This question is as old as electric power itself. It has been asked almost as often as “Do you love me?” and with equally unconvincing replies.
The common answer, “The birds aren’t electrocuted because they’re not grounded”, doesn’t get to the root of the question. Does everyone who walks away after that explanation really know what “grounded” means? What’s so special about touching the ground?
As you know, an electric current is a flow of electrons. The key word here is “flow.” Unless the electrons can flow from one place to another, they can’t do anything useful, or harmful, any more than a stream can turn a waterwheel by standing still. To get electric light, for example, we make electrons flow through a thin tungsten filament, in one end and out the other. In forcing their way through the very thin tungsten wire under the influence of a 115–volt push, they heat it so much that it glows white hot.
Notice that the voltage is the push; that’s what voltage is: a force that pushes electrons from one place to another so they can do work for us. But no matter how high the voltage, the electrons can’t do anything unless they are given a path to traverse. The power transmission wires are that path. Under the influence of a high-voltage push, they conduct electrons all the way from the power plant to our houses, where they may be tapped off to flow through a light bulb, a toaster or a television set.
Where do the electrons go after they pass through our electric appliances? They return to Mother Earth, which is where the electric company got them from in the first place. Where else, for heaven’s sake, could they have gotten them? The moon?
So Mother Earth, whom we familiarly refer to as “the ground,” is the original source of electrons at the power company and their final destination when we’re done making them work for us. Earth is made of gazillions of atoms containing multi-gazillions of electrons. By rough estimate, the number of electrons on Earth is 1, followed by 51 zeros. That’s what I’d call an inexhaustible supply.
Now, back to the birds. Their little feet are certainly in contact with lots of electrons that are waiting to be drained off and returned to the ground via your electric toaster. But fortunately for the birds, their bodies offer no way of leading the electrons to the ground. The birds just aren’t connected to anything; they’re a blind alley, an electron dead end. The electrons thus have no way of using the birds as a conduit to the ground, and no electricity flows through them. That’s why we don’t experience a rain of electrocuted sparrows.
And by the way, what are those birds doing on the power lines in the first place, besides befouling your automobile? In the winter, at least, they are there because the electric current going through the wires generates a small amount of heat that keeps their tootsies warm. And while we’re at it, how can they sleep there without falling off? When their foot muscles are relaxed, they tighten up, rather than loosen like ours do. So never fall asleep while hanging from a tree branch.
You may have seen an electric company lineman, raised from a truck in a “bucket,” working on electric wires with his bare hands. He’s as safe as the birds, because the bucket is completely isolated, insulated, from the ground. Electrons can’t find a path through the lineman’s body to the ground, so they can’t make him glow like a white-hot tungsten filament.