In a message dated 2008/05/16, paleoartifact@......... writes:

> Am getting to the (shall we say a unqualified) opinion that with a choice 
> of round or square / rectangular coils with a magnet, that it seems like the 
> round coil approach is the more efficient route to take. 

Hi Meredith,

       If you want an accurately linear output from a Lehman, while allowing 
for the large sensor drifts that you always get, you definitely need to use a 
quad arrangement of square NdFeB magnets and a rectangular coil.

       Round coils will DEFINITELY NOT give you a constant sensitivity over 
the drift range of a Lehman sensor. 
       If you are using a vertical pendulum, you can use a quad arrangement 
of rectangular magnets with the width of a magnet ~ the ID of the coil. 
Alternatively, you can make up a magnet block and attach it to the pendulum to form 
part or all of the mass. But you will then still get a small sensitivity to 
environmental magnetic fields.

       You make the ID of the actual windings about 3/4 of the width of a 
magnet and the internal length about 1/5 greater. This allows for some position 
error either as the coil swings in an arc or for any setup position error.  The 
flux change for a given magnet movement is then CONSTANT over maybe 3/4" 
drift with 1" square magnets.

       Mount the magnets on 1/4" thick mild steel backing plates held apart 
with zinc plated mild steel bolts. You need to give the mild steel some etch 
primer and a coat of anti rust paint. I use Hermetite. You paint the whole 
surface of the plates, then leave them a couple of days for the paint to harden. I 
make up a cardboard cut-out, clamp it to the plate and slide the magnets into 
position over the cardboard. This is 'the easy way to do it'!

       One advantage of this construction is that you only need maybe 2,500 
turns on you coil. The voltage output is maybe 10x what you get with a U Alnico 
magnet.

       This arrangement is shown on the drawings at 
http://www.jclahr.com/science/psn/chapman/lehman/index.html

       It is quite easy to make a rectangular coil former out of thin glass 
circuit board / sheet glass fibre. DigiKey sell both. You can either use the 
construction shown, or you can make a wood / plastic inner coil former and screw 
/ glue it between two sheets to form the ends of the coil. I suggest winding 
it using a bolt through the centre gripped by a hand drill, held in a vice. 
Use some fine plastic tube to guide the wire onto the coil. Spray cans usually 
have suitable tube if you open one up. I use two part Acrylic glue - Devcon 
Plastic Welder or similar. 

       I defiitely avoid using 'coil dope' - I find that I get too many 
shorted turns. You can use coil dope OK if you very carefully wind a single layer 
of wire at a time and then use tissue paper to interleave the winding layers.

       With luck, you are only going to ever make one coil per seismometer, 
so it REALL IS worth a bit of extra effort to get the best out of your 
instrument.

       Regards,

       Chris Chapman   
In a me=
ssage dated 2008/05/16, paleoartifact@......... writes:



Am getting to the (shall we say=
 a unqualified) opinion that with a choice of round or square / rectangular=20=
coils with a magnet, that it seems like the round coil approach is the more=20=
efficient route to take. 




Hi Meredith,



       If you want an accurately linear output=
 from a Lehman, while allowing for the large sensor drifts that you always g=
et, you definitely need to use a quad arrangement of square NdFeB magnets an=
d a rectangular coil.



       Round coils will DEFINITELY NOT give yo=
u a constant sensitivity over the drift range of a Lehman sensor. 

       If you are using a vertical pendulum, y=
ou can use a quad arrangement of rectangular magnets with the width of a mag=
net ~ the ID of the coil. Alternatively, you can make up a magnet block and=20=
attach it to the pendulum to form part or all of the mass. But you will then=
 still get a small sensitivity to environmental magnetic fields.



       You make the ID of the actual windings=20=
about 3/4 of the width of a magnet and the internal length about 1/5 greater=
.. This allows for some position error either as the coil swings in an arc or=
 for any setup position error.  The flux change for a given magnet move=
ment is then CONSTANT over maybe 3/4" drift with 1" square magnets.



       Mount the magnets on 1/4" thick mild st=
eel backing plates held apart with zinc plated mild steel bolts. You need to=
 give the mild steel some etch primer and a coat of anti rust paint. I use H=
ermetite. You paint the whole surface of the plates, then leave them a coupl=
e of days for the paint to harden. I make up a cardboard cut-out, clamp it t=
o the plate and slide the magnets into position over the cardboard. This is=20=
'the easy way to do it'!



       One advantage of this construction is t=
hat you only need maybe 2,500 turns on you coil. The voltage output is maybe=
 10x what you get with a U Alnico magnet.



       This arrangement is shown on the drawin=
gs at http://www.jclahr.com/science/psn/chapman/lehman/index.html



       It is quite easy to make a rectangular=20=
coil former out of thin glass circuit board / sheet glass fibre. DigiKey sel=
l both. You can either use the construction shown, or you can make a wood /=20=
plastic inner coil former and screw / glue it between two sheets to form the=
 ends of the coil. I suggest winding it using a bolt through the centre grip=
ped by a hand drill, held in a vice. Use some fine plastic tube to guide the=
 wire onto the coil. Spray cans usually have suitable tube if you open one u=
p. I use two part Acrylic glue - Devcon Plastic Welder or similar. 



       I defiitely avoid using 'coil dope' - I=
 find that I get too many shorted turns. You can use coil dope OK if you ver=
y carefully wind a single layer of wire at a time and then use tissue paper=20=
to interleave the winding layers.



       With luck, you are only going to ever m=
ake one coil per seismometer, so it REALL IS worth a bit of extra effort to=20=
get the best out of your instrument.



       Regards,



       Chris Chapman