I think it is important to explain that the "touch with the back of your hand to see if it is live" advice should be taken in-context.

It is not a general-purpose method for testing if a circuit is live. It is just a good habit that MIGHT save you when you go to purposely touch something that is disconnected, turned off, verified with a meter or test light and SHOULD be perfectly safe to touch. As long as you intend to touch it anyway, make your first touch with the back of your hand.

Please don't use it to find out which breaker controls a circuit by having your wife in the basement turn off one at a time while you repeatedly touch the bare wire with the back of your hand.

I'm not an "electrical" guy, but I've been pondering this probe idea since yesterday and it makes me nervous and I'm not sure if it's for a valid reason. 0311, perhaps you can tell me if I'm worrying about nothing here. And I'm thinking about checking broken wires, not intact circuits here, because building collapse or some other calamity seems to be the scenario that you envision using it. Maybe I'm just confused about where the ground in an AC system needs to be.

Couldn't a wire be "dead" according to this probe, but then you actually get shocked if you touch it because you just completed the circuit to ground? I guess I'm thinking that this probe only works when current is actually flowing at the time you test it. No current = no EM field. It seems to me that it won't tell you if the line is actually not hooked up to a power supply or not. Does this question make sense?

No. Those probes actually detect an electric field, not a magnetic one. The voltage on the wire is still AC, and it will still generate a faint electric field, even if no current flows. I have had one, and they work well. The only problem is that the probe needs to be less than an inch away to detect anything. It is pretty useless for detecting the electrified puddle or tree, unless you are paranoid and use this thing on every object before you touch it.

Interesting... So, I could hold this probe up to an empty household socket and it would detect that it is live, right?

I know that this is an AC probe, but can an open DC circuit also be checked this way? Seems like DC characteristics tend to be more "common sense" than AC, so maybe my layman's thinking is correct for DC.

AC in the US alternates at 60hz. That means that the polarity reverses 60 times/second. So, you have 1/60th of a second for the reversal, & the voltage drops to zero for an infinitly short period of time. We rarely see DC at high voltages, so AC is the think to be concerned about.

The back of the hand idea is a good one for taking no chances with a circuit that is "known" to be disconnected. This is the last test for safety AFTER you are certain that you have disconnected it.

The probe will detect that an extension cord is live if it is plugged into a AC outlet but has nothing plugged into it. The probe will detect flowing AC as far away as 8". Yes, it will detect a live condition in an empty light socket. Loose building wire is what I am concerned about. A house may be destroyed in a tornado, but the electric may still be connected. Before I start chopping debris to free a victim, I want to know that the power is off. You might not get a shock, but crossing the conductors with an axe head could start a fire. AFAIK, DC cannot be detected this way.

Correct. You can detecd DC current using a hall-effect sensor, but if there is no current, then you can't see it until it bites you.

No worries, though. In a household situation, you will never see any DC voltage large enough to kill you. You only have to start worrying in an industrial setting, but even that would be rare.