"DE-GAUSSING" SHIPS IN NEW ZEALAND

New Zealand Listener, Volume 3, Issue 59, 9 August 1940, Page 3

"DE-GAUSSING" SHIPS IN NEW ZEALAND

How the Magnetic Mine was Overcome

(By "23/762")

HE Minister of Defence, the Hon. F. Jones, stated recently that the "de-gaussing" of ships has been and is being undertaken in New Zealand. By fitting this apparatus to seagoing vessels, they are made immune from destruction by magnetic mines. When Hitler’s secret weapon, the magnetic mine, was first released, fear for our ships was greatly increased. Then a mine was washed ashore. Officers of the Royal Navy, risking their lives, carefully took the stranded mine to pieces. It was no longer a "secret weapon"but an antidote had still to be found. Science to the Rescue Eminent scientists came to the aid of the authorities; within the space of two or three months the "D.G." apparatus was born and the danger was over. "D.G." brought a new word to the British language and complete immunity to our ships from the evil effects of the Magnetic mine. "D.G." implies the process of "degaussing" a ship, and that in turn means de-magnetising or de-polarising a shipany ship, from the tiniest launch to the largest warship or transport. This "degaussing" equipment consists of a girdle, or a "snake" of wire round the hull of a ship at the level of the upper deck. This wire is ordinary insulated electric cable, energised in a special way which still remains a secret. Its effect is to demagnetise the "field" through which a ship is passing. The highly sensitive Magnetic detonators of the magnetic mine are not affected when the ship passes above it. How the Mine Works And this is how the magnetic mine works. First, it is laid only in comparatively shallow water, by parachute from an airplane or from a mine-layer or submarine. When a ship passes over a sunken mine, the magnetic field of the ship’s metal hull affects a magnetic needle in the mine. This closes an electric circuit and the detonator is automatically fired. The high explosive with which it is filled is thus touched off. A magnetic mine is 8 feet long, 2 feet in diameter, and weighs 1,5001b., which includes 700Ib. of high explosive. One of the greatest advantages of the "de-gaussing" apparatus is that it can be fitted to a sea-going vessel of any size. A ship can be equipped with it very swiftly, the time depending on the size of the vessel, and taking anything from one to four days. So far as cost is concerned, it is not unduly expensive. In fact, considering its powers and the ships it has saved from destruction, the "degaussing" apparatus may be reckoned as an extremely cheap "gadget."

It may be explained that a "gauss," in the technical language of’ physics, is a unit of magnetic flux. Like the volt, the watt, the ampere and the ohm, it is named after one of the pioneers of electric research-in this instance, Kar! Gauss, a German professor, who died in 1855 after devoting a quarter of a century to the investigation of the problems of magnetism. Since the introduction of the "degaussing" apparatus, not one ship so equipped has fallen a victim to a magnetic mine. Courageous Seamen The Rt. Hon. Winston Churchill was First Lord of the Admiralty when the secret of the magnetic mine was revealed. He described that and the introduction of the "de-gaussing" process as instalments of a detective story. Slowly but surely, it was unfolded, with all the attendant thrills of a mystery story re-

vealing itself. When the final chapter ended, the British Admiralty breathed a sigh of relief. Here, briefly, is the story of how the secret was revealed and the courage of the men who revealed it. On the evening of November 22, German airmen dropped a magnetic mine in shoal water off Shoeburyness. Fortunately, it could be reached at low tide. After firing e few shots at it, fortunately without effect, the Royal Artillery stationed at Shceburyness informed the British Admiralty. At 2.30 o’clock in the morning the mine was photographed by flashlight. Two hours later a party of experts arrived from H.M.S. Vernon (the Portsmouth Torpedo and Mining School), under the command of Lieut-Commander J. G. D. Ouvry, R.N. After securing the mine so that it could not be moved by the tide, paper rubbings were taken of its various outer fittings. This enabled special tools of non-magnetic material to be made in the experimental department at Shoebury-

ree om. -_- ness during the time the mine was submerged by the rising tide, As soon as the mine was again uncovered, Lieut..Commander Ouvry began his perilous task of taking it to pieces. He explained to the rest of the party what he was going to do at each step, so that if he caused the mine to explode, which would have blown him to pieces, the others, who remained at a safe distance under his orders, would have known what to avoid when another mine was secured for investigation. Fortunately no accident occurred. By the time Lieut.-Commander Ouvry had found and extracted a small fitting, which was evidently the detonator, the remaining members of the party joined him, laughing over their success, confident that there was no longer any danger of an explosion. Second Detonator A few minutes later they were startled to find a second detonator of a different

type, but this was also safely extracted and by 4.40 p.m. the following day the mine was no longer harmful, It was then loaded on to a lorry and taken to Portsmouth for further dissection in the worke shops of the Vernon Torpedo: School. The mine itself is constructed of nonmagnetic material and is detonated by the delicately-balanced electric needle referred to above. The second detonator is a cruder contrivance and comes into play only when the mine is dropped on a hard surface. Lieut.-Commander Ouvry showed great courage when he took the mine to pieces. Nothing was known about it; no one knew how it operated, and he had nothing to guide him except his knowledge of mines and explosives, yet he set to work on the cold beach, knowing that any moment might be his last. One of his assistants in the hazardous task, Chief Petty Officer Baldwin, has since been killed in an explosion while on similar experimental work.