One for the Sparky’s. Why Negative Earth

[Off da Grid]

Don’t know why this popped into my head.

Why do we use negative instead of positive earth?
I seem to remember way back when fuel was less than 50p a gallon yes that far back, that some vehicles were positive earth.

 

[David and Sally]

Was it because it's better to fuse the +ve? Less corrosion at the earth point? I remember -ve earth motorbikes when mageneto ignition and dynamos were common.

 

[Vipar]

My guess is anything touching the chassis would become a short

 

[David and Sally]

Most vehicles actually have a negative earth- the negative terminal of the battery is connected to the body. This means all the return current can pass through the structure of the vehicle rather than needing discrete wires. It simplifies the wiring as components can be connected to a convenient point on yhe body.24 Aug 2017

This explanation seems a bit contradictory:-

Why did cars change from positive to negative earth?
Originally, the voltages on the wires were positive with respect to earth. ... Then engineers discovered that with positive voltage on the copper wires, copper wires age quickly, due to electrolysis. With negative voltage on the wires, in respect to earth, (called positive ground) the copper is protected from corrosion.

 

[tonyidle]

There are advantages and disadvantages to each. Galvanic corrosion affects electrical components in one case and bodywork in the other (Google if you want a long and complex explanation) but I suspect the real reason is that the car or component maker with most clout won and so, as is the way of these things, a standard was born. In the '50s & '60s many cars were +ve earth but in those days of simple electrics changing from one to t'other was simply a case of swapping battery and ignition coil connections & flashing the dynamo (and switching the radio if it was a type where that was possible). Changing an alternator car was much more difficult - IIRC the Lucas 11AC was available in either flavour.

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[Millcourt]

I have a vague recollection that the preference for negative earthing to chassis came about when spark plugs in petrol engines were found to erode less with the spark travelling from the tip to the earthing body. This might of course just have been an "urban myth" (called "Fake News" today).

 

[tonyidle]

My guess is anything touching the chassis would become a short

Most vehicles actually have a negative earth- the negative terminal of the battery is connected to the body. This means all the return current can pass through the structure of the vehicle rather than needing discrete wires. It simplifies the wiring as components can be connected to a convenient point on yhe body.24 Aug 2017

This explanation seems a bit contradictory:-

Why did cars change from positive to negative earth?
Originally, the voltages on the wires were positive with respect to earth. ... Then engineers discovered that with positive voltage on the copper wires, copper wires age quickly, due to electrolysis. With negative voltage on the wires, in respect to earth, (called positive ground) the copper is protected from corrosion.

The last explanation above is one of the reasons. The chassis would no more be a short circuit than if -ve earth was in use.

Whichever way round the battery is connected current to (or from) the battery can conveniently use the chassis as a conductor.

Electrically the only way a vehicle knows which way round the battery is connected is if you tell it. Operationally in doesn't matter - there is no difference in the way anything works.

 

[tonyidle]

I have a vague recollection that the preference for negative earthing to chassis came about when spark plugs in petrol engines were found to erode less with the spark travelling from the tip to the earthing body. This might of course just have been an "urban myth" (called "Fake News" today).

That is correct. However it is fixed by connecting the coil the right way round depending on earth polarity. Or it was - most ignition systems don't work as they used to in the past - but spark polarity remains important.

 

[Off da Grid]

Ta very much for your explanations.

 

[Buzz67]

What really confuses things is the fact that in reality the electrons flow from negative to positive.
But we only discovered that after all the scientists and engineers of the time had drawn everything the other way round.

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[Charlie9038]

Or is it for safety, fuel pumps are earthed so to avoid a spark when filling up?
just a thought from my aircraft electrician days, when filling an aircraft had to have external earth to avoid static

 

[mitzimad]

i thought it was the introduction of alternators that started the change over

 

[eddie]

My first job at the Longlife Tyre and Battery Company was re polarising Positive Earth cars so people could use Harry Moss radios, which unlike some were only available negative earth

 

[Jev88]

I maintain an old cricket roller machine that uses a 10HP Ford sidevalve engine as used in Ford Prefects and Pops.
Positive earth (6 Volts)
Points and Condensor
No oil pump
No fuel pump
No alternator
No water pump

Try that in a modern car .

Battery terminals gain a coating very quickly and have to be cleaned off or current becomes poor and body contact for positive earth does corrode quickly.

It also does my brain in when fixing it to remember to work backwards electrically speaking.

 

[Rapido925M]

Or is it for safety, fuel pumps are earthed so to avoid a spark when filling up?
just a thought from my aircraft electrician days, when filling an aircraft had to have external earth to avoid static

Earth in this instance is only a relative term: there is no physical connection between the ground and the chassis of the vehicle (unless there is one of those strips to "prevent" car-sickness ). To a limited degree, you might consider the car to be a capacitor but I don't think that would help anyone's understanding.

As stated in other responses, old cars were usually positive earth but this didn't result in immense conflagrations at petrol stations.

Gordon

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[The Wino]

My first job at the Longlife Tyre and Battery Company was re polarising Positive Earth cars so people could use Harry Moss radios, which unlike some were only available negative earth

On my dads car the radio was fitted on a wooden block. The rest of the car was positive earth. It was probably looking back a bit dodgy!
Makes me wonder if we ought to have swapped to the other side of the road like Sweden in the past. It would sure make motorhomes cheaper.

 

[Rapido925M]

On my dads car the radio was fitted on a wooden block. The rest of the car was positive earth. It was probably looking back a bit dodgy!
Makes me wonder if we ought to have swapped to the other side of the road like Sweden in the past. It would sure make motorhomes cheaper.

I remember a TV programme (memory says "Tomorrow's World") about that: all roads closed over a weekend while newly installed signs were uncovered, junctions remarked etc. Brilliant!

Gordon

 

[Off da Grid]

When I visited Sweden on a trip we were told by the guide that the Swedish government held a referendum about changing to driving on the right.
The vote came in a resounding no but they did it anyway.

 

[EdwardFT]

I have a vague recollection that the preference for negative earthing to chassis came about when spark plugs in petrol engines were found to erode less with the spark travelling from the tip to the earthing body. This might of course just have been an "urban myth" (called "Fake News" today).

Actually the current (of electrons) flows from negative to positive.

 

[Rapido925M]

Actually the current (of electrons) flows from negative to positive.

There are some words which are widely accepted all over the world.

CURRENT (in electricity) is a concept of "something" which travels from positive to negative. It was decided upon long before sub-atomic particles were discovered.
ELECTRON FLOW (which is the major way in which energy is transported in metals, be that electricity or heat) is the actual movement of a physical entity (ignoring wave-mechanics) and it happens to be from negative to positive for electricity.

"Current of electrons" is only going to add to confusion .

Gordon

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[icantremember]

I maintain an old cricket roller machine that uses a 10HP Ford sidevalve engine as used in Ford Prefects and Pops.

I bet the windscreen wipers keep slowing down!

 

[PeteH]

i thought it was the introduction of alternators that started the change over

I think it had a part in the change but not the whole.

When I visited Sweden on a trip we were told by the guide that the Swedish government held a referendum about changing to driving on the right.
The vote came in a resounding no but they did it anyway.

I did not know that about the referendum ?. But when it happened, I was sailing on the Gothenburg run that weekend All the R-Ro vehicles where held in the docks for 48hrs. They had built a bridge, to the "new" layout, which had stood unused for over a year, waiting, as to use it would have caused traffic chaos!

 

[ctc]

What really confuses things is the fact that in reality the electrons flow from negative to positive.
But we only discovered that after all the scientists and engineers of the time had drawn everything the other way round.

Quite right, it's only a convention and mostly doesn't matter.

 

[EdwardFT]

Quite right, it's only a convention and mostly doesn't matter.

If you are speaking of a physical effect then it does matter what is really happening. You can use the convention that "current" is said to flow from positive to negative without having any problem with that, and I use that convention like everybody else, but there is not actually any physical entity that flows from positive to negative, and there is actually a physical entity (electrons) flowing from negative to positive. I am not just trying to be argumentative, I made that point in response to the suggestion that spark plugs work better when the "current" flows from tip to base, and that doesn't seem to be likely because nothing is actually flowing from tip to base in terms of the laws of physics. And ye cannae change the laws of physics.

 

[Richard n Ann]

But nothing physical can travel at the speed of light, it would require infinite energy. Hmm, so what are electrons?

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[EdwardFT]

But nothing physical can travel at the speed of light, it would require infinite energy. Hmm, so what are electrons?

It depends on what level you would like the answer
At the level of pre-quantum atomic physics, they are tiny sub-atomic particles. And they don't travel faster than light in a vacuum. (physical particles can travel through a medium faster than light travels through that medium).

 

[Rapido925M]

But nothing physical can travel at the speed of light, it would require infinite energy. Hmm, so what are electrons?

Amazingly, electrons travel at ridiculously low speeds in electrical circuits, perhaps only a millimetre per second. And if you are discussing alternating current (ie mains) then they simply go backwards and forwards, changing direction 100 times per second in Europe (incl UK, even after Brexit!).

However, the effective wave which they produce by "shunting" each other (very vague statement but hopefully understandable) does travel at the speed of "light" (an electro-magnetic wave) for the medium through which that wave is propagating. [For a vacuum light travels at approx 300 000 000m/s whereas for "plastic" it is about 200 000 000m/s.]

That is why your light comes on at "the instant" that you press the switch, even though millions of electrons queuing up "behind" the switch may never get to your lamp .

Gordon

 

[Gromett]

What really confuses things is the fact that in reality the electrons flow from negative to positive.
But we only discovered that after all the scientists and engineers of the time had drawn everything the other way round.

It was Benjamin Franklin I believe who set the positive/negative polarities. This was before the discovery of electrons. He had a 50/50 chance of getting the direction right and unfortunately got it wrong.

When learning electronics you are taught using the classical view of electricity flowing from positive to negative. It isn't until you study theory that you are taught that electrons actually flow from the negative to the positive. It caused a little confusion when learning semi-conductor theory and learning about valency electrons and positive charge carriers (mobile holes). Yup it was as weird as that sounded

In the real world it makes no practical difference though.

 

[EdwardFT]

Amazingly, electrons travel at ridiculously low speeds in electrical circuits, perhaps only a millimetre per second. And if you are discussing alternating current (ie mains) then they simply go backwards and forwards, changing direction 100 times per second in Europe (incl UK, even after Brexit!).

However, the effective wave which they produce by "shunting" each other (very vague statement but hopefully understandable) does travel at the speed of "light" (an electro-magnetic wave) for the medium through which that wave is propagating. [For a vacuum light travels at approx 300 000 000m/s whereas for "plastic" it is about 200 000 000m/s.]

That is why your light comes on at "the instant" that you press the switch, even though millions of electrons queuing up "behind" the switch may never get to your lamp .

Gordon

Very well put.

 

[Clarky]

I bet the windscreen wipers keep slowing down!

Now you’re showing your age.
Just like me who understands what you mean............they were a right bugger when going up long hills.
Of course, a lot of youngsters on here won’t have a clue what we’re on about.

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