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 -----
Sent: Saturday, February 03, =
2007 6:12=20
PM
Subject: Re: Orientation of =
magnet with=20
Lehman setup?
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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