He said there was some kind of device they could attach
to *one* end and it could tell you how many feet down
the wire the break was.

He didn't know how it worked, he said.

My probably-incorrect guess is that it broadcasts some microwave
frequency down the wire, and vary the frequency transmitted
and see if you can get a resonance -- and do that for several
frequencies (relatively prime to each other? -- I make this
up as I type it in) and if there is some cheap computer
hooked to it, maybe it -- well -- tries to disambiguate
how long the resonating part is????


Sounds like a TDR, they're used a lot in repairing computer networks in
large buildings. For something like this though a simple Fox & Hound would
probably work fine, we have one at work and it's nothing but a little box
you connect to one end of the cable and an inductive probe that picks up a
transmitted tone, run it along the wire until the tone goes away. Same can
be done with a radio and some sort of noise generator or oscillator. You
can't use the fence controller itself as they normally shut down when the
loop is open and you need it connected to only one end so the signal will
not be present after the break.