AN UNDER-WATER DETECTOR

Press, Volume LXXIII, Issue 22109, 3 June 1937, Page 9

AN UNDER-WATER DETECTOR

TRACKING A CABLE INGENIOUS ELECTRICAL METHOD In laying the heavily-protected shore-end of the new Cook Strait cable from Lyall Bay last week. Post Office engineers used an ingenious electrical detector to track the course of disused cable laid 30 years ago. This cable had found for itself a thoroughly safe bed where it had rested without damage throughout its useful life. Therefore, the engineers came to the conclusion that the route would be ideal for the new cable. To track the exact Course of an old cable, now covered with sand, involved the use of an ingenious electrical detector. The disused cable was electrically energised, regular signals being sent out and. at the same time, the motor-vessel Hokitika cruised a zig-zag course over the probable route trailing a submerged electrode. When the electrode passed over the cable its signals were picked up. and this process gave such clear indications of the position that the safest oossible route for the new cable was quickly marked by a line of buoys from the shore to a point two and a hall miles out from Lyall Bay. where the cable ship Recorder will eventually pick up the shore end, join it to the deep-sea section, and proceed along a carefully surveyed course across the strait south-east to the mouth of Blind river. Marlborough. This principle is utilised in some ports to guide shipping during foggy conditions, a cable being laid in the centre of the navigable channel and its course being followed by ships which keep in electrical contact with the signals sent out beneath the water. The New Zealand Post Office regularly uses the same method for locating faults in telephone cables. Some of them carry within their lead covering as many as 1000 pairs of telephone wires insulated with paper. When a breakdown in the insulation occurs, an electrical “trailer” is run over the cable, picking up a series _ of morse signals where the circuit is normal. Immediately the fault is reached the signals become indistinct or disappear, and this enables the point of action to be located by the repair gang to within a few inches.