Merideth's comments call to mind an old paper by Tom Leiper on  a horizontal 
seismograph he built using the magnet from a horn driver. It also  used a 
circular coil. See:
 
   _http://psn.quake.net/leiper/seismograph.html_ 
(http://psn.quake.net/leiper/seismograph.html) 
 
  In comments to me about the article Tom said:
 
  "I just happened to have a speaker from my boat hailer that was  "old" and 
needed to be replaced with a new model (later this spring) and could  be 
"sacrificed" to scientific experimentation. After unscrewing the horn from  the 
transducer, I removed the cover plate and voice coil assemblies. The magnet  
assembly is the usual "wrap around case" design which results in a circular gap  
for the voice coil with extremely high field strength in the gap between the  
inner pole plate and the outer pole ring. There was virtually no fringe field,  
but once my "test" block came in contact with the center pole it was almost  
impossible to remove. The magnet weighs five pounds and the gap is about 1.5  
inches in diameter.
I spun down an old chunk of iron rod to the same  diameter as the inner pole 
plate and adjusted the length (thus weight) down to  the point where it would 
balance the whole assembly at the face of the outer  ring of the magnet when 
placed on edge. A bead of epoxy is all that is required  to keep the iron bar 
from slipping once you have it perfectly centered in the  pole plate. The epoxy 
also seals the gap from foreign bodies (and domestic, for  that matter). The 
outer pole ring conveniently has a ring of 8-32 pem-nut  fasteners included at 
no extra charge, so a simple aluminum plate can be  fashioned to attach the 
whole mess to the end of your boom. No additional mass  is necessary as the 
entire assembly weighs in at about 8 pounds."
 
---Bob






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  Merideth's comments call to mind an old paper by Tom Leiper=
 on=20
a horizontal seismograph he built using the magnet from a horn driver. It al=
so=20
used a circular coil. See:
 
   http://psn.quake.net/l=
eiper/seismograph.html
 
  In comments to me about the article Tom said:
 
  "I just happened to have a speaker from my boat hailer that was=20
"old" and needed to be replaced with a new model (later this spring) and cou=
ld=20
be "sacrificed" to scientific experimentation. After unscrewing the horn fro=
m=20
the transducer, I removed the cover plate and voice coil assemblies. The mag=
net=20
assembly is the usual "wrap around case" design which results in a circular=20=
gap=20
for the voice coil with extremely high field strength in the gap between the=
=20
inner pole plate and the outer pole ring. There was virtually no fringe fiel=
d,=20
but once my "test" block came in contact with the center pole it was almost=20
impossible to remove. The magnet weighs five pounds and the gap is about 1.5=
=20
inches in diameter.
  I spun down an old chunk of iron rod to the sa=
me=20
diameter as the inner pole plate and adjusted the length (thus weight) down=20=
to=20
the point where it would balance the whole assembly at the face of the outer=
=20
ring of the magnet when placed on edge. A bead of epoxy is all that is requi=
red=20
to keep the iron bar from slipping once you have it perfectly centered in th=
e=20
pole plate. The epoxy also seals the gap from foreign bodies (and domestic,=20=
for=20
that matter). The outer pole ring conveniently has a ring of 8-32 pem-nut=20
fasteners included at no extra charge, so a simple aluminum plate can be=20
fashioned to attach the whole mess to the end of your boom. No additional ma=
ss=20
is necessary as the entire assembly weighs in at about 8 pounds."
 
---Bob

 
 


See what's new=
 at AOL.com an=
d Make AOL Your Homepage.