Stan writes:

> Tracy writes:
>  > Even more humorously, the thought occurred to me, what if someone
developed
>  > an interface between networking and local power grid?  After all, the
only
>  > Then more ominously, I thought, "What if someone has already invented
it?",
>  > "Perhaps the Government?"  60 cycles..... hmmmmm.
>
>  Years ago.  I believe that there are at least two competing home
automation
>  products that use the AC wiring in your house for sending signals.  They
>  either assume that the signals won't get past your main electric panel
>  (or require a box at that end?).

Actually, they assume that the signals carried on a wire-transported,
radio-frequency modulated signal won't get past the step-down transformer
that connects your house connects to your distribution line that runs down
the alley. The power transformer acts like a giant radio-frequency choke,
thus radio-frequency signals don't easily make it from one side of the
transformer to the other.

(As in all things technical, the right and proper answer pertaining to what
portion of the signal makes it across the transformer is determined by the
"reflected impedence" of the electrical universe that lies on the opposite
side of the transformer -- and that can't be calculated perfectly well
without a great deal more information, but under normal circumstances, there
isn't going to be much signal pushed through the transformer).

However, the situation can be substantially improved, if you wished it to be.
If you did want whatever RF-modulated signals (RS-232, LAN packets, etc.) you
put into your house-wiring to go down the block to a neighbor's house also,
you would have to put a bridging capacitor across the two sides of the
transformer, both at your house and your neighbor's. What that means is that
you must climb the power pole and very carefully attach a sufficiently large
enough capacitor across the opposite sides of the transformer. Probably some
words of warning should be inserted here for anyone foolish enough to try
this with power live on the wires -- but it will work. This is the method
that non-licensed, non-over-the-air campus radio stations broadcast their
signals around the campus and keep their signals essentially confined to
their campuses.

While this thread started out as a bit of joke, it's not as screwy an idea as
it sounds. All of the circuits in a building that operate off the same
transformer are in direct contact with one another -- and thus represent a
very cheap bit of pre-wired wiring to carry a great variety of signals. The
fact that these same wires are carrying power at 60Hz is irrelevant to the
radio signals.

Wirt Atmar


But now, Ladies and Germs, -- and far more the point --

Q. Why do power transformers hum?

A. They don't know the words.

Ka-bing! Ka-bang! Ka-boom!