Some telephone companies have such a device for tracing wires. I may be able to obtain details for you.
As far as I can remember, it is just an oscillator that inputs a square wave signal into the cable and a detector that receives the signal.
I presume that the detector is simply an Audio amp with a high impedance input with a wand attached to detect the electric field. The wand would be a short wire or plate with a screened cable connecting it to the amp.