|RECEPTACLES (OUTLETS/PLUGS) - REVERSED POLARITY:
Polarity is a confusing issue. Within 120-volt household circuits, if the polarity is correct, the black wire is the hot wire (ungrounded conductor) and the white wire is the neutral wire (grounded conductor). If the polarity is reversed, the white wire is hot and the black is neutral.
Polarized Outlets Have A Wide And Narrow Slot - Some of the old two-slot receptacles were polarized, but many were not. You can tell by looking, because non-polarized plugs have two slots the same size, and polarized receptacles have different-sized slots. The small slot is intended to be the hot connection and the large slot is the neutral.
Polarized Appliance Plugs Have A Wide And Narrow Slot - Some appliances are polarized and some are not. If the appliance is not polarized, the two prongs on the plug will be the same size. (All of this assumes no ground pin on the plug). Polarized appliance plugs, on the other hand, will have a narrow blade and a wider blade. This polarized plug will only fit into a polarized receptacle one way, and wont fit into the old non-polarized receptacles at all. On these type of appliances, including lamps, polarity matters.
Light Bulbs - With some appliances, polarity doesn't matter. This would be true of a clock, for example. However, with lamps, polarity is important. When you screw a light bulb into a socket, the electricity is intended to flow into the light bulb from the button on the bottom of the socket. Its the black or hot connection. The electricity flows through button on the bulb, through the wire (filament) in the bulb, and out of the bulb through the threaded collar around the bulb. The electricity is picked up by the threaded collar of the socket, which is connected to the white or neutral wire.
Collar Neutral - Electrical continuity is achieved by having the button at the bottom of the bulb contact the button at the bottom of the socket to make the hot-side connection. Then, the threaded collar of the light bulb contact the threaded collar of the lamp socket to complete the neutral-side connection.
Power Only At Button - Consider a light fixture with no bulb. If everything is wired correctly, the only live electricity in the empty lamp is in the button at the base of the socket, even if the switch is turned on. A person is much less likely to touch this button than to touch the threaded collar around the socket.
Hard To Touch Button - While its always good practice to turn a lamp off to change the light bulb, many bulbs are changed with the power on. Its easy to touch the threaded collar when screwing in a new light bulb, but its hard to touch the button at the bottom of the socket.
Reversed Polarity Makes Collar Hot - This is why light fixtures are polarized. We don't want the threaded collar to be hot, since you're much more likely to get a shock. We want that threaded collar to be neutral. If the house wiring has reversed polarity, or the lamp is miswired, the threaded collars on light sockets can be electrically live, and people are much more likely to get a shock.
Switched Appliances - Polarity matters with appliances that have switches. When the appliance is plugged in, power should only go as far as the switch. If polarity is reversed, power will go through the entire appliance back to the switch. If a wire comes loose in the appliance, the entire case of the appliance may be electrically, even though the appliance is not on. This is a shock hazard.
You should have the polarity of receptacles checked by an electrician or when your home is inspected by a home inspector. If the receptacles are not polarized, this is not as issue. Polarized appliance plugs will not fit into non-polarized receptacles because the slots are both narrow. The wide blade for the neutral side will not fit in. We trust none of you will ever file off the wide flanges on the neutral blade, since this defeats the protection of polarization.
Test Polarity - Where you have polarized receptacles, which include some of the two-slot receptacles and all of the modern U-ground receptacles, they should be checked for polarity with a portable circuit analyzer. Reversed polarity receptacles need rewiring.
Black Wire To Brass Screw - One of the ways to identify reversed polarity, other than with a tester, is by removing the outlet cover plate. The black wire is typically connected to the brass screw terminal on the outlet, and the white wire is connected to the silver. This color convention is useful but not 100 percent reliable for identifying reversed polarity.
Sometimes the outlet will appear to be wired properly when you take the cover off. The black wire correctly goes to the brass terminal and the white wire correctly goes to the silver terminal. In this case, the polarity has been reversed somewhere upstream, perhaps right at the panel itself. That's why its not completely reliable to look at the receptacle wiring connections to see if polarity is reversed.
Reversed polarity outlets often go unnoticed for a long time. Many appliances will work just fine. However, the polarity has to be right to provide the protection designed into the system.
It's usually recommended that you contact a Licensed Electrical Contractor for repairs.