UNDER-WATER DETECTOR

Wanganui Chronicle, Volume 80, Issue 133, 7 June 1937, Page 9

UNDER-WATER DETECTOR

TRACKING A CABLE. INGENIOUS ELECTRICAL METHOD. I In laying the heavily protected shore-end of the new Cook Strait ; cable from Lyall Bay last week, post office engineers utilised an ingenious . electrical detector to track the course of another cable laid 30 years ago and now disused. This cable had found for i self a thoroughly safe bW where ]it had rested without amage i 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 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 proi cess gave such clear indications of the j position that the safest possible route | for the new cable was quickly marked j by a line of buoys from the shore to | a point two and a-half miles out from I Lyall Bay where the cable ship Recorder will eventually pick up the i shore end, join it to the deep-sea secjtion and proceed along a carefully purveyed course across the Strait ! south-east to the mouth of Blind I River, Marlborough. ■ This principle is utilised in some • ports to guide shipping during foggy i conditions, a cable being laid in the | centre of the navigable channel and jits course being followed by ships i which keep in electrical contact with I the signals sent out beneath the I water. The New Zealand post office i regularly uses the same method for j locating faults in telephone cables. Some of them cany within their lead covering as many as 1,000 pairs of telephone wires insulated with papef. 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 Jor disappear, ahd this enables the j point of action to be located by the i repair gang to within a few inches.