I just got a classic Ford 8N tractor running again. Someone trying to be helpful flipped the battery around, connecting negative to ground. This made the engine crank backwards! So I had spark, compression, and fuel but it wouldn't start. This got me thinking:
Positive ground was very common, especially on 6V system on work machines. Today almost all vehicles are negative ground.
Is this just a matter of convention, or are there reasons to choose one over the other?
It is partly a matter of convention and standards, and partly because positive earth encourages galvanic corrosion* of the surrounding bodywork - whearas with negative earth is is the much-easier-to-replace electrical contacts that corrode.
*Think back to school physics lessons with anodes and cathodes...
Hooray for Wikipedia:
Originally, the voltages on the wires were positive with respect to earth. This is called negative ground, since the negative side of the battery is grounded 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. This is referred to as cathodic protection.
Clearly, there is a trade-off. You're going to have electrically-motivated corrosion on one set of components or another. As Nick points out, corrosion of electrical components (e.g., wires and connections) is much easier to replace on a vehicle than the bodywork or frame itself. Combine this with the fact that, frankly, we're better at protecting wires than we used to be and it makes a lot of sense to run a negative ground.
Thirty years ago I asked the same question to an automotive engineer. Thirty minutes later his answer was complete. In the beginning there were basically two - Ford and Chevy. Ford was positive ground, Chevy was negative (mostly). When the ASAE (in the 50's I think) met to establish conventions, the big guy won. However, when you consider the electro-motive series of metals, negative ground is incorrect. Japanese and British manufacturers stick with positive ground in cars they manufacture for domestic sales. The advantages of positive ground lie mainly in the sacrificial anode being the vehicle itself. A few grams of lost iron is less devastating than the loss (or corrosion) of the brass or copper connections in the vehicle. I've seen electrical connectors on the doors of 30 year old RR's look just like the day they left the factory. The switch to 12 volts was a matter of economy (and watts required) to run starters and accessories. Small wires are cheaper. Unfortunately corrosion (think electromotive series again) is squared when the voltage doubles. Negative ground vehicles require a lot of grease and isolation from moist air to remain intact. Positive ground will always be better- but the big guy (GM) won. Conventions have been set that will be next to impossible to change. (Think Metric - that will never become standard here or in the UK. ) Think BETA vs VHS. BETA is better. But guess what- we like quantity over quality. In this case, the consumer made the decision. In the case of negative ground, GM bullied that without any convincing arguments. 12 (and 24) volts were wanted by everyone. It's cheaper. No argument there. Remember, Ben Franklin got positive and negative wrong. This was done because we didn't know what electrons were yet. 50 - 50 guess. Just got it wrong. Japan and UK got it right and are keeping it that way. When our cars rot away, we just buy a new one. Let me get off this little box I'm standing on.
I restore old tractors that were originally positive ground 6 volt, i think that is a better system than 12 v neg ground for these reasons. I lived where 40 below 0 was common and these tractors never failed to start unless there was a problem and since it has been mentioned about corrosion the units i see corroded the worst are the ones converted to neg ground and usually 12 v although i doubt that makes any diff. I think people have been sold a bill of goods on changing ground to neg and i would be interested to know who and why it came about, as far as computer industry they would naturally follow the trend for neg ground unless they found it was problematic. One reason i think tractor people changed is that 6 v cranks the engine over a lot slower then 12 v but 6v will crank for 30 minutes or more where 12v is on its face in a few minutes. been fun ron
The positive ground extends the life of spark plugs. The electrons flow from the center to the edge of the plug. Then most of the damaging heat of the spark is put into the spark plug body and not the center electrode which has limited heat dissipation. Battery polarity has absolutely nothing to do with corrosion , unless you are underwater, which is probably going to cause bigger problems for a car than corrosion.
There is also a safety factor.
If you have the positive terminal of your battery connected to the frame of the car, then it is very easy to make new circuits, through your body for instance.
There are a lot more negative ground sources out in the open then positive ones.
