## PSN-L Email List Message

Subject: Re: Geophone questions
From: ChrisAtUpw@.......
Date: Mon, 28 May 2007 00:33:35 EDT

```In a message dated 2007/05/28, tchannel@.............. writes:

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

Hi Ted,

No, this forms the return path of the magnetic field. Consider the
loudspeaker construction.

> 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?

Lets start again.
You put the 3/4" magnet on one end of a short 3/4" dia circular iron
rod.
You put the other end of the rod on the centre of a 1/4" thick iron
baseplate.
You put the outer iron tube centrally over the rod so that it rests on
the baseplate.
So you have a cylindrical steel cup with the magnet on a stalk in the
centre.
You lower the coil over the magnet so that it is 3/8" down - half in and
half out of the field.
If the top of the magnet is N, the S pole will be in contact with the
iron stub. The field from the S pole will go through the stub, spread out
through the baseplate, up the outer cylinder and appear across the annular cylinder
/ magnet gap. Similar to the loudspeaker construction

>  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?

Correct.

> 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?

Correct.

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?

The magnet does not move at all. It is the coil which moves in the
radial field between the top of the magnet and the outer iron tube.

>  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?

The top of the magnet is say the N pole. The outer steel tube is the S
pole and the radial magnetic field is in the annular gap between the two. The
coil moves further into and out of the tube. Exactly like a loudspeaker, but
with a much thicker coil.

Regards,

Chris Chapman
In a me=
ssage dated 2007/05/28, tchannel@.............. writes:

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

Hi Ted,

No, this forms the return path of the m=
agnetic field. Consider the loudspeaker construction.

2  "a flat 1/4" mild stee=
l 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?

Lets start again.
You put the 3/4" magnet on one end of a=
short 3/4" dia circular iron rod.
You put the other end of the rod on the centre of a 1=
/4" thick iron baseplate.
You put the outer iron tube centrally over the rod so=
that it rests on the baseplate.
So you have a cylindrical steel cup with the magnet on a=20=
stalk in the centre.
You lower the coil over the magnet so that it is 3/8" dow=
n - half in and half out of the field.
If the top of the magnet is N, the S pole will be i=
n contact with the iron stub. The field from the S pole will go through the=20=
stub, spread out through the baseplate, up the outer cylinder and appear acr=
oss the annular cylinder / magnet gap. Similar to the loudspeaker constructi=
on

3  "Sit the magnets on t=
op 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=20=
inverted T , made of PLATE, ROD, MAGNET?

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

Correct.

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

Correct.

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 T=
HE FINGER?

The magnet does not move at all. It is=
the coil which moves in the radial field between the top of the magnet and=20=
the outer iron tube.

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

The top of the magnet is say the N pole=
.. The outer steel tube is the S pole and the radial magnetic field is in the=
annular gap between the two. The coil moves further into and out of the tub=
e. Exactly like a loudspeaker, but with a much thicker coil.

Regards,

Chris Chapman
```