UNDER-WATER DETECTION

Taranaki Central Press, Volume IV, Issue 450, 4 June 1937, Page 3

UNDER-WATER DETECTION

TRACKING A CABLE

Ingenious Electrical Method

In laying the heavily protected shore-end of the new Cook Strait cable from Lyall Bay last week, Pott Office engineers utilised ah ingenious electrical detector to track the course of another cable laid thirty years ago and disused. This cable had found tor itself a thoroughly safe bed where it retted without damage throughout its useful "tife. Therefore, the engineers came to the conclusion that th e route would be ideal for the new cable. To track the exact course of an old cable covered with sand involved th e use of an ingenious electrical detector. The disused cable was electrically energked, 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 indicia, tions of the position that the safest Possible route for the new cable was quickly marked by a line of buoys from the shore to a point two and a half miles out from Lyall Bay, where the cable ship Recorder will eventually pick up the shore end, join it to the deep-seta 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 so/he ports to guid'e shipping during foggy conditions, a cable being laid in the centre of the navigable channel and it* course followed by ships which keep in electrical contact with tfte Bigwak 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 1,000 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 ,th e circuit is normal. Immediately the fault is reached the signals become indistinct or disappear, and this enabled the point of action to be located by the repair gang to within a few inches.