## PSN-L Email List Message

Subject: Re: Homade Geophone
From: ChrisAtUpw@.......
Date: Thu, 4 Aug 2005 21:09:16 EDT

```In a message dated 05/08/2005, charles.r.patton@........ writes:

This has  two problems.
1) Geometrically the difficulty is to have an output for any  direction
movement. It would be easier to have two coils set for orthogonal  output then do
the vectorial addition in the PC (which would take two A/D  channels also) .
The PC could also give direction at the same time.
It can be done with analog multipliers also if you're a bit  of a
masochist.
However, it is fairly easy to make an orthogonal  sensor using a square
end mirror and four 8 sq mm photo diodes. I can  get a noise level of about 15
nano metres. Maybe a square hole in an optical  shutter with a light on one
side and the photo diodes on the other?

2)  Having the magnet on the boom makes you very subject to variable
magnetic  fields all around, so then it becomes were you measuring a
magnetic field  variation or an earth movement.
Unless the field is enclosed, you will pick up a  lot of AC power
interference. Even with just a soft iron weight, you will  pick up some unwanted
magnetic noise.

So I  would suggest you set up your Foucault pendulum with two orthogonal
coils  standing up, and put the single magnet pole on the floor pointing directly
into the bottom of the bob.
Charles Patton
?? My magnets all come with two poles. You could  use two rectangular
coils and two sets of N+S NdFeB bar magnets pairs, at right  angles on the end of
the bob. Maybe with soft iron plates top and bottom and  mild steel bolts to
enclose the fields?

I would prefer to put the coils on the end of a  brass bob weight and sit
the magnets and the soft iron plates comfortably  on the ground.

Chris Chapman

In a message dated 05/08/2005, charles.r.patton@........ writes:
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>This has=20
two problems.1) Geometrically the difficulty is to have an output for=20=
any=20
direction movement. It would be easier to have two coils set for orthogona=
l=20
output then do the vectorial addition in the PC (which would take two A/D=20
channels also) . The PC could also give direction at the same time.=20
It can be done with analog multipliers also if you're a b=
it=20
of a masochist.
However, it is fairly easy to make an orthogona=
l=20
sensor using a square end mirror and four 8 sq mm photo diodes. I=20=
can=20
get a noise level of about 15 nano metres. Maybe a square hole in an optical=
=20
shutter with a light on one side and the photo diodes on the other?
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>2)=20
Having the magnet on the boom makes you very subject to variable magne=
tic=20
fields all around, so then it becomes were you measuring a magnetic fi=
eld=20
variation or an earth movement.
Unless the field is enclosed, you will pick up=20=
a=20
lot of AC power interference. Even with just a soft iron weight, you wi=
ll=20
pick up some unwanted magnetic noise.
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>So I=20
would suggest you set up your Foucault pendulum with two orthogonal coils=20
standing up, and put the single magnet pole on the floor pointing directly=
=20
into the bottom of the bob.  Charles Patton
?? My magnets all come with two poles. You coul=
d=20
use two rectangular coils and two sets of N+S NdFeB bar magnets pairs, at ri=
ght=20
angles on the end of the bob. Maybe with soft iron plates top and bottom and=
=20
mild steel bolts to enclose the fields?

I would prefer to put the coils on the end of a=
=20
brass bob weight and sit the magnets and the soft iron plates comfortab=
ly=20
on the ground.

Chris Chapman

```

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