In a message dated 27/03/2006, Bobhelenmcclure@....... writes:


Place pairs on quarter-inch or thicker steel plates and hold the plates  
apart with stove bolts. Assemble very carefully, using a thick shim to hold  the 
gap, and then jack the gap open with the bolts to release the shim. Wear  heavy 
gloves! For a gap of 0.250 inches, you get a field of 8000 gauss. A  
rectangular coil of 1100 turns yields 85v-s/m output. You won't need separate  damping 
magnets. A shunt resistor will provide the  damping.



Hi Bob,
 
    What are 'stove bolts' please?
 
    You may need rather thicker plate than 1/4" with  two side by side 18 mm 
x 6 mm thick magnets. The 5 mm plate that I used at  first was only just thick 
enough to take the flux of the 1/2" x 1/4" thick  magnets.
 
    Are you using a pair of magnets on one plate and  then putting a bare 
iron plate over the top, or do you use four magnets on  two plates? I have found 
using four magnets gives a more constant central field.  It also allows a bit 
more thread to fit the two central nuts.
 
    The reason why I do not use this sort of damping is  that I want the 
damping and the sensitivity to be independent. The damping that  you need is a 
function of the period. Setting up a system where the sensitivity  and the 
damping are both a function of the period is a bit more difficult the  second time.
 
    I use the wide copper damping plate as a mechanical  stop. The plate edge 
can never move into the high field region at the edge  of the magnets. If 
your coil drifted off the magnet, the damping would go  to zero. I have also had 
some variable force effects when the copper wire is  close to the edge of a 
magnet. Not all copper wire seems to have the same  diamagnetic properties.
 
    Is the 85V/m/sec figure correct? It does not sound  very high. 
 
    Regards,
 
    Chris Chapman






In a message dated 27/03/2006, Bobhelenmcclure@....... writes:
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Place pairs on quarter-inch or thicker steel plates and hold the plat=
es=20
  apart with stove bolts. Assemble very carefully, using a thick shim to hol=
d=20
  the gap, and then jack the gap open with the bolts to release the shim. We=
ar=20
  heavy gloves! For a gap of 0.250 inches, you get a field of 8000 gauss. A=20
  rectangular coil of 1100 turns yields 85v-s/m output. You won't need separ=
ate=20
  damping magnets. A shunt resistor will provide the=20
damping.

Hi Bob,
 
    What are 'stove bolts' please?
 
    You may need rather thicker plate than 1/4" wit=
h=20
two side by side 18 mm x 6 mm thick magnets. The 5 mm plate that I used=
 at=20
first was only just thick enough to take the flux of the 1/2" x 1/4" thick=20
magnets.
 
    Are you using a pair of magnets on one plate an=
d=20
then putting a bare iron plate over the top, or do you use four magnets=
 on=20
two plates? I have found using four magnets gives a more constant central fi=
eld.=20
It also allows a bit more thread to fit the two central nuts.
 
    The reason why I do not use this sort of dampin=
g is=20
that I want the damping and the sensitivity to be independent. The damping t=
hat=20
you need is a function of the period. Setting up a system where the sensitiv=
ity=20
and the damping are both a function of the period is a bit more difficult th=
e=20
second time.
 
    I use the wide copper damping plate as a mechan=
ical=20
stop. The plate edge can never move into the high field region at the e=
dge=20
of the magnets. If your coil drifted off the magnet, the damping would=20=
go=20
to zero. I have also had some variable force effects when the copper wire is=
=20
close to the edge of a magnet. Not all copper wire seems to have the sa=
me=20
diamagnetic properties.
 
    Is the 85V/m/sec figure correct? It does not so=
und=20
very high. 
 
    Regards,
 
    Chris Chapman