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

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

 It's not just the size of the antenna, btw, it's more ground effect
or reflections. You get a similar effect if you plot the output of an
ultrasonic transducer. From memory, it's 3/2 law close up, going to
square-law after 10 wavelengths or so. It's easier to understand with
a magnet, where you can look at the actual field with iron filings.
It's the cross-section.

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

 In my Amateur Radio days I just kept putting up more antennas until
the back yard looked like a spider's web and the shed was like a
pincushion. Everythign interracted with everything else, but in the
end I got them all tuned up. I used to tune the 15-metre wire by
rotating the 2-metre beam.

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

 My old TV set is 25 years old. A few years back, the picture tube
was going dim. I gave it a wash and doubled the brightness!

... (1) BL> Flux density is what translates to force.

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

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

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

 Of course that's true, but it's nothing like your original
statement, marked (1) and (2) above. I was trying to hint that you
were on the wrong track. You seem to have missed the hint, btw.

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

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

 ROFL! The worst multiple-choise experience I had was the AOCP exam.
They had five choices to each question, and none of them were right! I
had to take off my engineer hat and put on a technician hat. The joke
was thatthe pass-mark was 70%. A monkey could have got 50% at random.

 They had mad questions like: 10V at 1A needs a :

1.  1W resistor
2.  2W resistor
3.  5W resistor
4.  10W resistor
5.  20W resistor  

 The correct answer was 15W, unless there were peaks involved,
expecially when a 20W resistor more than doubled the price.

 10W was the answer they wanted, I think.

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

RM> We did not do the frog.

RM> Skin of a shark?

RM> and proceeded to describe a shark skin.

 ROFL! There is a certain sad piquancy about that. What a dreamer.

RM> Are you sure it's stainless?

BL> Why would they do it any other way?

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

 It's not 18/8, it's some other partially-magnetic alloy that comes
out black. It's etched btw, not punched, and they don't solder it. It
goes in a frame like a flyscreen.

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

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

 You're talking residuals of a gauss or so, about one-hundredth of
what you get when you magnetise a needle by rubbing it with a magnet.
It probably *is* the shadowmask, but it doesn't have to be. If you
stick a magnet in the middle, it can magnetise the surrounds in  way
that only affects the middle.

 We had a perfectionist at Pye, and after he'd set his picture tube
up perfectly I added hidden magnets. The trick was to spread them
around, so the impurity showed up where they weren't. That way, he
couldn't find them. The best one was a sliver of plastic magnet
*inside* the yoke. He could never get it right no watter what he did.
So I swapped tubes and set it up no trouble at all. ROFL!!    

 We did terrible things to poor little Russ. At one stage we had him
convinced he was colour blind! There's a trick of flashing a yellow
window on a blue background - it looks pink. We all swore we saw
yellow. There's a whole range of these optical illusions where the
colours look wrong. He twigged to that one, finally.

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

 I think that's true. God knows where I read it, though. The Chinese
were great sailors but lousy navigators. They kept in sight of land 
and worked right around to Iran. Henry the Navigator did pretty-much
the same thing, coming the other way from Portugal. The trick was to
be able to sail into the wind. The Arabs could do that too.

 China's history should be enough warning against a bureaucracy, but
we do persist. The reason Communism collapsed was the bureaucracy.
IMO, the only answer to limit governemnt payroll to 1% of tax, written
into the Constitution.

Regards,
Bob
       

--- BQWK Alpha 0.5