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Re: [T3] My Notch's Tranny

On 7 Nov 2002, at 19:27, Keith Park wrote:

> Really??  I probably should have started this up with Jim on rec.antiques
> r+p but the philco antique radio shadow meters from the 30's generally work
> so crappy now because the little horseshoe magents in them lose their
> potentcy... or this is what was explained in the past... are there other
> things at work here??

I'm not familiar with the Philco shadow magnets. 

Things that WILL demagnetize permanent magnets: mechanical shock, 
thermal cycling, magnetic cycling. The choice of magnetic material can 
generally be made to make these things stable against whatever enviroment 
they are in, but poor choice of materials is always possible.

Some magnets are very susceptable to being demagnetized by having them 
disassembled. If there is another part in the magnetic path and you remove 
that part, then the magnetic field is diminished. The material may not recover 
when you put the magnetic material back together. Sturmey Archer dyno-
hubs are this way: If you take one apart you MUST slide the magnet off the 
armature and directly onto a special keeper if you want to avoid reducing the 
field by half. Magnets like this are magnetized with something in their gap 
and kept there all their lives. If the object is removed without a keeper, you 
may lose as much as half the field strength. This is just one form of 
demagnetization by magnetic cycling.

Some magnets can also be sensitive to having a tool, like a screwdriver, 
stuck in them to see if they are magnetic. Each time this is done the field 
may come down a bit, until they reach a stable lower level.

> It doesnt stick on the pin, it comes off a little... then goes up rapidly
> between say 15MPH and 25 or 30... then behaves more normally again...

Is your foot just heavy? Our second gear acceleration is always a bit better 
than third. ;-)

Is there anything sticky about your pivot points? I agree that this sounds odd.

Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA

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