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.
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