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Comment by brandmeyer | Hacker News Reader

The degaussing facility uses shore power. They could mean 4000 ampere-turns, too.

Source: I went through one of these back in the day.

Oh, it's a degausser? So it's not running regularly while at sea?

Correct. The magnetic signatures seen are the permanent magnetic moments embedded in the steel of the ships, which are picked up by passive magnetic-anomaly detectors. These have to be removed by randomization in a normal all-at-once degaussing process.

It is hypothetically possible to have an active-detection process too, but presumably that would take the form of emitting a pulse of electromagnetic radiation and looking for its echoes, in other words it's just another form of vision. The standard problem applies: find a frequency at which water is mostly transparent. Unfortunately the absorption spectrum for water is https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_... , it has a minimum in the visible range.

In addition to being one of the more stunning confirmations of evolution (of course our eyes are only sensitive to the least-absorbed wavelengths of light in water -- our ancestors evolved eyes in the ocean! there weren't many other useful wavelengths down there!) this has the nasty property that it's unlikely that we'd be able to detect a sub this way with any better approach than shining a bright laser or spotlight or what have you, and watching what's reflected back with our own eyes.

The ships undergoing this look kinda crazy. Couldn't find the exact image I was thinking of but here is a sub with wires wrapped around it:

http://systems.polyamp.com/images/stories/DepermingSub.jpg

Huh. It's like naval rope bondage.

(Which is probably somebody's fetish)

I'm a bit unclear on how degaussing the hull prevents the sub from showing up as a magnetic anomaly. All that would do is keep the vessel from affecting nearby compasses. It still distorts the local geomagnetic field, doesn't it, just by virtue of being made from a ferromagnetic material...?

"All that would do is keep the vessel from affecting nearby compasses."

"It still distorts the local geomagnetic field."

These two statements are trivially identical.

Qualitatively, but not quantitatively. A large metal object might not affect a conventional compass reading in a noticeable way, but if the enemy is flying around with cryocooled Josephson-junction magnetometers or whatever, they'll still see a distortion in the local field.

I'd think that the permeability of a submarine hull -- magnetized or not -- would be very different from that of the surrounding seawater.

Oh it doesn't completely erase the signal, but it minimizes it. P8 Orions have a plasma induction magnetometer in the back that is supposed to be able to see the degaussed submarines. The Russians did something even simpler: they took photographs from space and saw where submarine propellers were causing churn in the ocean cyanobacteria.

I think your understanding of evolution is a bit off, since there are other creatures with common ancestor (birds) that can see into long UV.

And magnetic anomalies, apparently, if they're migratory, via polarisation of uv iirc.

another hypothesis is that spin-coupled electrons preferentially and directionally emit photons from a cryptochrome protein. But tbh, we don't really know yet.

http://www.ks.uiuc.edu/Research/cryptochrome/

Thanks for your aclaration, it helped me understand the idea :)

The degausser is a pier-like structure constructed at shore facilities for submarines.

Here is a degausser for the largest US submarines: https://www.google.com/maps/place/30°46'44.3%22N+81°29'30.6%...

Awesome, thanks for link!

"Oh, it's a degausser"

Ah makes me remember large CRT monitors that went "bonk" when you turned them on (to degauss). I actually still have a Sony 30" TV that does the same thing (and it's quite loud actually).

Going back even older, the first TV's didn't have degaussing coils built in. If you moved the TV, a technician would have to come to your house with a portable coil and do it for you.

Wow what was the reason that moving the TV meant you needed to degauss? Obv. has to do with magnetic field but I always thought it simply had to do with power being turned on (and only because that is when it seemed to degauss).

Simply that moving the TV exposes it to a different localized magnetic field -- just like a compass inside a building. This could also be the result of moving around big loudspeakers near the TV (especially if they aren't magnetically shielded).

Built-in degaussing circuits fire when you turn on the TV presumably because (a) it disrupts the image, so best to do it at turn-on than randomly while you are watching, and (b) because you are unlikely to move the TV while it's on.

It would have been really helpful if the article had made that clear.

It isn't even part of the ship. Its a big coil that the ship pulls into.

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