electricity - Why don't electric workers get electrocuted when only touching one wire?

I know that when electricians work on the poles on the streets, if they only touch one wire at a time they will be fine. However, from my understanding, the negative wire is connected to a large negative terminal, and the positive wire to a large positive terminal. Now for example, if you touch the positive wire, wouldn't all of the electrons get stripped away from your body? Another way to look at it is that current always flows from a higher to a lower potential. If the potential in the wire is 100V (I have no idea what it is) and your potential is 0, wouldn't current flow through you?

Answer

AC or DC, you only get electrocuted if current passes through your body. (Current passing through any part of your body can be dangerous, and possibly cause an electrical burn, but current passing across your heart is the one that's really dangerous.) Touching just one wire at a time gives the current nowhere much to go. You are right to think that some electrons can get stripped from your body when you touch a bare wire. But not many. Once they've gone, unless your body gets new electrons from somewhere else, the current stops. If you're standing in a pool of water, or touching a metal pole, or another wire that can conduct lots of electrons from somewhere else, you're fried.

So how many electrons get conducted away from the human if it has no other source of electrons? In this case, the human acts as a capacitor. Now, Wikipedia tells me that one standard for this approximates a human as having capacitance $C=100\,\mathrm{pF}$. This is pretty tiny. (For comparison, the capacitors you might see if you open up a computer or other electronics can easily have capacitance billions of times larger.) If the voltage is $V=100\,000\,\mathrm{V}$ (which is really quite high), the charge that would be transferred is $Q = C\, V = 10^{-5}\,\mathrm{C} \approx 10^{14}$ electrons. You probably have $10\,000$ times more electrons in a single eyelash, so that's not many. The danger there is that at such high voltages, a lot of things become conductive that aren't normally.

P.S. Most electrical wires that you see in a city (though not those for trams, or the really big high-voltage lines) are actually insulated. (I worked as an electrician through college, and have touched many such wires.) So the birds aren't frequently touching anything dangerous to begin with. But even if they were, they'd only be in danger if they had somewhere to get new electrons. There are plenty of places (especially around those big transformers you see) where you can find exposed power, though, so be careful.