I'm not sure exactly how much electricity would be required from hand-to-hand, but the LEAST amount of current required to kill an adult human is around 0.07 amps, going through the chest or head. As for WHY, there's a few things here that you should notice:
The first is that I said how much current
it would take. The amount of voltage (measured in volts) isn't that important here. People have been known to survive with a difference of literally millions
of volts between their body and the ground because there was very little current. So how can you get a lot of voltage with only a little current? This happens because people have high resistance (about 500,000 ohms on average). You may have heard of the equation V=IR. In this equation, V is voltage, I is current, and R is resistance. If R is really big, then I can be really small even if V is big.
The second thing to notice is that the current is most lethal when going directly across the chest or head. This is because the heart and brain are the most sensitive parts of your body when it comes to electricity. If the current were just passing over the skin or along, say, the legs, it could be much higher without being lethal (although it could certainly cause serious burns). But how about your example, from hand-to-hand, across the chest. Well, the longer the path is, the more the total resistance will be. So the resistance from hand-to-hand would be higher than just from back-to-chest, meaning that the voltage from hand-to-hand would have to be higher to give you the same current.
So now that it's starting to make sense, here's a sort of confusing bit of information for you... I said that the smallest amount of current that can be lethal is 0.07 amps, right? Well, the electric chair that was once used for many electricutions produces only 2000 volts. Given the resistance of 500,000 ohms that I mentioned before, this would be only 0.004 amps! So how could the electric chair be lethal? The reason that it works is because they dip the electrodes used for electrocutions in salt-water, which is very conductive to electricity. A large part of the human body's resistance to electricity comes from the skin. When the skin is wet, it is much less resistive and the total resistance drops from 500,000 ohms to only about 5,000 ohms, meaning that the current is increased to 0.4 amps, which is more than enough to kill a person.
(republished on 07/22/06)