Strait Cable May Deflect Compasses

Press, Volume CIII, Issue 30562, 3 October 1964, Page 14

Strait Cable May Deflect Compasses

The Cook Strait cable and its links to Benmore and to Haywards in the Hutt Valley was likely to upset ship and aircraft compasses, and would also affect the Magnetic Observatory’s station at Amberley, Mr P. J. Gill, a member of the observatory staff, said yesterday. Special difficulties would arise when, for one reason or another, one of the lines was temporarily out of commission. Normally, the lines would tend to neutralise each other in their effects on objects at a distance, but if only one were on load its magnetic field would have full play. The lines were normally at 250,000 volts positive and negative to earth respectively. Under the strait, the lines were separated by a sufficient distance for the magnetic fields of each to be almost unaffected by the other when measured from the ocean surface. Mr Gill said the maximum compass deflection in an aircraft flying at 100 ft above the power lines when only one line was working was about 18 degrees. If both

lines were working, the maximum deflection would be 3| degrees in one direction and then a similar amount in the opposite direction.. The effect fell off approximately linearly with distance. The effect on the compass of an aircraft crossing the line at rightangles at airport approach speed would only be momentary, but the effect could be prolonged if the approach and power line were parallel. “Could be Dangerous”

The effect of the undersea cable currents on ship’s compasses could be dangerous if not taken account of, said Mr Gill. The effect of the overhead sections of the line on aircraft compasses was not, he thought, likely to be so

serious because the risk of collision with the line would keep aircraft at such a distance that the compass variation would be minimal. Never theless, it was possible that some aircraft landing approach paths might have to be changed. The airfields which could be affected were Rongotai and Woodbourne. The maximum deflection in the compass of a ship crossing the power lines in 100 fathoms was li degrees. If the ship crossed at right-angles, there would be a noticeable deflection in the compass for about a mile, the needle swinging first one way anji then the other as the ship passed over the individual lines. Shipping coming into

■Wellington from the Tasman Sea would normally cross the cable at right-angles, where the depth was about 100 fathoms, but the inter-island ferry from Picton would cross at an oblique angle. Depths towards the South Island end of the cable were much less than 100 fathoms, but Wellington shipping would probably keep nearer the North Island at this point.

The lines ran about seven miles from the Magnetic Survey’s main New Zealand station at Amberley, which was certainly near enough to affect the readings when only one line was working. Early Tests

As soon as the lines were put on loads tests would be carried out to confirm the predictions as to the magnetic fields they would create, said Mr Gill. Officers of the New Zealand Electricity Department and the Civil Aviation Administration who had been looking into the matter were not available for comment last evening.