In a message dated 97-07-25 11:06:48 EDT, bnicho19@bellsouth.net (bill
nichol) writes:
> Trevor Boicey wrote:
> >
> > bill nichol wrote:
> > > The cure was so simple i almost hate to admit it. I went through my
> > > lights one by one checking restance to ground abd found each of my
tail
> > > lts pulling 1 to 4 Megohms (not a miss print)
> >
> > Just a note that high resistances such as this draw LOW currents.
> >
> > So if the resistance of the circuit is really in the megohms, the
> > draw would be almost nothing, the lights would never light, and you'd
> > never get any hot wiring. Essentially, nothing would happen except
> > your battery might go dead after 4 billion years of powering the
> > circuit.
> >
> > > instead of the more normal 4 or less ohms.
> >
> > Which will actually draw a lot of current.
> >
> > Of note though, filaments have very low resistances when cold, and
> > the resistance increases when hot. This is why a light bulb seems
> > like a dead short (ie: fuse) but doesn't draw infinite current.
> >
> > You can use resistance to test whether filaments are intact, but
> > you can't really speculate on the current draw of them when
> > running.
> >
> > --
> > Trevor Boicey
> > Ottawa, Canada
> > tboicey@brit.ca
> > http://www.brit.ca/~tboicey/
> Trevor,
> You went to a whole different school of electrical theory than the rest
> of the world. The high restance was not through the bulb(duh!) The high
> restance was because the fantastically poor ground points and
> terminations used by Lucas, allowing incredible amounts of corrosion to
> accumulate. Even promoting in some instances through dissimiliar metal
> corrosion. Corrosion causes restiance. resistance causes heat. If
> voltage is constant then current must increase to carry the increased
> load.
> If you still feel compelled to disagree, fine. But, please don't try to
> discount the FACT that removing the corrosion ended my fuse problems.
>
> All hail the prince of Darkness!
> Bill
> 78Spit
> 80Wedge
>
Bill, now, now! Trevor is right. In the total circuit -- I=E/R (I is
current, E= elctromotive force or voltage, and R=resistance) - so if E is
constant and R increases then I goes down. It can't go up ( an example
would be an open circuit - where the resistance is infinite and no current
flows). High resisrance means that the filiaments of the bulbs would not have
much current flowing through them and would not heat up. Which means they
would not "light" up. As for the blowing of fuses - increased resistance
would not cause that. Fuses blow when current increases beyond the rating of
the fuse. That's why fuses are rated in amps (the measurement for current).
Your statement " If voltage is constant then current must increase to carry
the increased load" is not true. Look at the formulae E=IR, I=E/R, R=E/I.
Anyway you slice it, if E is constant and R increases, I MUST decrease.
SO since constant voltage with increased resistance causes current to
decrease, the blown fuses were caused by something other than the increased
resistance (maybe a "short" in the wires or to ground somewhere else in the
circuit).
To be nice about this, and my mother always said to be nice, I think the
confusion may be partly a result of Trevor's throwing in a discussion of the
filiament resistance. The filiament resistance increase to which he referred
is after heating (which would not take place if the current in the circuit
were too low, which is what very high resistance would cause - as in your
case). Hence you would get a dim bulb but not a blown fuse.
I didn't read him as saying that the filiament resistance had any bearing in
your case. He appears to have thrown that in as an aside. But his comment
(in a later e-mail) that the increased resistance was not causing blown fuses
is correct. Something else cured your blown fuse problem. The fact that you
cleaned up the connections had nothing to do with that problem.
Please check out exactly what you both said and I think you'll find some
agreement.
BTW the "duh" comment might reqiure a little eating of crow on your part. :-)
Cheers.
Art Kelly
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