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DIYSteve
seeking hygge
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DIYSteve
seeking hygge
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 Mon Apr 09, 2018 10:56 am |
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| mb wrote: | | A friend reported that a new compass he bought was approximately 90 degrees off |
A needle compass? How is that possible?
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Bernardo
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Bernardo
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Mon Apr 09, 2018 12:44 pm |
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| DIYSteve wrote: | | mb wrote: | | A friend reported that a new compass he bought was approximately 90 degrees off |
A needle compass? How is that possible? |
Good point.
A compass with a built in declination correction mechanism has a arrow that can be set to not point to magnetic north. Could it be set to be 90 degrees off?
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DIYSteve
seeking hygge
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DIYSteve
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Mon Apr 09, 2018 12:50 pm |
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| Bernardo wrote: | | A compass with a built in declination correction mechanism has a arrow that does't point to magnetic north. |
Well, the needle points to magnetic N, of course. The correction happens via dialing to the appropriate declination setting.
| Bernardo wrote: | | Could it be set to be 90 degrees off? |
That'd be maximum user error. I'd avoid going into the mountains with that guy.
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Bernardo
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Bernardo
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Mon Apr 09, 2018 6:06 pm |
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As is usually the case, DIY Steve is right. The compass needle in a functioning compass always points to north even if a declination adjustment has been made. How could it be otherwise given that a compass can really only do one thing? The declination adjustments are mechanical hacks to make it seem like the needle is pointing somewhere else.
What about in a "malfunctioning" compass? I know a compass can be 180 degrees wrong if the needle reverses polarity, but if the needle can move freely and there is no magnetic interference, can it ever be partially wrong? This would require the polarity of the needle to not be along its length. Does this ever happen?
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Malachai Constant
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| Bernardo wrote: | | What about in a "malfunctioning" compass? I know a compass can be 180 degrees wrong if the needle reverses polarity, but if the needle can move freely and there is no magnetic interference, can it ever be partially wrong? This would require the polarity of the needle to not be along its length. Does this ever happen? |
The poles will migrate to the ends of an elongated object even if they were magnetized at an angle. It rectangular or circular they might be at an angle. As described above if the needle were glued on it might shift, but that sounds like a lousy way to make a compass but it may be possible in the type where the needle is attached to a larger card.
"You do not laugh when you look at the mountains, or when you look at the sea." Lafcadio Hearn
"You do not laugh when you look at the mountains, or when you look at the sea." Lafcadio Hearn
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Windstorm
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| Malachai Constant wrote: | | Bernardo wrote: | | What about in a "malfunctioning" compass? I know a compass can be 180 degrees wrong if the needle reverses polarity, but if the needle can move freely and there is no magnetic interference, can it ever be partially wrong? This would require the polarity of the needle to not be along its length. Does this ever happen? |
The poles will migrate to the ends of an elongated object even if they were magnetized at an angle. It rectangular or circular they might be at an angle. As described above if the needle were glued on it might shift, but that sounds like a lousy way to make a compass but it may be possible in the type where the needle is attached to a larger card. |
That's interesting. If the needle is magnetized at an angle, do you know if the poles will migrate perfectly to the ends of the needle? A surveyor was testing compasses for a navigation class I took and about 90% of the compasses in that particular batch (including mine) were off by about 2.5 degrees. It was easy enough to adjust the declination to compensate.
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BigBrunyon
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DIYSteve
seeking hygge
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DIYSteve
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Wed Apr 11, 2018 5:15 pm |
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| Windstorm wrote: | | A surveyor was testing compasses for a navigation class I took and about 90% of the compasses in that particular batch (including mine) were off by about 2.5 degrees. |
Can you be more specific? Are you saying the needles pointed 2.5* off of magnetic N? That makes no sense (unless some relatively nearby magnetized mass were pulling the needle off magnetic N, in which case all of the compasses should have been affected). As Mal noted, magnetic poles of a long thin magnet align along the lengthwise axis of the mass of magnetic material. They can completely flip N-to-S, but not partially.
Or were the calibration lines incorrectly marked?
Or were the needles bent?
Or was the surveyor working with the incorrect declination, e.g., working with an old map? That's possible: Magnetic declination in the WA Cascades has changed nearly 5 degrees since I moved to the PNW. Here's a cool calculator of magnetic declination by zip code and year: https://www.ngdc.noaa.gov/geomag-web/#ushistoric
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Brian Curtis
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Wow, I knew the declination had changed but I didn't realize how much. Thanks for the link, that was a cool site.
that elitist from silverdale wanted to tell me that all carnes are bad--Studebaker Hoch
that elitist from silverdale wanted to tell me that all carnes are bad--Studebaker Hoch
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Windstorm
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| DIYSteve wrote: | | Can you be more specific? Are you saying the needles pointed 2.5* off of magnetic N? That makes no sense (unless some relatively nearby magnetized mass were pulling the needle off magnetic N, in which case all of the compasses should have been affected). |
It was my understanding that the needles pointed 2.5 degrees off. We were pretty careful about possible magnetic interference from things like headlamp batteries, so I don't think it was that. I didn't know that magnetic poles could realign along the length of the needle, so I assumed that the needle was cut 2.5 degrees off. I'm open to being wrong about that.
| DIYSteve wrote: | | Or were the calibration lines incorrectly marked? Or were the needles bent? |
It's definitely possible, but I don't have enough information to know if either was the problem.
| DIYSteve wrote: | | Or was the surveyor working with the incorrect declination, e.g., working with an old map? That's possible: Magnetic declination in the WA Cascades has changed nearly 5 degrees since I moved to the PNW. Here's a cool calculator of magnetic declination by zip code and year: https://www.ngdc.noaa.gov/geomag-web/#ushistoric |
That's a nice calculator! I'm pretty sure the surveyor had the right declination. We were using ~16 degrees in the Seattle area in 2014.
Sorry I don't have more info on the actual problem with my compass. I figure that for most cases, 2.5 degrees won't make that much difference. If your survival requires you to navigate that precisely, you likely have other problems.
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