## PSN-L Email List Message

Subject: Re: Geophone questions
From: "tchannel" tchannel@..............
Date: Sun, 27 May 2007 21:31:57 -0600

```Hi Chris,   Thanks for your reply:

My Questions:
1  "Can you get a mild steel tube which will surround the =
coil,"................  Is this to shield?

2  "a flat 1/4" mild steel plate and a stub of 3/4" mild steel rod with =
flat machined ends?"......................Say a 1/4" thick plate, 1"x 2" =
as the top of the inverted T...........Centered on this the 3/4" rod?=20

3  "Sit the magnets on top of the rod and the rod on the center of the =
plate."........................    Now I place the magnet on top of the =
rod, so I have an inverted T , made of PLATE, ROD, MAGNET?

4  "Put the iron tube centrally around the magnets and lower in the coil =
so that the top face of the magnets is about half way down it." =
.............................     Now I place the steel tube over the =
inverted T?   Now I lower the coil, so that the CENTER OF THE COIL, is =
lined up with the top surface of the magnet?

This would result with the top half of the COIL, 3/8", above the top =
surface of the MAGNET, and bottom half of the COIL below the top surface =
of the MAGNET?  With no space below this point as the 3/4" rod continues =
down.
In other words, the magnet is moving OVER A FINGER, RESTING AT THE TOP =
OF THE FINGER?

If this is the idea,  I see the One Pole, say North Pole of the magnet =
positioned in the center of the coil, the South Pole of the magnet well =
below the coil, as the 3/4" rod has extended the pole to the rod and the =
plate.
Is the objective to only move One Pole in and out of the coil?

Thank you for you help,
Ted

----- Original Message -----=20
From: ChrisAtUpw@..........
To: psn-l@.................
Sent: Sunday, May 27, 2007 7:58 PM
Subject: Re: Geophone questions

In a message dated 2007/05/27, tchannel@.............. writes:

I have a few questions about magnets and coils:

1  I have seen a cross section of a Geophone, magnet, spring and =
mass etc.  When used in a Seismic Sensor, how is it dampened, or is it =
do electronically?

Hi Ted,

Geophone coils are usually wound on a copper former, but this =
usually gives only maybe 40% of the critical induced damping required. =
It is usual to solder a carefully chosen resistor across the output =
terminals, which adds the rest of the damping. PSN seismic amplifiers =
have a 10 K input resistor fitted already and this needs to be taken =
into account when calculating the value of the required resistor.

2 I am making a new vertical sensor, and this question has to do =
with the coil and magnet.  The coil is a spool with a 1" dia. hole in =
the center.  The thickness of the coil is 3/4"   The value of the coil =
is 500ohms.   The magnet will be suspended from a rod and centered in =
this 1" hole. Left to Right and Top to Bottom.

I can choose from three magnets.  One magnet 1/4" thick x 3/4" dia. =
centered leaving 1/4" of coil above and below the magnet......... OR Two =
magnets 1/4" thick, together to equal 1/2" x 3/4", leaving 1/8" of coil =
above and below the two magnets when centered........... Lastly I could =
use One magnet 1/2" thick x 3/4" dia. leaving 1/8" above and below.

This is a basic question about the magnet's surface and the N/S =
poles,  I do not know the science, but just curious about just these =
three scenarios, which arrangement is the best?.....I could try all =
three and measure the result, but in theory is it better one way or the =
other?

Putting a magnet on a seismometer arm will enable you to pick =
up transients from the building Utility Power Wiring, passing trucks and =
cars, changes in the Earth's magnetic field.... If you really want too =
monitor any of these, OK, otherwise put the coil on the arm and the =
magnet on the frame ! !

None of them will be satisfactory. The centralised magnet =
position will give you an output minimum.=20

Consider how a loudspeaker is made. You have an external =
cylindrical magnet with poles at either end. One end A has a flat iron =
plate with a large hole in it. The other end B is fitted with another =
flat plate with a central solid iron rod which projects a xially to the =
top of the hole in the plate A. The coil moves in the radial annular =
field between plate A and the central iron rod.=20

Can you get a mild steel tube which will surround the coil, a =
flat 1/4" mild steel plate and a stub of 3/4" mild steel rod with flat =
machined ends? Lap the end of the tube flat with emery paper on the =
plate. Sit the magnets on top of the rod and the rod on the centre of =
the plate. Put the iron tube centrally around the magnets and lower in =
the coil so that the top face of the magnets is about half way down it.  =
=20
Alternatively, get three strips of 1/16" mild steel, bend them =
in a U shape and mount the mild steel column in the centre, with the =
strips at 120 degrees. Cut the strips to length so that when stacked =
they end level with the top of the magnets. You could also use a taller =
stack of magnets, but 2 off 1/4" ones won't reach the centre of the coil =
and allow for any free motion. You will probably need to stick the =
strips together with acrylic glue to form a rigid cup structure. Then =
give it a coat of anti rust paint.

Regards,

Hi Chris,   Thanks for your=20

Your=20
notes:           &=
nbsp;           &n=
bsp;           &nb=
sp;           &nbs=
p;            =
;            =
&=
nbsp;      =20
My Questions:
1  "Can you get a mild steel tube =
which will=20
surround the coil,"................  Is this to =
shield?

2  "a flat 1/4" mild steel plate =
and a stub of=20
3/4" mild steel rod with flat machined =
ends?"......................Say a 1/4" thick plate, 1"x 2" as the top of the =
inverted=20
T...........Centered on this the 3/4" rod?

3  "Sit the magnets on top of the =
rod and the=20
rod on the center of the =
plate."........................    Now I=20
place the magnet on top of the rod, so I have an inverted T , made =
of=20
PLATE, ROD, MAGNET?

4  "Put the iron tube centrally =
around the=20
magnets and lower in the coil so that the top face of the magnets is =
way down it." ............................     Now I =
place=20
the steel tube over the inverted T?   Now I lower the coil, so =
that=20
the CENTER OF THE COIL, is lined up with the top surface of the=20
magnet?

This would result with the =
top half of the=20
COIL, 3/8", above the top surface of the MAGNET, and bottom half of =

the COIL below the top surface of the MAGNET?  With no space =
below=20
this point as the 3/4" rod continues down.
In other words, the magnet is moving =
OVER A FINGER,=20
RESTING AT THE TOP OF THE FINGER?

If this is the idea,  I see the =
One Pole, say=20
North Pole of the magnet positioned in the center of the coil, the South =
Pole of=20
the magnet well below the coil, as the 3/4" rod has extended the pole to =
the rod=20
and the plate.
Is the objective to only move One Pole =
in and out=20
of the coil?

Thank you for you help,
Ted

----- Original Message -----
From:=20
ChrisAtUpw@.......
To: psn-l@..............
Sent: Sunday, May 27, 2007 7:58 =
PM
Subject: Re: Geophone =
questions
In a=20
message dated 2007/05/27, tchannel@..............=20
writes:
I have a few questions about magnets and =
coils:1  I have =
seen a cross=20
section of a Geophone, magnet, spring and mass etc.  When used =
in a=20
Seismic Sensor, how is it dampened, or is it do =
electronically?Hi=20
Ted,       Geophone coils are =
usually=20
wound on a copper former, but this usually gives only maybe 40% of the =

critical induced damping required. It is usual to solder a carefully =
chosen=20
resistor across the output terminals, which adds the rest of the =
damping. PSN=20
seismic amplifiers have a 10 K input resistor fitted already and this =
needs to=20
be taken into account when calculating the value of the required=20
resistor.
2 I am=20
making a new vertical sensor, and this question has to do with the =
coil and=20
magnet.  The coil is a spool with a 1" dia. hole in the =
center. =20
The thickness of the coil is 3/4"   The value of the coil =
is=20
500ohms.   The magnet will be suspended from a rod and =
centered in=20
this 1" hole. Left to Right and Top to Bottom.I can choose =
from three=20
magnets.  One magnet 1/4" thick x 3/4" dia. centered leaving =
1/4" of=20
coil above and below the magnet......... OR Two magnets 1/4" thick, =
together=20
to equal 1/2" x 3/4", leaving 1/8" of coil above and below the two =
magnets=20
when centered........... Lastly I could use One magnet 1/2" thick x =
3/4"=20
dia. leaving 1/8" above and below.This is a basic =
the magnet's surface and the N/S poles,  I do not know the =
science, but=20
just curious about just these three scenarios, which arrangement is =
the=20
best?.....I could try all three and measure the result, but in =
theory is it=20
better one way or the=20
other?       =
Putting a=20
magnet on a seismometer arm will enable you to pick up transients from =
the=20
building Utility Power Wiring, passing trucks and cars, changes in the =
Earth's=20
magnetic field.... If you really want too monitor any of these, OK, =
otherwise=20
put the coil on the arm and the magnet on the frame !=20
!      None of them will be =
satisfactory. The=20
centralised magnet position will give you an output minimum.=20
Consider how a =
loudspeaker is=20
made. You have an external cylindrical magnet with poles at either =
end. One=20
end A has a flat iron plate with a large hole in it. The other end B =
is fitted=20
with another flat plate with a central solid iron rod which projects a =
xially=20
to the top of the hole in the plate A. The coil moves in the radial =
annular=20
field between plate A and the central iron rod.=20
Can you get a mild steel =
tube=20
which will surround the coil, a flat 1/4" mild steel plate and a stub =
of 3/4"=20
mild steel rod with flat machined ends? Lap the end of the tube flat =
with=20
emery paper on the plate. Sit the magnets on top of the rod and the =
rod on the=20
centre of the plate. Put the iron tube centrally around the magnets =
and lower=20
in the coil so that the top face of the magnets is about half way down =

it.           =20
Alternatively, get three strips of 1/16" mild steel, bend them in a U =
shape=20
and mount the mild steel column in the centre, with the strips at 120 =
degrees.=20
Cut the strips to length so that when stacked they end level with the =
top of=20
the magnets. You could also use a taller stack of magnets, but 2 off =
1/4" ones=20
won't reach the centre of the coil and allow for any free motion. You =
will=20
probably need to stick the strips together with acrylic glue to form a =
rigid=20
cup structure. Then give it a coat of anti rust=20
paint.      =20
Regards,       Chris =
Chapman=20

```

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