Two-prong polarised plugs have never quite made sense to me, and as I sit here and rewire these lights, it’s finally occurred to me where my error lay. Polarity isn’t quite the same as current - I’ve been thinking of AC as a river that changes direction sixty times a second, but that’s not it, is it? The current is always flowing from the same hot wire to the same ground, but it switches polarity at 60 hz. Is that correct?
No, AC is like a river changing direction at the frequency. Current is the movement of electrons, which always move from minus to plus if it is DC, although the current is said to move from plus to minus. But this could be pictured as the holes left when the electrons move.
Anyway, in alternating current the current changes direction, so it is not like a river 
, the hot (live) pole being just one pole and the ground being the other, but neither being plus or minus all the time. The hot pole is hot (live) because it is not grounded, whereas the other (called the neutral pole in the UK) is grounded, connected to the ground (earth) physically. That’s why you don’t get any potential (voltage) between yourself and the neutral pole, because you are grounded, standing on the earth, or a floor connected to it. And why you always get a potential between the live pole and yourself, because it has a potential to earth (the ground), therefore called hot or live. And this potential changes from plus 240V to minus 240V and back at the mains power frequency.
‘Live’ is a misnomer though, as touching it may kill you 
Perhaps a better way to explain what I mean is that the only thing that changes is the vector. Viewed as a scalar value, it is always the hot wire which contains all the potential energy; the ground is necessary but easily replaceable.
Yes. But the ground is not necessary. It is just convenient in some wiring schemes to use the ground/earth as the second wire. It is also convenient as a safety measure to have one pole neutral. But feeding directly from a generator one can have two live poles, and no neutral. Or four live poles from a three phase generator, which is the norm.
North American household current is 110v 60 hz AC. Historically, plugs were like this:
Which work either way you stick it into the socket. In the sixties or early 70s we got three-prong plugs like these
The round prong is a dedicated ground wire. This is still in use for higher power and utility devices.
In the 80s we started getting these:
..where the prongs differ because one is grounded. They’re not as safe as a three-prong plug, but they’re safer than option one because appliances that have a polarized plug are designed so that any short is supposed to go straight to ground via the neutral wire. It made no sense with my reversing river model, because with it each wire is equal. Like a lot of people, I did my share of cutting off the wides prongs to adapt new equipment to older plugs and extension cords. My new model says that sometimes the hot wire pushes and sometimes it sucks, but it is always the one in charge.
The bottom line to all of this is not to mess around with electrical wiring unless you understand what you are doing. I had a electrician install several new 120 volt wall sockets at several locations in my basement for new light and shop tools, even though I could probably have done it myself. He installed every one of the 3-prong receptacles with the ground plug on top, which I consider non-standard. I think that I am right (see diagram at the link in the above posting), but I decided not to argue with him when he told me that it didn’t make any difference. Sure. In the USA it doesn’t make any difference which side of the road that you drive on, but if you want to live very long, you had better learn that the ride side of the road is your best choice. For you, Hans, I wouldn’t pay any attention to that advice.
makes no difference?
current be upside/down!
take 'em out & turn 'em over 
worse’n havin’ the current run backwards, innit 
It’s like when you accidentally set up your stereo speakers upside-down, and all the music comes out upside-down. I hate when that happens.
Of course, I understand that it doesn’t matter whether the ground plug is on the top or the bottom of the 3-prong configuration, as long as all of the wires are connected correctly. I’m not too worried about the electricity coming out upside down. To me, it is merely a matter of convention. Similarly, a wall light switch is on when switched upward and off when switched downward. You can easily wire it the other way, but that is not the standard way of doing it. For faucets, the hot water goes on the left and the cold water on the right.
Denny, double wall electrical receptacles once wired are not easy to turn upside down unless you want to disconnect all of the wires and reconnect them. I left them just the way they were installed.
I’d have a go at turnin’ 'e over. It isn’t that big a deal (yeah, I’ve got a headlamp) and I do like them being all the same.
Oh yeah, I do prefer the ground being down. Odd habit I suppose
then he cut the wires too short also
an’ don’t get the little neurotic started on light switches, we’ve several 3-way…
Mine are sideways!
I think that it is left over from spending more time on the floor when younger and having fewer house pests.
ah, pets…wunnit 
The UK standard fused plugs have the earth pin topmost, cable coming out at the bottom. Makes sense when you try to pull them off the socket unintentionally.
The earth pin is also the strongest and longest, which will disengage last.
we drive on the wrong side too
better than in the middle
that would be the motor homes, on the lane and a half roads, in the mountains
I miss having a motorcycle 
It’s convention otherwise, but many of the wiring conventions are stupid. The advantage of third prong up installation is that this way anything falling down the wall onto a loose plug can’t hit a hot wire.
I saw a lecture from a former contractor who’d become a guru of senior-friendly housing standards, in which he explain that we all expect sockets to be where they (in NA, about 14 inches or so from the floor) because back when the nation was being electrified, everyone did as they thought best, and that offended the sense of uniformity of someone at the carpenters’ or electricians’ union. The union opted to tell apprentices to stick sockets at the height of their hammers from the floor, so henceforth they’d all be uniform. And 80 years later we all still do it, despite this being a position that a great many seniors and the disabled can’t reach.
You can still think of it as a river. The hot wire is the source, and the cold wire is the drain. Just like a pipe. It doesn’t really matter what the electrons are doing. AC is just a different kind of “juice”. 
We drive in the middle, head down, fast. You can take the boy out of Texas, but you can’t take the Texas out of the boy. 
-=chuckt=-
Edit: I don’t remember if the hot is the black wire or the white wire. Green is ground.
Hot’s black, but only if you can be confident that everyone who’s had a go at your circuit followed the convention religiously, and was working with wire new enough to distinguish black from white. If anyone along the way guessed wrong or didn’t care, it could be either way.
Hot/Live is red here. And black is neutral. Black is live in Germany though, and blue is neutral. Then we get brown as live as well. As you say, Simon, never be too sure! If you take a light switching circuit apart, the colour coding may be mixed up anyway. And also a neutral wire may actually carry live if a load/appliance is switched on.