## PSN-L Email List Message

Subject: Re: Round external coils and stacked internal magnets
From: "meredith lamb" paleoartifact@.........
Date: Sat, 17 May 2008 00:52:04 -0600

```Hi Gary,

Imagine (or use) a rectangular piece of paper as the flat "steel plate".  On
it, place/lay 3 pencils spaced apart
@ say 2 inches.  The pencils are the imaginary rectangular "magnets" stuck
to the " steel plate".  The magnets are polarized through their thickness;
and hence are arranged with say, the left magnet with the N pole up, then
the center "magnet" has the S pole up, and the right pencil has the N pole
up.  The imaginary coil goes
over the the center "magnet" (up-down motion) and in between the outer
"magnets".  This type of magnet is
often called a motor or wind generator magnet, as the poles are through the
thickness and not at the ends.

Every magnet has two poles.  Hence their is two attracting fields in between
the left "magnet" and the
center "magnet".  The same goes with the same center "magnet" and the right
"magnet"; hence you have
4 attracting fields acting on the single coil.  The fields are of course,
relatively straight across to the adjacent
attracting magnet.

The intensity of the fields depends on the magnets ratings, how close they
are together, and it also
depends on the coil itself.

For example; If you only have one magnet you "could" get for example, say 3v
max velocity.  With two, it
could jump to ~ 8v;  but with 3 it probably goes over 15v max velocity.  Of
course it all depends on the coil
used and the magnet rating and the magnet spacing.

Take care, Meredith

On Fri, May 16, 2008 at 11:07 PM, Gary Lindgren
wrote:

>  Meredith,
>
> I can't visualize the magnet layout you speak of with 3 magnets. Do you
> have a magnet placement diagram so that I could map out the magnetic fields.
>
> Gary Lindgren
>
>
>
>
>
>
>
>
>
> *From:* psn-l-request@.............. [mailto:psn-l-request@...............
> *On Behalf Of *meredith lamb
> *Sent:* Friday, May 16, 2008 9:22 PM
> *To:* psn-l@..............
> *Subject:* Re: Round external coils and stacked internal magnets
>
>
>
> Hi Chris and all,
>
> Thanks Chris for the extensive reply below.  Actually my impression with
> the stacked magnets came from
> trying to utilize a old surplus Texas Instruments rectangular coil form,
> and at the time, the stacked
> magnets approach seemed quite impressive at the time via a table top and
> voltmeter experiment.
>
> The problem with the coil is it has relatively thin walls, and I'am not so
> sure that a quad (4) magnet
> setup is the best or only approach.  Its possible to use 3 magnets and
> essentially get a similar "quad" (4 pole)
> arrangement, where the fields go through the sides of the coil walls.
>
> The coil form shape is quite similar to transformer coil forms, and in a
> sense there maybe 2 ways to
> use magnets with them.  Esterline-Angus also made similar coils back in the
> 1950's, of which
> I also have some of those.
>
> I put up a quick web page with 2 photos on the TI coil; along with my
> "notes/ramblings" related to such:
>
>
> Take care, Meredith Lamb
>
>  On Fri, May 16, 2008 at 2:56 PM,  wrote:
>
> 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
>
>
>
Hi Gary,Imagine (or use) a rectangular piece of paper as the flat "steel plate".  On it, place/lay 3 pencils spaced apart @ say 2 inches.  The pencils are the imaginary rectangular "magnets" stuck to the " steel plate".  The magnets are polarized through their thickness; and hence are arranged with say, the left magnet with the N pole up, then
the center "magnet" has the S pole up, and the right pencil has the N pole up.  The imaginary coil goesover the the center "magnet" (up-down motion) and in between the outer "magnets".  This type of magnet is
often called a motor or wind generator magnet, as the poles are through the thickness and not at the ends.Every magnet has two poles.  Hence their is two attracting fields in between the left "magnet" and the
center "magnet".  The same goes with the same center "magnet" and the right "magnet"; hence you have4 attracting fields acting on the single coil.  The fields are of course, relatively straight across to the adjacent
attracting magnet.The intensity of the fields depends on the magnets ratings, how close they are together, and it alsodepends on the coil itself.  For example; If you only have one magnet you "could" get for example, say 3v max velocity.  With two, it
could jump to ~ 8v;   but with 3 it probably goes over 15v max velocity.  Of course it all depends on the coil used and the magnet rating and the magnet spacing.Take care, Meredith
On Fri, May 16, 2008 at 11:07 PM, Gary Lindgren <gel@.................> wrote:

Meredith,

I can't visualize the magnet layout you speak of with 3 magnets.
Do you have a magnet placement diagram so that I could map out the magnetic
fields.

Gary Lindgren

From:
psn-l-request@.............. [mailto:psn-l-request@..............] On Behalf
Of meredith lamb
Sent: Friday, May 16, 2008 9:22 PM
To: psn-l@..............
Subject: Re: Round external coils and stacked internal magnets

Hi Chris and all,

Thanks Chris for the extensive reply below.  Actually my impression with
the stacked magnets came from
trying to utilize a old surplus Texas Instruments rectangular coil form, and at
the time, the stacked
magnets approach seemed quite impressive at the time via a table top and
voltmeter experiment.

The problem with the coil is it has relatively thin walls, and I'am not so sure
setup is the best or only approach.  Its possible to use 3 magnets and
essentially get a similar "quad" (4 pole)
arrangement, where the fields go through the sides of the coil walls.

The coil form shape is quite similar to transformer coil forms, and in a sense
there maybe 2 ways to
use magnets with them.  Esterline-Angus also made similar coils back in
the 1950's, of which
I also have some of those.

I put up a quick web page with 2 photos on the TI coil; along with my
"notes/ramblings" related to such:

Take care, Meredith Lamb

On Fri, May 16, 2008 at 2:56 PM, <ChrisAtUpw@.......> wrote:

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

Regards,

Chris Chapman

```