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"A good science fiction story should be able to predict not the automobile but the traffic jam."
- Frederik Pohl

Electrified Fence  
  A wire fence carrying an electrical charge sufficient to deter crossing the boundary.  

Well, how have you arranged the fence?"

"I start twelve immensely strong wires--naked, not insulated --from a big dynamo in the cave--dynamo with no brushes except a positive and a negative one--"

"Yes, that's right."

"The wires go out from the cave and fence in a circle of level ground a hundred yards in diameter; they make twelve independent fences, ten feet apart--that is to say, twelve circles within circles--and their ends come into the cave again."

"Right; go on."

"The fences are fastened to heavy oaken posts only three feet apart, and these posts are sunk five feet in the ground."

"That is good and strong."

"Yes. The wires have no ground-connection outside of the cave. They go out from the positive brush of the dynamo; there is a ground-connection through the negative brush; the other ends of the wire return to the cave, and each is grounded independently."

"No, no, that won't do!"

"Why?" "It's too expensive--uses up force for nothing. You don't want any ground-connection except the one through the negative brush. The other end of every wire must be brought back into the cave and fastened independently, and _without_ any ground-connection. Now, then, observe the economy of it. A cavalry charge hurls itself against the fence; you are using no power, you are spending no money, for there is only one ground-connection till those horses come against the wire; the moment they touch it they form a connection with the negative brush _through the ground_, and drop dead.

From A Connecticut Yankee in King Arthur's Court, by Mark Twain.
Published by Not known in 1889
Additional resources -

Many sources credit Twain as the earliest source for this idea - but I think there is at least one earlier reference that should count.

In his 1875 epic 20,000 Leagues Under the Sea, Jules Verne writes about electrifying a stair rail or cable to prevent unwanted intrusions onto the Nautilus.

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Additional resources:
  More Ideas and Technology from A Connecticut Yankee in King Arthur's Court
  More Ideas and Technology by Mark Twain
  Tech news articles related to A Connecticut Yankee in King Arthur's Court
  Tech news articles related to works by Mark Twain

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