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A dozen years ago or so a friend of mine was restoring an old Studebaker pickup, and in the process of aksing about converting it from 12 negative ground back to the original 6 volt positive ground, he accidentally started quite a debate on one of the early e-mail lists about this very subject. Eventually one Simon Favre claimed that the idea of positive-to-negative electrical current flow orginated with Benjamin Franklin. He did not give any citations. It ought not to be too difficult to find it in Franklin's experimental writings. This is confirmed by the following, which I found on a NASA web page: "Franklin knew of two types of electric charge, depending on the material one rubbed. He thought that one kind signified a little excess of the "electric fluid" over the usual amount, and he called that "positive" electricity (marked by +), while the other kind was "negative" (marked -), signifying a slight deficiency. It is not known whether he tossed a coin before deciding to call the kind produced by rubbing glass "positive" and the other "resinous" type "negative" (rather than the other way around), but he might just as well have.
Later, when electric batteries were discovered, scientists naturally assigned the direction of the flow of current to be from (+) to (-). A century after that electrons were discovered" (in 1897 by J.J. Thompson at Cambridge - A.) "and it was suddenly realized that in metal wires the electrons were the ones that carried the current, moving in exactly the opposite direction. Also, it was an excess of electrons which produced a negative electric charge. However, it was much too late to change Franklin's naming convention."
(End of citation from NASA web page.) I don't know the reasoning that Charles Kettering used when decided to employ postive ground in the system he developed for the 1912 Cadillac (which became the pattern for almost all manufacturers for the next sixty years); but he seems to have been under the influence of prevailing engineering practices of 1910-11 (when the system was actually developed and finalized for mass production). I am sure most automotive historians would agree that the predominance throughout the industry of electrical systems either made under license from Kettering, or simply bought wholesale from Kettering's Dayton Engineering Laboratories Company (Delco) was the original reason for the standardization of postive-ground systems. In my friend's e-mail discussion, Mr. Favre also claimed that the switch to negative ground was prompted by the fact that this had long been the standard in radio engineering practice (true to my personal knowledge) and that as silicon devices and other electronic improvements were invented in the field of radio, the collective opinion in the automotive industry was that it was better to change to negative ground so as to more easily accomodate these devices and techniques. Mr. Favre's explanation for the switch to negative ground sounds more likely to me than the idea that the switch was undertaken specifically to reduce corrosion, but then, I also have a background in radio electronics and that may degrade my objectivity...
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