It is possible to design networking systems that only use two wires: one for data and one for a common ground. Examples include 1-wire and Pin&amp;Play. These are called single wire systems because the requirement for an earth wire is implied too. But you can also get systems to extend home networks that use the home earth to connect network points, like Power Line Communication. How is this possible over just one wire?

== EDIT ==

From the answers (thanks!) I think I failed to word this question clearly. Let me try again.

Can you send data usefully over one wire, literally one wire? Radio is zero, 1-wire is two, but is it possible with one? "No, and here's why" or "Yes, here's how it is done in X" are the kind of answers I am hoping for.

(N.B. I'll also change the question title from "Single wire systems need two wires; so how does ethernet over ground work?" to "Can you send data usefully over one wire, literally one wire?")


Question: Unclean -- Consequences
Answer: Nowadays there is mostly no implication of not being Tahor (spiritually clean).

Only 2 related laws still apply:


A Cohen may not come in contact with a corpse, nor be under the same roof as a corpse, nor come within 4 Amot (~2 meters) of a grave. If he does, he has transgressed, and needs to repent, but no other practical consequences.
Nidah - menstruation, as you already mentioned. Nowadays, after menstruation a wife may not have relations (or any other physical contact) with her husband until after she has counted 7 clean days and gone to a Mikvah.


In the time of the Bet HaMikdash (the Temple, may it be speedily rebuilt in our times) it was important to remain Tahor for various reasons. Here are some:


You were not allowed into the Temple unless you were Tahor.
You could not touch Terumah and Challah (the tithing and piece of dough given to a Cohen) unless you were Tahor. Otherwise they became impure and had to be burned. Nowadays we burn them, as we are all considered impure.
You had to keep away from people and items which were Tahor in order not to make them impure.


The only person who was sent into isolation was a person who had Tzara'at (Biblical-leprosy). All other people were expected to act civilly and not make each other (or their items) impure.