PSN-L Email List Message

Subject: Re: Orientation of magnet with Lehman setup?
From: "Connie and Jim Lehman" lehmancj@...........
Date: Sun, 4 Feb 2007 07:47:29 -0500


CHRIS--thanks for the info--very logical--I remember a project where we =
aligned a suspended magnet freely in the Earth's field and slightly =
stressed its position with another magnet nearby.  This was in an =
attempt to note changes in the Earth's field with time due to Sun's =
activity etc.--well, the building "chiller" used to cycle on & off, =
about 100 ft. away, and we could sense that activity.  WWV in those days =
gave the solar flux index which was a reference for such activity.  We =
never had any definitive results!!!
  As ever,  Jim
  ----- Original Message -----=20
  From: ChrisAtUpw@..........
  To: psn-l@.................
  Sent: Saturday, February 03, 2007 6:12 PM
  Subject: Re: Orientation of magnet with Lehman setup?


  In a message dated 03/02/2007, lehmancj@........... writes:
       You mention your design has the coil on the boom, and magnet on =
the
    ground.  That is quite ok I guess, but I have never understood the =
advantage
    over the coil on the ground - with the coil on the ground, the boom =
is free
    of any attachments..... Maybe I am missing something---

  Hi Jim,

      There are several different situations if you mount a magnet on =
the boom, use an iron mass, or make the boom from steel, some of which =
are worse than others. You are very likely to be sensitive to magnetic =
field changes, or to changes in the field gradient, all of which can =
result in unwanted noise. You don't have to use magnetic components! It =
is advisable to 'design out' such problems in the first place where =
possible, rather than having to 'find and eliminate' them later on. =
Commercial mechanical seismometers are usually designed with integral =
magnetic shielding - even the Ni-SpanC springs are magnetic.=20

      The principle effects are due to changes in the Earth's field, to =
the magnetic attraction / field changes produced by cars, trucks, =
trains, kids cycles, lawn mowers and especially to pulses on the utility =
power wiring in the house. TVs, refrigerators, cookers and central =
heating systems are common culprits. A seismometer is extremely =
sensitive - it doesn't take much force to move the arm by over 50 nano =
metres!

      You may pick up RF noise / static with the sensor coil / onto the =
wiring. I favour the quad NdFeB magnet arrangement on mild steel backing =
plates for producing the sensor field. This construction shields the =
pick up coil from both external magnetic and electric fields and you =
don't need very large coils.=20
      Co-axial microphone cable gives good screening. I use the sort =
which has a black plastic conducting layer in between the woven screen =
and the central polythene insulation. This reduces signals due to =
vibration / cable movement / thermal expansion.=20

      Regards,

      Chris Chapman







CHRIS--thanks for the info--very logical--I remember a project = where we=20 aligned a suspended magnet freely in the Earth's field and slightly = stressed its=20 position with another magnet nearby.  This was in an attempt = to note=20 changes in the Earth's field with time due to Sun's activity etc.--well, = the=20 building "chiller" used to cycle on & off, about 100 ft. away, and = we could=20 sense that activity.  WWV in those days gave the solar flux index = which was=20 a reference for such activity.  We never had any definitive=20 results!!!
  As ever,  Jim
----- Original Message -----
From:=20 ChrisAtUpw@.......
To: psn-l@..............
Sent: Saturday, February 03, = 2007 6:12=20 PM
Subject: Re: Orientation of = magnet with=20 Lehman setup?

In a message dated 03/02/2007, lehmancj@........... = writes:
   You mention your design has the coil on the = boom, and=20 magnet on the
ground.  That is quite ok I guess, but I have = never=20 understood the advantage
over the coil on the ground - with the = coil on=20 the ground, the boom is free
of any attachments..... Maybe I am = missing=20 something---
Hi Jim,
 
    There are several different situations if = you=20 mount a magnet on the boom, use an iron mass, or make the boom = from=20 steel, some of which are worse than others. You are very likely to be=20 sensitive to magnetic field changes, or to changes in the field=20 gradient, all of which can result in unwanted noise. You don't = have to=20 use magnetic components! It is advisable to 'design out' such problems = in the=20 first place where possible, rather than having to 'find and = eliminate'=20 them later on. Commercial mechanical seismometers are usually designed = with=20 integral magnetic shielding - even the Ni-SpanC springs are magnetic. =
 
    The principle effects are due to changes = in the=20 Earth's field, to the magnetic attraction / field changes produced=20 by cars, trucks, trains, kids cycles, lawn mowers and especially = to=20 pulses on the utility power wiring in the house. TVs, refrigerators,=20 cookers and central heating systems are common culprits. A = seismometer is=20 extremely sensitive - it doesn't take much force to move the arm by = over 50=20 nano metres!
 
    You may pick up RF noise / static with = the sensor=20 coil / onto the wiring. I favour the quad NdFeB magnet arrangement on = mild=20 steel backing plates for producing the sensor field. This construction = shields=20 the pick up coil from both external magnetic and electric fields and = you don't=20 need very large coils. 
    Co-axial microphone cable gives good = screening. I=20 use the sort which has a black plastic conducting layer in between the = woven=20 screen and the central polythene insulation. This reduces signals = due to=20 vibration / cable movement / thermal expansion. 
 
    Regards,
 
    Chris=20 Chapman

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