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Brooke Clarke asked-

>I have seen the terms "Soft Iron" and "Hard Iron" used with reference to
>calibrating a magnetic compass, like is used in a car.

Yes, though there are not actually two types of iron (or steel) involved.
The one piece of steel can have its own built-in magnetism which stays with
it and turns as the heading of the vehicle (or vessel) turns; and can ALSO
show induced magnetism, in which it picks up, and concentrates, the Earth's
magnetic field. Describing it in terms of hard and soft iron is just a way
of portraying that concept, though hard steels certainly do retain
permanent magnetism better than soft iron does

>I think "Hard Iron" implies that the iron is acting as a magnet causing
>an error in the indicated compass bearing that's independent of the
>heading of the car.

Correct, except that the error isn't independent of the heading; it's like
two vectors summing together, the main one fixed to the Earth, the smaller
one turning with the vehicle. The error is greatest when the two are at
right-angles. There will be two opposite headings at which the compass is
unaffected by this permanent magnetism, often (not always) when the heading
is near magnetic North or South. It depends on whether the permanent
magnetism is along the centreline of the vessel (or not).

>I think "Soft Iron" implies that the iron in the car distorts the
>Earth's magnetic filed causing an error that depends on the car's heading.

True. The effects of this induced magnetisn are usually minimal or zero
when the heading is near magnetic North, South, East, or West, and maximum
in between, at NE, SE, SW, NW.

With these two effects giving rise to such different error-patterns, which
sum together as a complex overall compass-error, it's up to the
compass-adjuster to sort out how much of each component there is, and to
compensate the resulting magnetic field around the compass accordingly. He
uses permanent magnets to compensate for the permanent component, and
non-magnetised iron balls to compensate for the induced component. Those of
us with non-steel vessels will usually do no more than measure and record
that compass-error, and then allow for it when we use the compass.

George

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