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From: palmer@tybalt.caltech.edu (David Palmer)
Newsgroups: sci.physics,comp.terminals
Subject: Re: terminals in southern hemisphere
Message-ID: <9580@cit-vax.Caltech.Edu>
Date: 15 Feb 89 18:33:13 GMT
References: <1274@sri-arpa.ARPA> <6569@geac.UUCP>
Sender: news@cit-vax.Caltech.Edu
Reply-To: palmer@tybalt.caltech.edu.UUCP (David Palmer)
Organization: California Institute of Technology
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In article <6569@geac.UUCP> sean@geac.UUCP (Sean Phelan) writes:
>I've just received a message from a colleague in Australia.  The local
>representative of Wyse Technology, whose terminals we are starting to 
>use all over the world, is saying that the terminals have to be aligned
>differently in the Southern hemisphere, because the magnetic flux of the 
>Earth goes the opposite way.
>
>Now, this sounds to me like complete b*llsh*t, perhaps the kind of story
>a salesman uses to get a local order.  When I try dividing the Earth's
>magnetic flux into it's x,y,z vector components and look at how they
>affect a terminal, I conclude that a North facing terminal in the Northern
>hemisphere is going to be affect in EXACTLY the same way as a South facing
>terminal in the Southern hemisphere.

The vertical component of the magnetic field is the opposite in the
two cases (south facing in south hemi. vs. n-facing in n-hemi) even
though the horizontal components are the same (roughly).  This causes
the electron beam in the CRT to curve to the side in opposite directions
in the two cases.  The question is, 'Is this important?'

For a quantitative understanding, the radius of gyration of a particle
with charge Z (-1 for an electron, ignore the minus sign) and energy
E keV (~15 keV for a monochrome terminal, more for color, all numbers
are approximate) in a field of B gauss (it is ~1/2 for the Earth,
the z-component is less still, call it 1/4 gauss just to be simple) is:

		R (cm) = E (eV)/ (300  Z B (gauss))

R = 200 cm. Over a 20 cm path from the gun to the screen, the beam is
deflected by ~ 1mm  (20 cm * (1 - cos(20/200 radians)).  This means
that the whole image is shifted (horizontally) on the order of a millimeter.

This may be important for color systems with their shadow masks,
but I doubt that it makes much difference in a monochrome system.

As a sanity check, pointing the terminal East instead of West should
cause a similar vertical shift.  This seems to work okay.

Just remember, the creature you meet when you talk to customer service
is designed to sell, and he doesn't care whether its toothpaste, terminals
or weapons systems.  As long as he extracts money from your pocket into
his company, he is successful.

Giving him the benefit of the doubt, he may have noticed that the terminals
he gets are often badly aligned.  This may be due to rough shipment, but
he may think that it is due to the magnetic field.

		David Palmer
		palmer@tybalt.caltech.edu
		...rutgers!cit-vax!tybalt.caltech.edu!palmer
	"I was sad that I had no shirt, until I met a man with no torso"