Barrie Robinson wrote:
> Naturally no spares were at hand. My cousin told me he took out the tail
> light and then connected the wire at the bulkhead end to the condenser
> connections....and away they went! He maintains that the capacitance of
> the length of wire used to connect to the rear light was sufficient.
> Hmmm, always wanted to check this one out on an old Army lorry.
There is some truth in this, any double connector wire will
have capacitance, the question is, is it enough to be
meaningful here? Often wire is rated this in in picofarads
per foot.
For very small capacitors on RF circuits, sometimes you
can just use two one-inch sections of thin-insulated wire
spun together extending up off the circuit board. Easy, but
only a tiny capacitor useful at radio frequencies on low
current circuits.
As well, in the design of memory chips and other microscopic
circuits, capacitors are made on the surface just by using
careful geometry. Extremely small capacitors, but again
low current and high frequency.
Back to the car....
Notably, the car can run without a capacitor at all. A
duff capacitor can short internally which will stop you
cold, but if you remove the capacitor the car will still
run. The points won't last very long, but you can get home.
Tying the two above points together, I'd be skeptical
that the wire really did have enough capacitance to
really replace the condensor.... Without calculating, I thing
this would require the wire to have parallel strands that
ran VERY close together for a long distance.
Without going into the details, a capacitor works when
two surfaces are close together but not touching. A
charge appearing one one conductor repels a charge on
the other conductor. So if electrons flood down one
wire, it has a negative charge, which repels electrons
out the other wire, so the current "flows" through the
capacitor even though they don't touch.
However, once the wire is charged the flow stops. When
the voltage is removed, the charge flows "out" and the
capacitor can be said to have stored the charge.
What's very important here is that the distance between
the conductors has to be very small for the charge to
have any effect. A standard capacitor is made by using two
sheets of metal foil with a thin insulating film between,
then rolled up into a can-shape. The surface area is
large and the insulating film is VERY thin, so you can
get a fair bit of capacitance in a small volume.
Even 10 feet of wire doesn't have a lot of surface
area, and the spacing between the wire is likely
at least 1/8th inch which is quite a lot.
So...
It's possible the capacitance of the wire wasn't a lot
and perhaps not enough to really help, and the car
would have ran just the same without it once the shorted
condensor was removed.
My 2c.
--
Trevor Boicey, P. Eng.
Ottawa, Canada, tboicey@brit.ca
ICQ #17432933 http://www.brit.ca/~tboicey/
"Shamu go boom boom!" - Murk
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