Hello,
In addition to what has been said already, I'd like to add / clarify a couple of points.
After a lightning strike or nearby, the actual compass is normally functioning fine...i.e. it is still magnetised along the normal axis of the 'needle'.
(Some older marine compasses have been known to have their polarity completely reversed, but the axis is still the same.)
The problem with the compass error after the lightning strike is usually due to the 'ferrous' metals around (in the cockpit and on the aircraft) being magnetised. The mechanism for this happing has been explained already.
To answer Cron's comments:
All magnetic objects near or far will affect your aircraft's compass.
Magnetic force between two objects is inversely proportional to distance between them. So if you increase the distance, the magnetic attraction / repulsion decreases. (I forget the formula.)
Bearing this in mind, the relationship between magnetic objects and the compass deflection is simply explained
Edited: Or rather....let me try to explain it simply!
If an object's magnetic strength is greater than that of the earth's, then it will cause a deviation from magnetic North. While individually, they may not be enough, an accumulation of weak magenetic objects in one area of the aircraft may be enough to influence the compass. Also, in particluar orientations, the lines of magnetic flux may be enough to cause an error.
This is a
simple explanation, but might help.
Regards,
cl12pv2s
Nick: Thanks for the book suggestion...I'm going to order it tomorrow!
Some futher reading I found...
Lightning strikes are due to flow of charges between the ground and the clouds. The flow of current creates a magnetic field around it and the direction of the field is given by what we called the right-hand rule: point the thumb of your right hand in the direction of the current, and your fingers indicate the direction of the circular magnetic field lines around the current flow. If we imagine that the lightning flows through a straight wire that is connected from the clouds to the ground, we can use the Ampere's law or the Biot-Savart Law to find the magnetic field due to a steady current flowing in this very long straight wire. Let us assume that the wire is infinitely long. For an infinitely long straight wire, the magnetic field at a point a distance r from the wire carrying a current I has magnitude given by B = µ0 I / 2pr where B is the magnetic field strength (units in tesla) and the constant µ0 is the permeability of free space with a value of 1.26 x 10-6 tesla metre ampere-1. The current in an average lightning bolt is as high as 30,000 amperes and its short duration of about 0.1 seconds might just deflect the compass needle momentarily. The Earth's magnetic field strength is about 0.6 x 10-4 tesla on the Earth's surface at the poles. It is about 0.3 x 10-4 tesla on the Earth's surface at the Equator. Let us also assume that the compass is placed such that the magnetic field created in a lightning is perpendicular to the Earth's magnetic field and that a magnetic field strength of about 10% of the Earth's magnetic field strength is able to cause an observable deflection in the compass needle. Using the Earth's magnetic field strength of about 0.6 x 10-4 tesla and the formula for B given above, we get a value for r =1002 m. This implies that if the compass is at the position as assumed above, there should be an observable deflection of the needle even at a distance of about 1 km from the lightning bolt, though only momentarily.
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