I came across this quote in a Web Forum discussion about magnetic mines. It just shows you should never expect a spy-novel author to write sensible science (unless he's perhaps someone like Nevil Shute)! Aside from his first statement about the mines being a surprise, just about everything he says is totally wrong. My own comments are bracketed in blue.

"In the opening days of the war, the German magnetic mine gave the Royal Navy one of its first big shocks. It was a simple weapon, but the method of its activation demonstrated some of the curiosities of the natural world. The mines sat on the seabed and came to the surface to explode against the hulls of ships that passed over them. [No... they didn't. They sat on the seabed, and exploded there. It was the blast wave from there that holed the ship.] Inside each mine there was a dip needle which was pushed down by the downwards North Pole of the ship passing over it. Such mines were activated by ships built in the Northern Hemisphere. Ships built South of the equator had a downwards South Pole which pulled the contacts further apart, so they could pass quite safely over magnetic mines. [Wrong again. The 'remanent magnetism' of a ship was totally unimportant. It was the distortion of the earth's magnetic field that was detected by the mine. On the other hand, you couldn't lay one of the early mines designed for the northern hemisphere in the south, because the earth's field is reversed. And actually later mines (with a mechanism like that illustrated here) didn't care — they responded to any significant change in the field.]

"In fact it was more complex than that: the magnetism of a ship's hull was not simply North or South. Each ship was different. The hulls varied according to the direction, relative to magnetic North, in which the ship's keel had been laid down when built. Even more surprisingly, it was discovered that ships sailing to the Southern Hemisphere and back again changed their magnetic signature. Prefabricated ships were sometimes assembled from two halves made in different places; the halves then had different magnetic properties. [Although technically true, this again is utterly irrelevant. The magnetism left in the ship by its manufacture is tiny conpared to its concentration of the earth's own field.] Once a ship's signature was known, it could be demagnetized by means of a fluxmeter." [And this last sentence had me on the floor...! A "Fluxmeter"?! Bwahaha...! A fluxmeter is a little gadget used to measure fields — not change them. A far cry from the massive cables carrying large currents actually used to demagnetize the ships.]

Len Deighton, "Blood, Tears and Folly", (Pluriform Publishing Company, 1993) - ISBN: 0-7858-1114-1

To elaborate a bit: Deighton's basic misuderstanding is that it is not the magnetization of a ship's hull that's important, but the way in which it concentrates the earth's field. A ship does get magnetized — slightly — during construction, because working on steel shakes it up enough to let the 'domains' (that give it its magnetic properties) align themselves a bit in the direction of the existing field. However, it's simply the iron in the ship that essentially magnifies the effect of the earth's own field. The best way to demonstrate the mine mechanism shown in the photos was always just to wave the 'mu-metal' sheath of the mechanism a foot or so above the detector. Mu-metal has a high 'permeability' (it concentrates a field well), but it never holds any permanent magnetism of its own.

In the Northern Hemisphere — at least in European Atlantic latitudes — the field is tilted downward toward the north, and the ship passing over the mine simply increases this "north pole down" sufficiently to trigger the mine. To protect a ship, you had to prevent any apparent change in the field, and this did involve actually magnetizing the hull — in the reverse direction! [So actually Dad's introduction of the word "Degaussing" meant something slightly different from its usual use today of removing magnetization.]

One way to cancel the field is to have a coil around the ship that carries a suitable current (all the time), but this is naturally hard to do, so the idea of 'wiping' was developed: a cable carrying a thousand amps or so was wiped up and down the hull until a suitable effect was achieved. (And yes, a fluxmeter might have been used to check the result!)