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I believe you will find that it is the heeling magnet that must be
changed end for end when crossing the magnetic equator - neglect of this
nicety will adversely affect the magnetic compass. The Flinders Bar is
utilized as a soft iron corrector, as are the navigator's balls, for
induced magnetism and is itself subject to induced magnetism - no purpose
being served by reversing it. The heeling magnet is a permanent magnet
and thus may well over-correct if not maintained in proper position. I
must confess to having sailed from Artic to Antartic Oceans by magnetic
compass and never experienced any problem with tilt on a 7-1/2" card
compass mounted in a conventional compensating binnacle, with the heeling
magnet reversed on crossing the magnetic equator - please take note that
I did not say there was no tilt whatsoever, just that it never was a
problem.

On Sun, 20 Jun 2004 21:06:16 +0000 "Trevor J. Kenchington"
 writes:
> Bob Peterson wrote:
>
> > Back to that world traveler:  what are they to do?  My
> recommendation is
> > to purchase additional "cores" for their binnacle compass.  Then
> as they
> > change zones and the card tilt bottoms out and the card binds,
> change
> > out the "core" to a new zone.  In theory, the compass correction
> should
> > not change, in practice, it does.  So best to check it and build a
> new
> > deviation card.   In my book, data always outweighs theory.
>
>
> When I moved to Australia, in the 1980s, I took with me a sighting
> compass originally purchased in England (one of the Morin "hockey
> puck"
> type). It had worked well enough in Nova Scotia but in Tasmania the
> card
> dipped so far that it was impossible to take bearing sights. I
> figured
> out that the card must have been balanced for a north-down dip and
> was
> thrown off by the south-down dip around 45 South latitude. (When I
> moved
> back to Nova Scotia a few years later, the compass became fully
> function
> again, so the problem wasn't some sort of breakage of the
> instrument.)
>
> One day when out in the Tasman Sea with nothing better to do, I
> mentioned the problem with my compass to our research-ship captain
> (who
> held a British Master Mariner's ticket) and he initially denied that
> there could be any such problem, on the grounds that he had taken
> ships
> from one hemisphere to the other without their compass cards ever
> tilting in response to magnetic dip. Then he relented and said that
> there was one vertical magnet in a ship's binnacle (the "Flinders
> Bar"
> perhaps?) which had to be reversed, end-for-end, when crossing the
> Equator and he suggested that maybe that adjustment prevented the
> dip
> problem that afflicted my sighting compass.
>
> So ... do big-ship magnetic compasses (still carried as back-up to
> their
> gyros, so far as I know) dip more than Captain Sheridan realized? Is
> there some routine of changing cores when crossing zonal boundaries,
> as
> Bob suggests, which the captain did so automatically that he had
> forgotten its significance when talking to me? Or does a single
> adjustment when crossing the Line suffice for a binnacle compass?
>
>
> No doubt the answers are in Bowditch and other textbooks. But
> digging
> them out of such sources is beyond me just now.
>
>
> Trevor Kenchington
>
>
> --
> Trevor J. Kenchington PhD                         Gadus@iStar.ca
> Gadus Associates,                                 Office(902)
> 889-9250
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> 889-9251
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>
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>

   

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