## PSN-L Email List Message

Subject: Re: Seismograph for school
From: ChrisAtUpw@.......
Date: Sat, 30 Sep 2006 23:58:52 EDT

```In a message dated 2006/10/01, gmvoeth@........... writes:

> Hello Chris;
> Can you tell us how to get a uniform magnetic field
> to cut the coil at right angles (normal) to the windings ?

Hi Geoff,

I use 4 off 1" sq x 1/8" rectangular NdFeB magnets on two 3.5" x 2" x
1/4" thick mild steel backing plates and mount the coil in between. You have
N+S magnets on one face opposite to S+N magnets on the other face. This gives a
high central field and a linear movement response. I use a rectangular
pancake coil of about 2000 turns. It is quite effective!

See  http://jclahr.com/science/psn/chapman/lehman/index.html

> can you use two opposing horse shoe magnets
> with a coil in the middle ?

This is likely to give a lower sensitivity and is more difficult to
make / mount. Large Alnico U magnets are quite expensive. You benefit from the
relatively high flux of the NdFeB magnets.

Regards,

Chris Chapman
In a me=
ssage dated 2006/10/01, gmvoeth@........... writes:

Hello Chris;
Can you tell us how to get a uniform magnetic field
to cut the coil at right angles (normal) to the windings ?

Hi Geoff,

I use 4 off 1" sq x 1/8" rectangular Nd=
FeB magnets on two 3.5" x 2" x 1/4" thick mild steel backing plates and moun=
t the coil in between. You have N+S magnets on one face opposite to S+N magn=
ets on the other face. This gives a high central field and a linear movement=
response. I use a rectangular pancake coil of about 2000 turns. It is quite=
effective!

See  http://jclahr.com/science/psn=
/chapman/lehman/index.html

can you use two opposing horse=20=
shoe magnets
with a coil in the middle ?

This is likely to give a lower sensitiv=
ity and is more difficult to make / mount. Large Alnico U magnets are quite=20=
expensive. You benefit from the relatively high flux of the NdFeB magnets.

Regards,

Chris Chapman
```