Hi Bob

 BL>  Obviously... to degauss a picture tube you need a long magnet to
 BL> give a greater field 50mm from the end, not a horseshoe. This explains
 BL> why we make degaussing coils like that.

Which means, btw, that the inverse square law is only an
approximation. In antenna terms, we're in the "near field" region,
where the dimensions of the antenna are significant compared with
the distance we are from it.

Tangent - this week I was in a radio hut (not one of ours) on a
hilltop. This hut is fairly well populated, there are something
like 20 UHF repeaters on it. A normal 9600 baud modem on our new
setup was interfering with a 490 MHz receiver (carrier squelch
repeater, no subaudible tones, I've no bloody idea how such a
simple system survives at all on such a crowded site). I'm pretty
sure there's another interfering signal, with the modem just
pushing it over the edge, but I digress: The interference can be
made to come and go just by walking from point A to point B inside
the (metal) building.

 RM> Me magnet, btw, is no amateur. The poles measure 20 x 32 mm,
 RM> and the main body measures 25 x 50 mm. It weighs a bit under a
 RM> kilo.

 BL>  Very impressive... (ho, hum). I think you'll find a degaussing wand
 BL> weighs a kilo too, and is ten-times as effective. 

I have a small one at work - it's a three inch long half inch bolt,
with a pile of wire wrapped round it (50 or 100 turns at a guess).
I hook it up to the secondary of a scope iron transformer, and use
it to degauss colour echo sounder picture tubes (some of them are
pretty gross: to get any brilliance at all a layer of grease,
nicotine, and THC has to be removed with solvent)


 BL>  For your 20 * 32 mm and the wand's 25 * 25mm the area will be the
 BL> same. Of course, the wand is the wrong shape (long and thin) for this,
 BL> just as your horseshoe is the wrong shape to put a high flux density
 BL> into the picture tube and surrounds.

 BL> Flux density is what translates to force.

 RM> Wouldn't half the density over twice the area give the same
 RM> force, given a large target?

 BL>  Not when we are talking about bending a large flat sheet. It's the
 BL> force in the middle that counts, the differential does the deflection,
 BL> not the integral. If it were a round ball, okay.

If I put one bar magnet end-on 2 inches from the sheet, and it
pulls with a force of 10 grams, and then put a second magnet 2
inches away from the first, so a larger area of the sheet feels the
flux, won't the two magnets pull harder on the middle of the sheet
than one? The "middle" of the sheet isn't a point, it's a fair
sized area.

 BL> ... funny story

 BL>  One beautiful late-spring day I turned up for the final exam in
 BL> Theory of Structures 2.13 at UNSW running late, and when I turned over
 BL> the paper I didn't recognise any of the questions! I looked around and
 BL> 
 BL>  But Bob was a clever lad in 1960, and I figured that everyone would
 BL> be as stuffed as I was, so I faked my little heart out for the full 3
 BL> hours and got a Distinction! ROFL! My guess is that the pass mark was
 BL> about 20. I did not have a clue.

I remember a half hour multiple choice test in year 11 - most of it
was dead easy, I did it in ten minutes. In question 7, though, I
didn't have a clue if the answer was b or c. So, making sure the
teacher could see, I wrote a "b" on one side of a rubber, a
"c" on
the other, and tossed it like a coin.

I can't remember if I got it right, though.

I also remember an exam in year 9, sitting next to a year 12 bloke
doing the state-wide public Senior exam. His exam was biology, and
the question that stopped him was something like "Describe, with
diagrams, the skin of a frog." He looked at it for several minutes,
then carefully wrote

 We did not do the frog.

 Skin of a shark?

and proceeded to describe a shark skin.



 RM> Stainless?? Isn't stainless nonmagnetic?

 BL> ROFL! That's the general idea...

 RM> Are you sure it's stainless?

 BL>  Why would they do it any other way?

There's no oxygen in there to rust it, it's a bit softer so your
drill bits or punches don't get blunt as quickly, and it's a lot
easier to stick things to than to stainless steel. Solder, for
instance.

 RM> I just tried my magnet on our main tv again, and from 250mm
 RM> it's made the centre of the screen purple. Just the centre 10%,
 RM> not the edges. After ten frantic fruitless futile minutes
 RM> searching for the little tiny tape head degausser I'm sure i
 RM> had ten years ago, I took the plunge and tried the big magnet
 RM> the other way round, and removed most of it.

 BL>  ROFL! You're a demon for experiments. It'll eventually degauss
 BL> itself over the next few weeks. You have to let the posistor cool down
 BL> (ten minutes) before the internal degaussing will work again.

It's going away, but very slowly, as you say.

 BL> I can
 BL> guarantee that you have not bent anything, or magnetised the centre
 BL> of the shadowmask (except slightly). God knows what you've magnetised
 BL> to affect the centre,

Presumably the shadowmask? The discoloured patch was even slightly
rectangular, oriented horizontally, which is how I was holding the
magnet.


 BL>  I tried to explain, and you said you already knew. It's about
 BL> angles.

I do know. Wasn't born yesterday. Day before.


 BL>  Our display technology seems to have reached a dead end. The Japs
 BL> are great at pushing the limits and refining, but the profits are all
 BL> going to the wrong place... Asia where no one has ever had an original
 BL> thought since Ghengiz Khan made pyramids of human skulls.

 BL>  He didn't take the technology far enough, IMO.

I read somewhere unreliable recently that the Chinese navy had
reached India, Ceylon, and bits of East Africa before the Europeans
did, but gave it up when bureaucrats in the government, scared that
they were losing influence, shut down the navy's big ship program.

Cheers

--- PPoint 1.88