drawing insulative layer 130 is formed with an u201c overhanging u201d section 130 a which will effect the conduit gap of the present invention the inner and outer conductors are electrically connected at some point during the run of the well so that current injected at the surface by source 24 via lines 60 and 61 through encoder decoder 25 has a closed path within which to flow along illustrative current line 12 in this embodiment the connection between inner and outer conductors is accomplished at the bottom of the well by shunt 150 a toroid 126 is disposed in the insulating layer 130 i e u201c sandwiched u201d between the inner conductive casing 122 and the outer conductive layer 140 as in the earlier embodiments such as fig4 measurement device 28 is installed on the casing 122 along with encoder decoder 27 device 28 receives electrical power from toroid 126 where the current flowing through the casing 122 inductively generates a voltage in the toroid that is used to power the sensor the device is connected to the toroid 126 via a lead through feed through nonconductive seal 160 the signal sensed by measurement device 28 is encoded into a second alternating current in the toroid 126 at a frequency distinct from that of the current injected at the surface thus creating a second current in casing 122 and conductive layer 140 which is decoded by surface electronic encoder decoder 25 and recorded or otherwise processed fig1 illustrates another which utilizes direct downhole coupling like the embodiment of fig1 a three layer.