Polarized Plugs (2/3) old
If you look at where electricity comes into your house, you'll notice 3 wires. Since the grounding is done with a pipe at your house, why do you need three?
It turns out that most of the plugs in your house connect a "hot" wire (usually colored black) to the device, and a "neutral" wire (usually colored white) as well. In the hot wire, the voltage is 120V AC, which varies from zero to a maximum 60 times every second. By putting these two wires on, current will flow in the device and switch direction as the voltage in the hot wire does.
The neutral wire is inherently safer, because it's at the same voltage as the ground and a person. If you accidentally touch a hot wire, you'll get shocked because of the voltage difference. If you accidentally touch only a neutral, nothing will happen.
Thus, we wire switches to devices in the hot wire. This means when it's off, nothing in the device is energized. If we put the switch in the neutral, stuff inside could still be at high voltage and dangerous if we touch it.
A good discussion of the safety benefits of polarized plugs can be found in this thread:
arstechnica.com/civis/viewtopic.php?t=828563
The other wire, seen at the bottom of this board, is also hot. But it's the mirror image sine wave. Half the outlets in your house are wired between neutral and one hot, and the other half between neutral and the other hot.
You also can get 240VAC by wiring a device across the two hot wires. Note that this is different than the 240VAC you get in other countries, where you have one hot and one neutral (Seen at right in this photo.) The device may work, but if having a neutral wire is essential for safety then you won't have it.
On the next picture you'll see a common 240V plug found in older buildings in the United States. One of the pins is wired to neutral and the other two to the opposite hot wires. A more modern outlet has replaced this that has a separate grounding pin.
Polarized Plugs (2/3) old
If you look at where electricity comes into your house, you'll notice 3 wires. Since the grounding is done with a pipe at your house, why do you need three?
It turns out that most of the plugs in your house connect a "hot" wire (usually colored black) to the device, and a "neutral" wire (usually colored white) as well. In the hot wire, the voltage is 120V AC, which varies from zero to a maximum 60 times every second. By putting these two wires on, current will flow in the device and switch direction as the voltage in the hot wire does.
The neutral wire is inherently safer, because it's at the same voltage as the ground and a person. If you accidentally touch a hot wire, you'll get shocked because of the voltage difference. If you accidentally touch only a neutral, nothing will happen.
Thus, we wire switches to devices in the hot wire. This means when it's off, nothing in the device is energized. If we put the switch in the neutral, stuff inside could still be at high voltage and dangerous if we touch it.
A good discussion of the safety benefits of polarized plugs can be found in this thread:
arstechnica.com/civis/viewtopic.php?t=828563
The other wire, seen at the bottom of this board, is also hot. But it's the mirror image sine wave. Half the outlets in your house are wired between neutral and one hot, and the other half between neutral and the other hot.
You also can get 240VAC by wiring a device across the two hot wires. Note that this is different than the 240VAC you get in other countries, where you have one hot and one neutral (Seen at right in this photo.) The device may work, but if having a neutral wire is essential for safety then you won't have it.
On the next picture you'll see a common 240V plug found in older buildings in the United States. One of the pins is wired to neutral and the other two to the opposite hot wires. A more modern outlet has replaced this that has a separate grounding pin.