Hi, I have recently brought an Austin Seven Ruby, I have noticed that the 6v battery is connected up negative earth, The wiring diagrams I have looked at all say Positive earth, My Question is can I reverse these to put back to original or will this cause me more problems? Any information you could pass on would be much appreciated . Thank you Trev

Location: West Midlands

Trev, Rubys were negative earth in 1934/5 and positive earth 1936/39.

Ian Mc.

Location: Shropshire

Hi, Thanks for a fast reply, I believe my Austin to be a 1936 ruby mk2 june if I have Interpreted the Pitman book correctly so with that in mind is my battery connected right?. Thanks again

Location: West Midlands

Hello Trevor,
It is a simple matter to polorize the dynamo, so If the ameter shows a charge when the engine is running, and a discharge with the lights switched on then all is well.
J

Location: As far east in Kent as you can get!

Hi John, thanks for the Info, as the Ammeter shows a charge I'm going to Leave it as is. Thanks to all contributors . Trev

Location: West Midlands

That is exactly what I would do, regardless of perceived authenticity of polarity.

IAN mC.

Location: Shropshire

Good evening Ian. Why did Austins change from negative to positive earth? I understand that it was to counteract corrosion but was this so, especially as post war most manufacturers went negative earth?
Also just when did in 1935/6 did Austins change? I have been asked this question many times.
Regards from the creative county
Stuart

Location: Staffordshire

Hi Stuart, I don't know a definitive date but, it is suggested in the "Companion's" Table of modifications to the specification, that September 1935 was the introduction of positive earth.

In period there were definate ideas about cathodic protection based around the understanding that electron flow (from negative to positive) resulted in erosion of the negative side of the circuit. I believe that for this reason that even tramcars were converted to have the rails as positive and the overhead wires as negative polarity; the rails being by far the most expensive part of the circuit to replace. I must say though that my belief in this about tramcars is based on a vaguely remembered snatch of "A" level Physics!!

Ian Mc.

Location: Shropshire

Electrons are attracted to the positive, but they are ethereal almost weightless thingies. Any atom which has lost an electron is attracted to the negative, so actual material migrates there. Ships hulls, pipelines etc are rendered cathodic ie negative to make sure they do not function as an anode.

In the early 60s, when I was working as a radio tech, I was asked at an assessment interview why cars were pos earth. Fortunately I had heard the legend. But I could not resist pointing out that my neg earth RP in the carpark seemed none the worse than many of the (relatively) modern Fords, Hillmans etc of my smug contemporaries.
(And in those days they all leaked oil too)

I have never read or fathomed just which part of the car is supposed to be protected. Of a myriad Jowett Javelins dismembered, pos earth had never prevented a large proportion of the alloy crankcase appearing as sludge in the radiator.

For pos earth cars it is prudent to place a very obvious notice near the battery.
It can avoid just one of the many problems which seem to plague Seven charging circuits.

Bob Culver

Location: Auckland

The Idea around positive earth was not protecting the body, but to do with the wiring. The rubber/cotton insulation used at the the time could absorb moister and could allow electon flow where the wire was in contact with the body etc. By using positive earth it was found that the corrosion to the wire was less than if the car was negative earth.

Hope this makes sense

Location: NZ

The question of "why positive earth ?" has been asked many times over the years, and I have never seen a full and satisfactory reply. In my opinion the only thing which just might be true is determining which parts of the car and wiring are prone to corroding away due to any leakage current flowing in an electrolytic cell, i.e. the situation where two dissimilar metals are separated by an electrolyte such as damp insulation,impure water or acid fallout from the battery. I suspect that Lucas based their recommendations more on percieved theory than any extensive practical investigations.

The question of needing negative spark plug polarity is a red herring, as it is just as easy to manufacture an ignition coil for either system. Even if you are forced to use the "wrong" coil, swapping the LT connections will restore the desirable negative spark. The only downside is that the autotransformer action is then reversed, which will reduce the HT voltage by about 2%, hardly worth worrying about. When ignition systems were feeble the reduction in firing voltage for negative spark (only when the plugs were hot mind you) was helpful, these days it is of less importance.

The theory I most like is that positive earth cars emit red smoke rather than black when there is an electrical fault, which helps pinpoint the location.

Location: New Forest

Thanks Bryan. Makes complete sense. Another of life's great mysteries solved. The Forum has its uses. (But I am still not sure why early models had such absurdly short brake levers!)
If I had known the full answer 50 years ago I might have been promoted and made redundant years earlier!

Did you work it out, or had you read it somewhere? The nearest I got was pondering the battery leads!

Bob Culver

Location: Auckland

My copy of the reprinted Austin Service Journal announces the change over to positve earth in the January 1936 edition page 3. It gives the reason " with a view to reducing the possiblity of corrosion at the positive terminal"
In May 1936 it announces the "positive pole earthed" It says to achieve this "the battery is revered and the starter cable terminals changed over at the battery end". It then gives a list of part numbers for battery cables (and price!)It finally finishes with " The alteration commenced at chassis No. 236,210 (December 1936)
Whether this is the new cables or not, it doesn't make clear.

Location: At the desk again