What is the device designed to detect a difference in current between circuit wires

D=distance

VS=velocity through steel

VA=velocity through air

ts=time through steel

ta=time through air

Let's just get some equations out there:

D=VS*ts=16500*ts

D=VA*ta=1100*ta

ta=ts+6 (you can reason that it will take longer through air because the speed is slower)

So, D=16500*ts=1100*ta=D so 16500*ts=1100*ta. Substituting ts+6 in for ta we get 16500*ts=1100*(ts+6), so 16500*ts=1100*ts+6600. Subtracting 1100*ts from both sides, we get 15400*ts=6600, so ts=6600/15400=3/7 seconds. So, to find the distance, we need only plug this time back into the equation D=16500*ts=16500*3/7=7071.43 feet.