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MG TD TF 1500 - History of polarity

Do any of you MG historians know when the positive terminal of the battery was adopted as the standard for the T-Series (or earlier) cars, and if it was ever changed on pre-war models for any reason?

Was it with the arrival of the MGB that a negative earth was introduced for the remainder of the MG production run?
Geoff Love

I don't know about earlier cars, but the MGB was positive earth until about 1968. I believe that the first negative earth MGB was the first MGB with an alternator, but I could be wrong about that. Of course, quite a few positive earth cars have been 'converted' so that they can use modern radios and accessories.

My conjecture is that the advent of semiconductors made it desirable to standardize polarity across markets, and since more cars were sold in markets that were already using negative ground, MG switched to negative ground.
Mark B.

+ve ground supposedly improved the life of the points and plugs.

Guess metallurgy has improved a lot since then.
Will

Goeff,
I finally located this for you. I remembered that + groung (earth) reduced corrosion and was used on most prewar car everywhere ( It is still used on big trucks)

Tips and Comments from:
The Octagon Car Club Bulletin, No. 391, December 2002
Positive/negative earth
In Septembers Technical Torque there was a comment concerning positive and negative earth (No. 388 p.40).

When I was a sixth former at school in the sixties, and being interested in cars, I asked my Engineering teacher why cars had changed from positive to negative earth. He didnt know, and suggested I write to Lucas Ltd. I still have the reply (dated 4 May 1966) from Christopher Wilson of their Technical Information Department. I quote in full:

Electrical equipment will only function while there is current flowing in it. Because of this it is essential to provide a return path for the current that has Passed through the component. Originally the majority of vehicle manufacturers utilized either a special wire, or the chassis for a return path. To distinguish between the two wiring systems, the type that used special return wire was designated insulated return or IR, and the chassis return as earth return or ER.

Earth return would be dangerous where high voltages are used, but earth return is quite safe for low voltages between 6 and 12 volt, but any leakage between wiring and chassis causes corrosion at the leaky point.

Pre-war insulating material was usually basically either wood, rubber, ebonite, glass and later Bakelite. All but glass had tendencies to minute leakage which over a period of time encouraged corrosion of the terminals. Battery connections were most prone to corrosion because of the added effect of the acid. Earth return is only effective if the battery has one terminal connected to the chassis, and for certain chemical reasons less corrosion occurs if the positive terminal is earthed.

It was because of the advantages of earth return that by about 1935 nearly all manufacturers were using this system and to reduce corrosion the positive battery terminal was earthed. An additional advantage of positive earth was that a sparking plug sparks over better with the earth electrode positive.

Since the war, plastic insulation has been used extensively, and because of this the leakage current no longer prevails. As there is no leakage to bother about, it does not matter which battery terminal is earthed from a corrosion point of view. Sparking plugs still operate more effectively with a positive earth electrode but now that ignition coils can be made which reverse the earthing potential of the power flowing through them, it is possible to use a negative earth battery and still obtain positive earth high tension. In theory the last few facts are a little more complicated than stated but in practice the effects are still the same.

Post-war negative earthing has many factors in its favour, and the most predominant are given below:-Television interference is easier to suppress with negative earth, and of course this was no bother before the war. All Europe and America have reverted to negative earth, and it would help our Agents abroad if our vehicles used there were of the same earth as those of the country concerned. Electronic equipment is now being used as standard in many applications and now covers not only radio (probably the first electronic device used in an English car), but dipping mirrors, fluorescent lights, dynamo control boxes, ignition systems, automatic lighting control, shaver power supplies etc.

It is characteristic of all electronic apparatus, as opposed to electric, to be very polarity conscious, that is affected by which supply terminal is earthed. If a transistor is connected the wrong way round it will be immediately irreparably damaged. Universal negative earthing would ensure that all power supplies to electronic gear were of the same earth and therefore reversed connections impossible. Electrical components like bulbs, etc., are not affected by which terminal is earthed, therefore these can be made unchanged and universal in their application.

To help avoid any confusion which may arise in the difficult in-between changeover time, when vehicles of both types of earthing are produced, components are being marked + and . To consider an ignition coil, for instance with positive earth, + is earthy, so this is for distributor contacts and is live, so this is for the ignition feed. The same coil when used with negative earthing would be earthy at and so this is for the contact breaker, and live at +, making this the ignition feed.

When connecting or changing a battery, always ensure it is correctly earthed for the particular vehicle. Take care with battery chargers for they must always be connected the right way round.
So, to summarize:
Positive earth gives a better spark and less corrosion when insulators leak current
With negative earth TV interference is easier to suppress and electronic components are less likely to be damaged.

Harvey Cole, Marsh, Huddersfield.

Don Harmer

Many thanks indeed, Don for that extremely informative response to my question. As a member of the Octagon Car Club and an avid reader of "The Bulletin", ( a most valuable resource of practical information and, in my opinion, required reading for any MG T-Series owner), I obviously did not require myself to read the relevant issue thoroughly otherwise I might have been better informed!

Very grateful for the information and I am sure other readers of this BBS will think so too.

If any other member (or non-member like me) can contribute further to this, I shall be interested to hear about vehicles made much earlier. For instance, does anyone know what Number 1's polarity is?
Geoff Love

Don - That was a neat find! Thanks for sharing it with us. FWIIW, I can add one piece of information for those not versed in electronics or physics. The statement "that a sparking plug sparks over better with the earth electrode positive" is the result of the center electrode is acting like a cathode in a vacuum tube. When the cathode is heated (hot engine in a car, heated by filiments in a vacuum tube), electrons are boiled off forming a negatively charged area around it. If the other electrode (earth electrode on the spark plug) is positive in relation to the catohde (center electrode of the spark plug), current will flow between the two more easily, going from negative to positive. You are all now half way towards an EE degree in the university of MG.
Cheers - Dave
David DuBois

The positive ground reduces corrosion at ground locations ( connections to iron chassis from copper wires) because the copper-iron junction is subject to electrolsis. When damp or wet it acts as a battery (with Cu + and Fe -). By making the iron positive and the copper negative it reverses this action, which would liberate iron ions, Fe++ & Fe+++ at the iron electrode, which plus water = iron oxide or rust.

Dave is correct on the action in spark plugs, negative center electrode is better.
Don Harmer

And yet, like in the battle between Betamax and VHS... the technically-inferior standard prevailed. :(
Will

This thread was discussed between 26/06/2003 and 04/07/2003

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