This tree branch is screaming — seriously, it actually is making screaming noises — as it catches fire on a group of power lines. But why does this branch spontaneously combust when it hits them, and birds that land on power lines don't?
The power lines above your head aren't insulated. This video is a graphic illustration of that fact. The branch connects the two wires, and things immediately get eerie. Escaping gas and steam make the branch shriek as it catches fire. The spectacular arcing flame at the end is, I think, the electricity moving through plasma — a state of matter in which electrons come unhooked from their atoms and the two swim around in a kind of extremely conductive soup. As the last of the plasma dissipates in the air, the connection between the two wires subsides. It's a good show.
We aren't treated to the same when birds land on a power line only because they don't straddle the wires they settle on. While living bodies make decent conductors, electrons only move if there is a voltage difference between one side of a conductor and the other. When a bird sits on a wire it is essentially connecting the positive end of a battery to another spot on the positive end of the battery. They're not worth moving through.
Any bird that sits on a power line is relatively small, and that certainly helps them keep safe. Just as sparks will sometimes fly off a doorknob just before your skin touches it, sparks can fly between the two wires in power lines, provided a conductor reaches most of the way between them. They can also fly between someone who has just climbed the main pole — which is grounded — but hasn't actually touched the cable. So be careful even getting close to power lines. Let the branch be a lesson to you.
[Via Ask An Engineer]