At what height do navigation compasses stop working?
Mike Follows Sutton Coldfield, West Midlands, UK
Advertisement
A compass needle is essentially a thin bar magnet. If it were free to move in all three dimensions, a magnet would line up along any local magnetic field line. Field lines are graphical representations of the strength and direction of the magnetic field in a region of space.
In principle, a navigation compass will work anywhere there are magnetic fields (which exist throughout the universe), although this doesn’t guarantee that it would be helpful as a navigational aid.
Earth’s magnetic field, the magnetosphere, extends to at least two or three times the radius of the planet and is limited by the influence of the solar wind.
Using a compass for navigation on the surface of Earth has its problems, particularly at the poles or if you switch between the hemispheres.
The compass needle is suspended on a tiny plinth. From side-on, this looks like a miniature see-saw. It is immersed in a transparent fluid, usually oil, to cushion the delicate mechanism from shock and dampen the movement of the needle.
The field lines are almost parallel to Earth’s surface at the equator and are inclined to Earth’s surface at an angle that increases with latitude until they are almost perpendicular to Earth’s surface at the poles.
This means that, in the northern hemisphere, the north end of the needle will dip downwards (and the south end will rise up). But navigators are taught to hold map and compass horizontally and, to ensure that the needle is free to swing in the horizontal plane, a weight is added to the south side. This means that a compass designed for use in the northern hemisphere shouldn’t be used in the southern hemisphere and vice versa.
The dipping of the needle is one of the reasons why magnetic compasses are next to useless at the poles and why the Vikings resorted to other navigation aids, probably including the use of Iceland spar. These crystals can be used to detect the polarisation of sunlight and infer the position of the sun even just after sunset.
To answer this question – or ask a new one – email lastword@newscientist.com.
Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.
¿ìè¶ÌÊÓƵ retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.