Silva Compass problem

[Drew]

Thanks for the replies. I've emailed Suunto. I'll let you know what they say.

[Drew]

Quite amusing reply from Suunto below. I think based on this I'll assume that my compass will work anywhere in the southern hemisphere. As Grace from Suunto has helpfully pointed out for me, Peru can be found below the equator line, so I think I'm good.

Thank you for contacting Suunto.

Upon checking this compass has different hemisphere wherein we can us this.

Since you purchase this in Australia there is a change that this compass is set to be us in the Southern Hemisphere.

This information where we can know if where the compass is set is located on the box containing the compass.

If you do not have the box anymore than that is alright. What we can do instead is to test the compass once you are in Peru.

When we check it on the map Peru is located just below the equator line. So there might be a chance the compass might not work depending on the location.

I am sorry that this information is not much but this is the only information I have. I was able to partner with the group and we cannot specify which hemisphere the compass is set as both Northern and Southern hemisphere compass has the same face as the one on the photo.

Sincerely,
Grace
Suunto Customer Support Team

[sailfish]

Many people don't seem to think it amounts to much but here are the Silva a Suunto magnetic compass zones for what it's worth.

Silva magnetic compass zones


Suunto magnetic compass zones


Ken

[Orion]

From NOAA's website:




The difference in inclination between Victoria and Peru is about 60°, which is a fair amount. But if you have an inexpensive compass I'll bet it won't matter.

I have a $15 compass I bought in Australia and a $15 compass I bought in the United States. They both work just fine in Australia and in the U.S., despite a 120° inclination difference. How can that be? I believe it is because they are designed to use gravity to compensate for the magnetic torque induced by the inclination. If I tilt either one of these compasses the needle drags. But an inclination of +60° or -60° has very little effect on the needle.

More expensive compasses are more sensitive to the inclination, which is why they come optimized for different locations. They are more sensitive since relying on gravity to correct for the inclination results in a needle that is slower to respond and will have a larger dead zone around magnetic north. At least that's my theory.

[Orion]

You could simply take your compass to Peru and find out. That's the easiest way. Or you could test it at home with a magnet. The best way to do that would be with a Helmholtz coil since they produce nice uniform magnetic fields. But you can get a pretty good idea of the sensitivity of your compass with a simple magnet.

The idea is to find the distance from the compass where the magnet is producing a magnetic field approximately the same as Earth's. Then you can place the magnet in a location to simulate being in Peru. You can't do it exactly because the Earth's magnetic field will be there as well as the field from your magnet. But you can get a pretty good feel for how sensitive your compass is to inclination change.

In Victoria, the inclination is -60° (field points up into the air 60° above horizontal). This means the horizontal component of the Earth's field is 1/2 the full field strength. So if you place the magnet at the same height as the compass and due east of it, it will produce a field at the compass approximately equal to Earth's when the distance from the compass is such that the compass needle points at 60° NE (skipping over the trigonometry I hope I didn't get wrong).

So get the strongest magnet you have and try that. Put the compass on a table and bring the magnet in from the east until the compass needle points to 60° NE. You might have to flip the magnet around if the needle wants to go NW. When I did this with a 5cm magnet from an audio speaker I had to place it about 30cm away from the compass.

Now put the compass near the edge of a table, pointing north (with the magnet far away). Then bring the magnet the same distance as before but now due north and at 60° below horizontal. This will produce a total magnetic field pointing north at 0° inclination, just like you'll find in Peru. Well, kind of. The total field will be stronger, about 1.4 times that of Earth.

But playing around like this you'll quickly see whether your compass responds to inclination changes. When I tried it with my two compasses they showed no signs of tilting at all. I had to bring the magnet a lot closer. The needle didn't drag until I got to about 10cm. Magnets like this have a strength/distance relationship that goes as 1/r^3, so at 10cm the field strength was something like 25 times stronger than the Earth's.

Have fun in Peru!

[Drew]

Wow Orion, that's quite an explanation! Thanks for that.

I have a Suunto A10 compass, which is neither super cheap nor expensive (just checked and it's $40 at Bogong). With a million things to do before I leave I don't think I'll get around to testing with magnets, but hopefully someone will find those instructions helpful! I'll just take my compass and see how it goes in Cusco, and buy a cheap one there if it doesn't seem to work well.