have a PhD in Electrical Engineering, and
another in Physics, so I may well be a bore on this issue.)

> Then why do the coil manufacturers mark the leads for + and - if it
> makes no difference?  Just to make sure that the + and - weren't
> confused early Lucas coils were marked sw (switch) and cb (contact
> breaker).

I agree that if one is reversing the polarity of the car's entire
electrical system, one should be complete and consistent about it
so as to not confuse yourself later, but I don't see any practical
tangible electrical advantage that would matter to any of the actual
original components in a typical early British car with no modifications
or additions to complicate or confuse the electrical system.

> It's not an old wives tale.  Reversing the coil leads maintains the
> correct direction of the spark jump for the plugs...

I'll agree on this, but only in the special case of extreme neglect combined
with extraordinary luck, where spark plugs would somehow remain in service
for decades of daily use.  Any spark will eat away a tiny bit of metal, but
only from the more negative "jumping off" point, never from the "landing" point.

If the coil is connected in a way consistent with the polarity of the car, the
(negative) center spark-plug electrode wears slowly away.  While I have never
seen this happen, or even know anyone who has had the problem, the wearing away
of the center spark-plug electrode avoids any corrosion or mechanical weakness
the body of the plug, which is subjected to some serious force when removed and

But this scenario is the product of a highly creative imagination, the sort of
thing that men in white coats study and prescription drug companies offer pills
to cure.  :)

If the plugs were not replaced every so often, this would indicate a level of
care so low as to result in a non-running MG in a matter of less than a year,
long before any damage was done.  If nothing else, the points will wear out
or carbon-foul long before any "damage" is done to the plug that one could see
without a very good microscope.

That said, there is NO CONSENSUS IN SCIENCE as to the actual "direction"
of current flow given a simple battery and a light bulb.  The argument
started in Maxwell's lab, and has continued to this day.

Just in case anyone jumps on the observable direction of a spark jumping
as some sort of "proof" that current flows on way or the other, look up
some technical texts on lightning, such as the (1960s?) MIT Press book by
Peter Viemeister, and realize that what you think you see when you see
lighting or a spark is not what actually happens.

> and distributer points.

I'll also disagree on this, using the same exact reasoning as above.
I don't care which contact wears out on the points first, as both are
identical, and one is no more important than the other, nor is one
any larger than the other.

> Not reversing the coil wires has been know to cause early
> failure of the distributer's points and or condensor.

I have to also disagree on this point. A condenser in an MG is simply a
non-polarized capacitor, so it won't care one bit which "direction" the
current goes through.  It certainly will not wear out any faster due to
a reversal of current flow.

So I don't buy a single reason given as valid in a "real world" application.

But darn it, the battery will have a "+" and "-" on it, and the coil will
also, so they should be consistent with each other, and with the wiring


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