battery polarity

The specific tractor is an Oliver 550. The operators manual states that it is to be positive ground. Mine is negative ground and it works fine. It has a generator (not alternator) and the only difference I see from stock is that it has a different voltage regulator.

On a system such as this, what determines the polarity and theoretically would it matter if it is negative or postive ground?

As to the flow of electrons is concerned, they do go from negative to positive. The hole theory as someone mentioned has to do with semiconductor material IE transistors. The newer theory being taught now is that ENERGY flows from positive to negative. That is how most newer engineers will look at the situation. Nothing has changed as electrons still move from positive to negative. It is all to do with how one views the problem. It is often easier to view the energy flow from positive to negative in a circuit.

By the way electrons don’t move through material at a very high speed, they are actually pretty slow moving. The energy itself does move near the speed of light. Most people think the electrons move at the speed of light but they do not. They bump into other electrons in the material and cause those to move and so on. You may remember the demonstration of six or eight solid balls hanging from strings and each ball touching one another. Striking the first ball does not cause all to move but only the last one in line. That is similar to what happens with electricity in a wire.

It does make some difference in the ignition system as to how polarity is set up. The spark from the coil should be negative in polarity as it will arc slightly easier if it is. So the coil connection should be observed at its primary.

As far as points arcing and building up deposits, that is a function of the condenser across them. Too much capacitance and the metal (buildup) will move to one side. Too little capacitance and the metal will move to the other side.

As far as any galvanic corrosion and polarity is concerned, that would depend on the type of metal on each side of a joint.

Regards
Gary

As far as points arcing and building up deposits, that is a function of the condenser across them. Too much capacitance and the metal (buildup) will move to one side. Too little capacitance and the metal will move to the other side.

I would say - too little capacitance and sparking will be excessive and burn points rapidly (metal transfer). Too much capacitance and the system will not work properly due to the primary coil magnetic flux collapsing too slowly, thus reducing the magnitude of the induced voltage in the secodary HT winding. With reduced sparking (eventually to zero?) of course. So size of capacitor is a compromise, really.
Regards, RAB
Regards, RAB

Tooooo much capacitance, a weaker spark, and if its too much, no spark at all. It has to do with that di/dt business and the capacitive and inductive time constant stuff. An oscillograph trace would show the inductive capacitive ringing that occurs right after the points break open which eventually dampens out.

Correct amount of capacitance, maximum spark PLUS fairly good point life, the compromises and trade offs RAB spoke of.

Tooooo little or NO capacitance, rapid point burn up PLUS a weaker spark.

As far as the deposit build up, I could swear the artice I read showed more build up at one polarity then the other, but I fail to understand that and would think it would only matter as to which contact it happened on. Electrons have a mass and when current arc jumps across the points gap, it looks like build ups would occur on the side its jumping to leaving pits/voids on the other side???? Darned if I know I gotta dust off my olddddd dd textbooks if this keeps up lol

John T, a retired sparky

Aint lectricity funnnnn nnnnn n

Yes it is interesting! In the operation of making a spark the ringing that occurs is just a by product and not a necessary ingredient for the spark plug to fire. Only a voltage transition in one direction at a rapid rate causes the plug to fire.

When the points open an arc occurs between the contacts as they open. An arc once started presents a low resistance across the points as long as the arc is there and sustains the arc. Actually a small plasma trail is formed in the arc which has a lower resistance than air. When switching, DC arcs are hard to put out without changing the polarity of the voltage or widening the gap of the switch (points).

Switching AC is much easier as it is constantly changing polarity and the change in polarity, voltage swinging down through zero volts, extinguishes the arc. That is the reason that you will see different ratings on a switch that may have both AC and DC current ratings. The AC rating will be much higher than the DC rating.

With a DC arc metal gets transferred from one side to the other of the contacts when the contacts open. I believe what happens to transfer metal back to the other side of the points is when the charged capacitor gets shorted by the points closing and releasing its energy which causes the points to arc again. This arc has a polarity opposite of when the points open so metal transfers to the opposite contact.

If the capacitor is of the proper size the same amount of material will transfer in one direction with points opening as the amount of material transferring in the opposite direction when the points close. The net result is no pitting or buildup of material on either side.

But since nothing is perfect there is always erosion of the point surfaces. During the arc some molten material does not transfer but gets boiled or thrown of the contact surface and lost.

If one looks at the points after some time in use usually one side or the other will have a spike (built up material) and the other side will have a matching pit from which the spike material came from. If new points and capacitor are installed and inspected some time again later it may be found that the pit may be on the opposite side of the points that it was on the last time. The capacitor may have gone from too small to too large.

With contacts switching an AC voltage the contacts last much longer as the random switching of the AC voltage results in sometimes one contact being positive and at other times the other contact being positive so the metal transfer kind of equals out and you usually only see small pits on each contact from lost metal that has gotten thrown clear during the arc times. Otherwise the metal is transferred back and forth between contacts depending on the polarity of the voltage at switch time.

There of course methods to limit the amount of arc in AC switching too, which is done on higher power circuits.
There other methods to limit arcs in DC switches also in addition to just the capacitor.

Regards
Gary

Electrons are negative charged particles that have mass and I understand sort of?? how they move n why its not the speed of light, thats over the heads of most this old tractor work fer sure lol

Take care, John T (long retired EE)

Before there was an established standard, Ford and I believe Chrysler and many tractor vendors used Positive ground. The reasons if I recall from articles were there was less galvanic corrosion between joined metallic frame members when the battery Poisitive was tied to frame ground,,,,, ,,,,,the points pitted less and didnt build up such high deposits (I know I read an article on that, wish I had saved it) ,,,,, ,,and theres the thing about its easier to emit electrons from a relatively hotter electrode to a relatively cooler ground strap on the spark plugs, not sure what polarity that would favor, however, and coil configuration also affects that.

Later the industry standard became Negative grounding and solid state devices are polarity sensitive so now almost all is at Negative ground.

NO WARRANTY on any of this I may have to research the question someday, no time now.

John T (Long retired and rusty electrical engineer)

As a tech and engineer I found that as a tech, troubleshooting was easier chasing the electron.

As a designer, I found that it was easier sourcing holes from the + terminal of the power supply and delivering them where needed.

So I use both; just tell me which one you want to talk about when conversing. Grin.

On engines, there is reasoning at the ignition points and spark plug terminals for which way the molecules are a running. Don't remember which way was the least damaging to the parts.....i.e. + or - grounded electrical system.

Mark

John T

I read this somewhere and it might be true , the material used on the contact points is not the same material on both contacts. In other words if positive Gr. the point material on one side will be the opposite if used as neg. gr. No guarantees.

John T