Mr. Steven Tritle, Joe Porter, TATerry (and other gentlemen of like persuasion),
Since no one has replied to Mr.Tritle's query of March 12th concerning the
determination of spark coil polarity and Mr. Porter and Mr. Sanders were
concerned with a different aspect of the same question on June 25/26, allow
me to submit this belated bag of technical prattle on the subject of coil
polarity.
The symbols Lucas molds into the caps on its coils are there to insure that
the primary current is in the correct direction. The words 'correct
direction' refer to that direction of primary current whose interruption
will generate a negative pulse at the H.T. terminal on the spark coil.
This interruption occurs when the distributor points open. A negative H.T.
pulse is a little more desirable than a positive H.T. pulse. Your MG will
run equally well with either pulse polarity. Mine has run on the less
desirable positive pulse for decades,
When there are no symbols molded into the cap on the coil, a problem arises
in determining the direction of primary current required to generate the
negative H.T.pulse. Perform the following Physics 101 experiment and all
will end well.
Connect the NEGATIVE terminal of a D'Arsonval movement voltmeter to the
H.T. terminal on the spark coil. Connect the POSITIVE terminal of the
voltmeter to either of the two primary connections on the spark coil. It
will not matter which primary connection the voltmeter connects to. Next,
obtain a 1.5 volt flashlight battery (a AA battery is quite acceptable) and
attach a wire to each terminal on the AA battery. Connect the two battery
leads to the two primary connection posts on the spark coil. A second
later, disconnect either (but not both) of the battery connections AND
observe which way the needle on the voltmeter flicks. When the battery is
connected in one direction, the needle will flick upscale - when the
battery is connected in the other direction, the needle will flick
downscale. When the needle flicks upscale, the H.T. pulse has been
demonstrated to be the more desirable negative pulse. When this upscale
flick is observed, inspect the battery connections and determine which
primary post on the spark coil was connected to the negative side of the AA
test battery. The experiment has now been completed. The negative
terminal on the coil will be that terminal that was connected to the
negative side of the battery when the upscale flick was observed.
- - - For the Electrical Enthusiast, detailed considerations are provided
below.- - -
(I) A D'Arsonval meter movement is the old electromechanical meter
movement that consists of a small coil suspended in the field of a
permanent magnet. Forget the new wonderful digital meters (unless they
posses a peak picker) as they have sampling problems with pulse
measurements. D'Arsonval movements are universally found in older
multimeters. Whether old or new, set the multimeter on its, say, 0-10 volt
D.C. scale. Don't worry about a 30,000 volt output from the spark coil and
an anticipated destruction of your multimeter when set on its 0-10 volt
scale. Destruction will not occur as 30,000 volts cannot be generated
under the conditions of this experiment. (Your multimeter is being employed
as something akin to a ballistic galvanometer.)
(II) The center electrode of a spark plug typically runs hotter than the
ground electrode. Since it is easier for the electric field (between the
two electrodes of the spark plug) to strip electrons off of a hot electrode
than off of a cold electrode, the center electrode is chosen as the
negative electrode because it is the hotter of the two. Additionally, the
edge of the center electrode also provides a sharp edge (90°) that
facilitates the stripping of electrons. Again, the result of this polarity
choice is that the spark discharge across the electrodes can be initiated
at a slightly lower potential when the center electrode is made the
negative electrode. Why should anyone care whether the plugs will fire at
a slightly lower potential? There are at least two weak reasons: The
lower operating potential results in lower electrostatic fields in the
ignition components. This is probably a good thing when one is concerned
with bottom dollar components. The lower firing potential also provides
the coil with a slightly greater reserve voltage.
(III) The reason the multimeter didn't burn out when connected across the
30,000 volt secondary is that the spark coil secondary never remotely
reached 30,000 volts. The loss of this 30,000 volts occurred under the
conditions of this experiment because of the sum of two phenomena: (A) The
primary voltage was reduced from 12.6 volts to 1.5 volts. As a result, the
primary current and thus its magnetic field were also reduced in the same
proportion. (B) In addition, the voltmeter was always connected across the
secondary winding of the spark coil and accordingly drew current when any
voltage, however small, was induced in the secondary. This current served
to load down the secondary and thus very severely attenuate the voltage
inducted in the secondary.
(IV) After the spark discharge (plasma) has been initiated, the potential
across the spark plug drops down to several thousand volts (an instant
later it even becomes oscillatory). The initial negative polarity is only
useful in initiating the arc between the spark plug electrodes. As soon as
the arc is established and the plasma between the electrodes is in the
process of igniting the air-fuel mixture, the spark plug couldn't care less
what its initial polarity was. So much for the Kettering ignition system.
Since it has so few components, it appears deceptively simple. It is not.
Its mathematical analysis is anything but simple.
(V) Be aware that when the needle of the voltmeter is driven downscale
rapidly that it will hit its stop and then mechanically bounce upscale.
Although this superficially may appear as an upscale flick, it is not a
legitimate upscale flick. It is a mechanical bounce. This should not be
confusing as the needle first goes downscale, then bounces upscale.
Additionally, disregard the weaker and oppositely directed flick and bounce
of the needle that occurs when the battery is initially connected to the
primary of the spark coil. Also disregard the steady reading that will
occur when the positive lead of the voltmeter is connected to one of the
primary posts (but not to the other). This steady reading is eliminated by
switching the positive lead from the voltmeter to the other primary post on
the spark coil. (the voltmeter had been connected so that it also read the
battery potential. This was a steady reading.)
You are to looking for the single strong UPSCALE FLICK of the needle that
occurs when the primary circuit is interrupted - NOTHING ELSE. Don't be
bamboozled by the pointer bouncing off of its stop or the steady reading
resulting from the voltmeter accidentally being connected so as to also
read the primary potential of 1.5 volts.
(VI) A more expensive meter movement will display less bounce as it
will be closer to being critically damped. The movements incorporated in
cheapie multimeters can be expected to bounce more as they are generally
poorly damped.
(VII) Terry, I also vaguely recall some kind of a pencil test. It would
assumedly be less of a set-up than the preceding Physics experiment. I
tried several 'logical' pencil experiments and obtained negative results.
I was looking for a yellow end on the arc that I would have associated with
ions of carbon. No such luck. Perhaps pencil 'leads' are no longer
suitable as they are no longer made from carbon and clay but from a plastic
binder and whatever. Can someone provide words of wisdom?
This discussion is too long, so I will stop here - enough is enough.
Best wishes to all, Carl Cederstrand / Orange, California
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