On the pos/neg polarity of sparks, if you've got a single, negative sparks are preferable - for being stronger - because electrons like going from the centre electrode to the earth one if they can. But not an option we get with a twin mag, as one spark is always going to be positive. Folk say you should swap the plugs round now and again to compensate, but have to say I never do. The plugs in my A (NGKs as it happens) have been in there for 30K+ miles now, and you'd have to be a lot smarter than me to know which had run pos and which neg. They both look good as new.
Just a throwaway line, but I reckon a lot of plug trouble is actually symptomatic of other issues with the mag (or the carb). (But I agree some plugs do prematurely fail for no obvious reason.)
On the pos/neg earth Q for the electrical system generally etc, I really dunno. 'They' said years ago, and the 'continentals' followed it, that negative earth avoided corrosion of bodywork on cars, and didn't I read once that Rolls Royce were always Neg earth? But frankly, a lot of cars just rusted away whatever you did after they'd abandoned chassis. None of my bikes ever has (rusted away), despite terrible neglect, and I run pos earth, except on a couple of things with electronic ignition conceived for neg earth. I don't really think it makes a lot of odds.
But I'm sorry your mag's died again Morris - that's depressing. They shouldn't die like that, they really shouldn't!
