Depends how old they are
Old electric fence units had a mechanical timer like the balance wheel of a clock that closed a contact and connected up a coil. When the wheel swung round and broke the contact it produced a big spark which the coil stepped up to a couple of thousand volts at a tiny current.

Modern ones do the same thing but it's all electronic and works like a flashgun circuit. A little oscillator drives a transformer that charges a big capacitor, and when it's charged it is discharged through a big transformer like a car ignition coil.

Incidentally, most animals are smart enough to learn to stay away from the bright orange electric fence "string" which has stainless steel strands to carry the voltage. They're not smart enough to learn the difference between electric fence string and ordinary orange baler twine, though ;-)
(gordonjcp, Mon 28 May 2012, 22:02, Reply)