JONO, thanks for that clear and concise post.
The battery holder was originally designed for the spring to go to the negative (flat) end - that's why it gets the black wire. Fortunately, most alkaline D cells today have all metal at the positive end, so you can flip them and still get a reliable connection. That battery holder design goes back to the days of carbon-zinc cells, which had a central carbon rod with a metal cap for the positive connection with some kind of goop (tar perhaps?) surrounding it. The entire outer casing was the negative terminal, pure zinc. It would rot through as the cell died, and leak the corrosive insides all over your electronics if you did not monitor them. Even nastier than the electrolytic caps of those days, though not quite so explosive. Components have come a long way since I was a kid. :^)