Recently got a dose of laziness and bought a robot mower. Works really nicely

One thing I haven't got clear in my head is how it knows whether it is inside or outside of the charged boundary wiring.

I can power down the whole system, move the robot to just outside the boundary loop, and when it is all powered back up it instantly detected as being outside the wire.

Anyone know how this works? What goes down the low voltage cable that is the boundary wire to allow the robot unit to detect where it is in relation to being inside or outside?