The presented instrument is perfect to find out the depth of metallic casing of wells and drill-hole. It can be used even in very narrow wells of minimum 2-inch diameter. The principle of its work lies in the phenomenon of magnetism: when the probe is inside the magnetic field of the coverage, the audio-signal as well as the light are active, but when the magnetic field doesn’t affect the probe anymore, all the signals stop. The results are read from the tape.
Note: an indicator of datum point is behind the probe holder.