"mc" wrote ...
"Richard Crowley" wrote ...
The power that runs the computer you are reading this on
is likely transmitted through the ground (on the return path
of the high-voltage side). When you see a single wire on
the power poles with taps going into the transformers, note
that the other side of the transformer is connected to a ground wire.
The electricity comes from the generator,
through the single wire along the poles, into the transformer
(the "destination"), and then returns through the earth's
crust back to the generator.

Are you sure about that?

Absolutely.

I've heard of some telephone systems that used the earth as a
conductor, but surely it would be hazardous to transmit substantial
amounts of AC power through the earth,

Why? By the most obvious definition, it is at "ground-potential"
and cannot cause harm (unless you take a long metal antenna
mast and touch the other side of the circuit 35 feet up in the air!)

and I've never heard of it being done.

Nevertheless, millions of watt-hours are flowing through
your planet's crust as you read these words. Perhaps even
under your feet.

People walking barefoot on the ground would be electrocuted.

You need a complete circuit to be electrocuted. If you are not
touching the other side of the circuit, no current can flow and
you are safe from electrocution.

There would be a constant risk of setting things on fire on the
ground.

How? (Hint: this is clearly not the case as we don't see
examples of spontaneous combustion from current flowing
through the earth.)

And the conductivity of the earth is very variable --
your power would fail when the ground got dry.

Think deeper.

As far as I know, all high-voltage power transmission systems are
3-phase systems, with a fourth wire connected to the earth for safety
purposes, but no substantial current flowing through the earth.

http://en.wikipedia.org/wiki/Single_wire_earth_return

http://science.howstuffworks.com/power9.htm shows a
ground-referenced power distribution system which also
includes a ground wire strung along the poles. In my part
of the country, they don't use the extra ground wire shown
in the photo, they rely on actual earth ground for the return
path.