You can buy a polarity checker at the hardware store for about $5...plugs in and has lights that confirm if it's ok or what's switched...
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Well I ain't no lectrician like some folks around here but the large slot on the left is neutral and the other narrow one on the right is hot.
Also, this is an area that gets lots of talk up, but an AC source has no "polarity." AC is by definition an equal and repeating plus and minus voltage swing. Switching the plugs or improperly wiring them would have no effect on "polarity." Could be ground/safety issue.
Sincerely, I remain
Two comments about polarity: 1) polarity is important to maintain for TV transmission synchronization between the TV camera and the TV set. 2) many new electrical codes require that the outlets be installed "upside down", that is, the grounding plug is now on top. It was explained to me that under the old standard with the hot/ground plug slots on top, there was the (remote) possibility that if the plug was not pushed in all the way, it was possible for a metal object to fall down behind a dresser or a stand and end up shorting the hot/ground terminals. Point being, that the large slot is not always on the left! Happy Tunes!
Whose new electrical codes, I wonder? Sounds kind of silly. Maybe we should change the plumbing code to require that a shower head be placed no higher than 3 feet from the bottom of the tub because of the remote possibility that some fool will get hot water in his eyes upon entering the shower. Don't even get me started on those enviro-nazi low-flow toilets! ;-)
Here's an interesting twist. In my house, only the outlets that are controlled by a wall switch are mounted up side down so you know immediately which ones they are.
Polarity can be an important safety issue, especially if the ground is floating. Reversing polarity on some tube equipment I've personally owned will place 110VAC on the chassis. If you touch the chassis and another part of your body is grounded, you become the least resistant path to ground. In other words, major shock!
Actually AC IS very asymmetrical with respect to ground, although Clueless is obviously correct that if you ignore ground and only check between the two AC terminals, you can't tell which is which. In the vast majority of cases, though, the "hot" side (black/narrow) will be around 120V from ground and the "neutral" side (white/wide) will be very near but not exactly at ground. This has both safety and electrical implications. You can usually grab the white side without much shock, whereas the black side will get you every time (no, I'm not recommending you try it!). And in the electrical case, it can be the source of ground loop problems if different components are hooked to the power source in different ways or grounded differently.
It doesn't mean anything current-wise, they only went with one narrow and one wide blade so plugs could be oriented and not inserted backwards. In things where they use nongrounded plugs, like electric drills, TV's, some pieces of audio gear, etc, they still want to design the piece to protect you from the "hot" side as much as possible, which means orienting the plug. The current through each blade is identical no matter what.