Question: In the Flashpoint episode "The Farm," there's a scene where you can see one of the officers run a jumper cable from one contact on an electric fence to another, then cut the line of the fence (not breaking the circuit, mind you). Based on context clues, one could assume the fence surrounds a large, multi-acre plot of land. Another bit of information: the cable they used to keep the circuit complete was longer than the original cable on the circuit, which would mean it would have a higher resistance when the circuit was rebuilt.

I have two questions:

1. How feasible is this?
2. And would a (powerful, that is) fence controller be able to detect the change in resistance? Or would it be so small that it's undetectable in the grand scheme of things?

From my (limited) electronics/electrical schooling, adding the cable to jump between the contacts would change the resistance, as resistance for parallel circuits is modeled as:

$$r\_{total} = \frac 1 {\frac 1 {r\_{a}} + \frac 1 {r\_{b}}}$$

Which means the addition of the extra cable would have an impact on the resistance of the circuit itself (although very minimally).

If I recall correctly, another one of the officers was counting down to when the first officer should splice into the circuit. I don't know enough about electric fences to come to any conclusion, but are electric fences constantly charged? Or do they have a delay between pulses?

Answer: 
Theoretically, yes. You can measure the amount of resistance and determine the length of the fence.

However, there are some practical limitations to this. Doing this accurately would require quite an investment while the resistance won't be stable. Even the weather would influence it. Compensating for all those fluctuations would require some pretty advanced equipment while an electric fence is essentially a very low-tech device.

> 
> are electric fences constantly charged?
> 
> 
> 

No, they are usually pulsed. This has a couple of advantages, one of them making it easier to generate such a high voltage. They are continuously powered, but the circuit is not designed to sustain the charge.

As [@PlasmaHH mentions](https://electronics.stackexchange.com/questions/192839/would-an-electric-fence-be-able-to-detect-being-bypassed/192840#comment399881_192839), if you want to measure your fence the proper way, impedance would be more valuable than resistance. I have never seen anyone do this on an electric fence, but you can measure the length of a coax cable by sending a pulse through it. The time it takes to come back and the form of the pulse will tell you a lot about the characteristics of the cable, including the length. However, this wouldn't be foolproof on an electric fence.
Some car engine controls look at wave forms in ignition secondary wires to learn if & how fuel-air mixture is burning. Some can spark again if burn is not as desired. A fence monitor could measure wave forms of voltage applied & current to detect fence changes. Measuring between pulses is not the only option.
Sorry I do not remember makes & models.