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Hi Dale,
Your question and one or two of the posts in answer seem to beg a bit of an
explanation about magnetic fields in general, and units in particular. So
if you are interested, bear with me.
All magnetic fields result from (are manifestations of) movements of
electric charge. This is true of solenoids, permanent magnets, or any other
kind of magnetic device. In the case of a wire or solenoid, it is easy to
visualize the flow of electrons and the resultant magnetic field. It gets a
little more obscure with permanent magnets and magnetic core materials. But
even there, relatively simple and workable understandings are possible
without worrying too much about quantum mechanics and such. All materials
are made of atoms, and all atoms contain moving electrons. These electron
movements (electron spins, etc) are, of course, moving charges and therefore
have an associated magnetic field, just like a single-turn solenoid. In
most materials, these magnetic moments are randomly oriented in space (due
to thermal random movements) and therefore the resultant external magnetic
field is zero. And just as one energized solenoid exerts a force or torque
on another nearby energized solenoid, these atomic-scale magnetic moments
experience physical aligning forces and torques when an external magnetic
field is applied to them. At this stage, it is useful to differentiate
three different basic kinds of atomic magnetic moments. The kind that align
themselves in parallel with an external field, but opposing that field (i.e.
opposite polarity) are called "diamagnetic" materials. The kind that align
themselves in parallel but adding to that field (i.e. same polarity) are
called "paramagnetic" materials. Then there is a special kind of
"paramagnetic" material which aligns not only with a strong external field,
but even with the relatively weak neigbouring atomic-scale magnetic moments;
these are called "ferromagnetic" materials. These "ferromagnetic" moments
are rather like a military platoon on parade, whereas the "paramagnetic"
moments are more like a crowd at a fairground (the applied field might be
thought of, in this case, as a barker crying "free beer!"). So the
resultant magnetic field of a structure comprising a winding and various
magnetic materials nearby, is simply the summation of the little magnetic
fields from an almost infinite number of little electric loop currents.
These currents are all tending to line each other up, and fighting thermal
activity tending to randomize that alignment.
The above explanation also helps with problems in understanding magnetic
fields with and without poles. A long bar magnet is easy to think of in
terms of physical regions commonly called poles, and a long solenoid is
completely analogous. A single-turn solenoid is not a large conceptual
stretch from a multiturn solenoid, and as you "unwind" even that last turn,
and get a short piece of wire, you realize that you are simply looking at a
complete turn of larger radius (you cannot have an electric current without
a complete turn, even if that turn passes through the battery or power
supply). So now you can understand POLARITY as being, not the top or bottom
region of a permanent magnet, but rather an indication of the direction of a
resultant field (sort of like left-handedness vs right-handedness). The bar
magnet is, after all, just the summation of billions of tiny atomic-scale
electric currents or electron spins.
Now I explained that these atomic-scale magnetic moments align themselves
with external fields. When they do, the observed external field changes in
magnitude as a result. When paramagnetic and ferromagnetic materials are
introduced, the observed magnitude rises. When diamagnetic materials are
introduced, the observed field magnitude decreases. The increase or
decrease in observed field, as a fraction of the applied field, is called
the "magnetic susceptibility". It is rather small (parts per million) for
most materials, but can be very large (much larger than unity) for
ferromagnetic materials.
Above, I said that individual magnetic moments within a ferromagnetic
material (like iron or nickel) will align even with only the applied field
of neighbouring individual magnetic moments. These alignments, which I
compared to military platoons on parade, can grow quite large, but their
natural growth is limited by more subtle crystal-structure parameters that
are more complex to explain. Suffice to say, that the growth of these
magnetic equivalents to platoons on parade are called "magnetic domains".
When an external field is applied, any such domains which are in
near-alignment with that field tend to grow in size at the cost of
neighbouring domains which are in poor alignment. Sort of like someone
moving a fence between neighbouring properties. As the applied field
becomes stronger, these domain boundaries (fences) move so far as to "meet"
other boundaries coming in the opposite direction. As the applied field
becomes stronger still, and the boundaries cannot move any further, the
entire domain (that is, the physical crystal structure itself) moves into
alignment with the applied field. If you listen carefully near a
magnetically energized piece of ferromagnetic material, you can actually
hear the sound waves produced by this movement (it is called Barkhausen
noise). The hum from a power transformer is largely caused by this effect.
The magnetic field of the earth is quite similar to the above. It is
produced by large convection currents in the earth's molten core (due to
thermal gradients produced by nuclear reactions in the earth's core). These
convection currents carry electric charges and cause magnetic fields. The
magnetic field is not that of a simple solenoid, but multiple complex loops
with a strong tendency to align close to the earth's axis of rotation.
There is thought to be a self-energized dynamo action which produces the
resultant field, and the field has been well characterized in terms of its
distribution in space and its change in time. The time rate-of-change is
really very considerable (just compare a magnetic map from 1900 or 1950 with
one from today).
Now for units. Think of a single loop of electric current as being a
magnetic "force". This force or field strength is expressed in
amperes/meter (or oersteds in the old cgs system). This magnetic force in a
magnetic circuit is analogous to electric potential (in volts) in an
electric circuit. If this magnetic force exists in free space, there
results a magnetic "flux" (expressed in webers) which is analogous to the
electric current (in amperes) in the electric circuit analogy. There is a
related parameter called "flux density" (expressed in webers/square meter,
also called a tesla; the old cgs unit was the gauss) which is equivalent to
current density (amperes/square meter) in the electric circuit. In that
electric circuit, the ratio between potential (volts) and current (amperes)
is resistance (ohms). In the magnetic circuit analogy, the equivalent of
resistance is called "permeability", and its value in free space (the
"permeability of free space" is 4*pi*10E-7). This permeability is
determined directly by the "susceptibility" of the materials within the
region of interest.
Lastly, in direct answer to your question about measuring susceptibility, it
can be measured by observing the change in inductance of a solenoid placed
around the material in question. If that solenoid is incorporated into a
resonant circuit (electronics jargon here) you can note with even
inexpensive equipment that there is a measurable phase shift (the angle
between alternating voltage and current) as the susceptibility changes, even
by parts per million. The phase-change observations can be done with
bridge-like (nulling) circuits.
Your bit of ferromagnetic material in the center of your wire would be
magnetized according to the applied field at that point. In the absence of
other turns of wire nearby, the field would form a loop centered about the
axis of the wire, and the domains in the magnetic particle would therefore
align in a circular array. This would minimize, but not eliminate, external
observable field change. In a long solenoid, the magnetic field is not
symmetric around each turn of wire, as you can easily prove for yourself by
just tracing the resultant magnetic loops from several nearby turns of the
solenoid.
I hope some of this helps.
Best regards,
Peter Boetzkes
----Original Message Follows----
From: "Dale Seppa"
Reply-To: "The Proton Mag Forum"
To: "The Proton Mag Forum"
Subject: Re: tests for magnetic materials
Date: Fri, 16 Jun 2000 02:00:15 -0600
The Proton Mag Forum
Many thanks to Cris, Peter and George for the valued input.
Some of it quite a bit above my head but I feel I have already
learned something and hope to learn more if you are kind
enough to answer this posting.
Cris: I did not quite understand if the Neodymium "Super Mag"
is a magnetometer, gaussmeter, metal detector or what. Also
not clear if it is the same thing that sells for 99 Quid. I
can afford that amount so if it is simple enough that there is
a reading and a big reading is bad and a small reading is good
(or vice versa), I want one and would appreciate knowing where
I can get product literature and order one as I could find no
reference on the web.
I keep coming back to the "hand held digitial gaussmeter shown
at
http:www.ascscientific/gauss.html
and it seems to me that this or something similar would
work??? (Does anyone know why this web address does NOT
highlight and the one below DOES highlight???
Peter: The only item that I know of that shows it is
specifically to measure the "magnetic susceptibility" of
something is a rather pricey item (I think US$2000.00 or so)
made in Australia and found at
http://www.geoinstruments.com.au/main.html. I also know
TerraPlus and some other firms have them but I think price
even greater than that shown above.
At the risk (or probably certainty) of sounding really
stupid - is a "magnetic suceptibility meter" the same as a
"gaussmeter"???
George: Your method sounds like the most exciting of the
bunch, but considering the knowledge base that I am starting
from and the fact that I am the only man in the world who has
frittered away six months and haven't even selected my
toroidal form yet, if I got started on your project, I'd be
six feet (1.82880 metres) under before I finished. I am not
depreciating or denigrating your greatly appreciated answer -
only trying to interject a bit of humor into MY own personal
failings. Or possibly "LMF".
Again thanks and very best regards to all,
Dale
PS I regret boring you with these things you all know so well
but I just can't seem to learn it from books. In fact for
whatever reason I cannot even read a full page in a technical
book anymore without my mind wandering so bad that I start
the same page five times before I give up.
----- Original Message -----
From: "George Davidson"
To: "The Proton Mag Forum"
Sent: Thursday, June 15, 2000 11:40 AM
Subject: Re: tests for magnetic materials
> The Proton Mag Forum
>
> Mag forum,
>
> Another angle:
>
> Am late on this discussion as I have been maggin in
Mocambique
> and have returned without malaria, cholera or yellow fever.
>
> I have wound a number of coils mostly solenoids and found a
> simple test for magnetic buildup which may be useful, as
follows:
>
> In the field, a magnetized solenoid will show a deviation if
swung
> from E-W to W-E, at worst up to 5 nT . Couldnt find any
effect
> of ferrous contaminated Cu wire and assumed it would cancel
> itself out within the wire**. Did get deviations when
mag probes
> were rubbing against anchor chain in the bilges of the boat
or
> dragged across a warehouse floor or deck of a steel survey
vessel.
>
> The test system I used was a hollow solenoid into which a
bottle
> could be placed for testing the various fluids and over
which a
> PVC housing with various plastics could be tested . Ran it
> overnight at 2 A from 24v DC to get some serious
polarization and
> tested it in the field on top of a 3 Metre wooden pole
which could
> be rotated and the trace observed. Using a notebook and a
> PICOSCOPE oscilloscope a number of measurements of various
> type s could be obtained
> . After that one can use the least offensive of the
various
> materials from the same stocks to build the final toroid
etc.
>
> I like the idea of bifilar windings but always think of the
field
> situation like what cable to use and where the earth will
be .
> Cable is always a problem and stray voltages on a boat can
realy
> become a character -building experience. In addition
,cables can
> be microphonic and change capacitance (pF) with wave
action ...
>
> George
>
> **PS Quiz: a ferrous particle in the very centre of a
current
> carrying copper wire will become polarized in which
direction?
>
>
>
__________________________________________________ ____________
________
Your question and one or two of the posts in answer seem to beg a bit of an
explanation about magnetic fields in general, and units in particular. So
if you are interested, bear with me.
All magnetic fields result from (are manifestations of) movements of
electric charge. This is true of solenoids, permanent magnets, or any other
kind of magnetic device. In the case of a wire or solenoid, it is easy to
visualize the flow of electrons and the resultant magnetic field. It gets a
little more obscure with permanent magnets and magnetic core materials. But
even there, relatively simple and workable understandings are possible
without worrying too much about quantum mechanics and such. All materials
are made of atoms, and all atoms contain moving electrons. These electron
movements (electron spins, etc) are, of course, moving charges and therefore
have an associated magnetic field, just like a single-turn solenoid. In
most materials, these magnetic moments are randomly oriented in space (due
to thermal random movements) and therefore the resultant external magnetic
field is zero. And just as one energized solenoid exerts a force or torque
on another nearby energized solenoid, these atomic-scale magnetic moments
experience physical aligning forces and torques when an external magnetic
field is applied to them. At this stage, it is useful to differentiate
three different basic kinds of atomic magnetic moments. The kind that align
themselves in parallel with an external field, but opposing that field (i.e.
opposite polarity) are called "diamagnetic" materials. The kind that align
themselves in parallel but adding to that field (i.e. same polarity) are
called "paramagnetic" materials. Then there is a special kind of
"paramagnetic" material which aligns not only with a strong external field,
but even with the relatively weak neigbouring atomic-scale magnetic moments;
these are called "ferromagnetic" materials. These "ferromagnetic" moments
are rather like a military platoon on parade, whereas the "paramagnetic"
moments are more like a crowd at a fairground (the applied field might be
thought of, in this case, as a barker crying "free beer!"). So the
resultant magnetic field of a structure comprising a winding and various
magnetic materials nearby, is simply the summation of the little magnetic
fields from an almost infinite number of little electric loop currents.
These currents are all tending to line each other up, and fighting thermal
activity tending to randomize that alignment.
The above explanation also helps with problems in understanding magnetic
fields with and without poles. A long bar magnet is easy to think of in
terms of physical regions commonly called poles, and a long solenoid is
completely analogous. A single-turn solenoid is not a large conceptual
stretch from a multiturn solenoid, and as you "unwind" even that last turn,
and get a short piece of wire, you realize that you are simply looking at a
complete turn of larger radius (you cannot have an electric current without
a complete turn, even if that turn passes through the battery or power
supply). So now you can understand POLARITY as being, not the top or bottom
region of a permanent magnet, but rather an indication of the direction of a
resultant field (sort of like left-handedness vs right-handedness). The bar
magnet is, after all, just the summation of billions of tiny atomic-scale
electric currents or electron spins.
Now I explained that these atomic-scale magnetic moments align themselves
with external fields. When they do, the observed external field changes in
magnitude as a result. When paramagnetic and ferromagnetic materials are
introduced, the observed magnitude rises. When diamagnetic materials are
introduced, the observed field magnitude decreases. The increase or
decrease in observed field, as a fraction of the applied field, is called
the "magnetic susceptibility". It is rather small (parts per million) for
most materials, but can be very large (much larger than unity) for
ferromagnetic materials.
Above, I said that individual magnetic moments within a ferromagnetic
material (like iron or nickel) will align even with only the applied field
of neighbouring individual magnetic moments. These alignments, which I
compared to military platoons on parade, can grow quite large, but their
natural growth is limited by more subtle crystal-structure parameters that
are more complex to explain. Suffice to say, that the growth of these
magnetic equivalents to platoons on parade are called "magnetic domains".
When an external field is applied, any such domains which are in
near-alignment with that field tend to grow in size at the cost of
neighbouring domains which are in poor alignment. Sort of like someone
moving a fence between neighbouring properties. As the applied field
becomes stronger, these domain boundaries (fences) move so far as to "meet"
other boundaries coming in the opposite direction. As the applied field
becomes stronger still, and the boundaries cannot move any further, the
entire domain (that is, the physical crystal structure itself) moves into
alignment with the applied field. If you listen carefully near a
magnetically energized piece of ferromagnetic material, you can actually
hear the sound waves produced by this movement (it is called Barkhausen
noise). The hum from a power transformer is largely caused by this effect.
The magnetic field of the earth is quite similar to the above. It is
produced by large convection currents in the earth's molten core (due to
thermal gradients produced by nuclear reactions in the earth's core). These
convection currents carry electric charges and cause magnetic fields. The
magnetic field is not that of a simple solenoid, but multiple complex loops
with a strong tendency to align close to the earth's axis of rotation.
There is thought to be a self-energized dynamo action which produces the
resultant field, and the field has been well characterized in terms of its
distribution in space and its change in time. The time rate-of-change is
really very considerable (just compare a magnetic map from 1900 or 1950 with
one from today).
Now for units. Think of a single loop of electric current as being a
magnetic "force". This force or field strength is expressed in
amperes/meter (or oersteds in the old cgs system). This magnetic force in a
magnetic circuit is analogous to electric potential (in volts) in an
electric circuit. If this magnetic force exists in free space, there
results a magnetic "flux" (expressed in webers) which is analogous to the
electric current (in amperes) in the electric circuit analogy. There is a
related parameter called "flux density" (expressed in webers/square meter,
also called a tesla; the old cgs unit was the gauss) which is equivalent to
current density (amperes/square meter) in the electric circuit. In that
electric circuit, the ratio between potential (volts) and current (amperes)
is resistance (ohms). In the magnetic circuit analogy, the equivalent of
resistance is called "permeability", and its value in free space (the
"permeability of free space" is 4*pi*10E-7). This permeability is
determined directly by the "susceptibility" of the materials within the
region of interest.
Lastly, in direct answer to your question about measuring susceptibility, it
can be measured by observing the change in inductance of a solenoid placed
around the material in question. If that solenoid is incorporated into a
resonant circuit (electronics jargon here) you can note with even
inexpensive equipment that there is a measurable phase shift (the angle
between alternating voltage and current) as the susceptibility changes, even
by parts per million. The phase-change observations can be done with
bridge-like (nulling) circuits.
Your bit of ferromagnetic material in the center of your wire would be
magnetized according to the applied field at that point. In the absence of
other turns of wire nearby, the field would form a loop centered about the
axis of the wire, and the domains in the magnetic particle would therefore
align in a circular array. This would minimize, but not eliminate, external
observable field change. In a long solenoid, the magnetic field is not
symmetric around each turn of wire, as you can easily prove for yourself by
just tracing the resultant magnetic loops from several nearby turns of the
solenoid.
I hope some of this helps.
Best regards,
Peter Boetzkes
----Original Message Follows----
From: "Dale Seppa"
Reply-To: "The Proton Mag Forum"
To: "The Proton Mag Forum"
Subject: Re: tests for magnetic materials
Date: Fri, 16 Jun 2000 02:00:15 -0600
The Proton Mag Forum
Many thanks to Cris, Peter and George for the valued input.
Some of it quite a bit above my head but I feel I have already
learned something and hope to learn more if you are kind
enough to answer this posting.
Cris: I did not quite understand if the Neodymium "Super Mag"
is a magnetometer, gaussmeter, metal detector or what. Also
not clear if it is the same thing that sells for 99 Quid. I
can afford that amount so if it is simple enough that there is
a reading and a big reading is bad and a small reading is good
(or vice versa), I want one and would appreciate knowing where
I can get product literature and order one as I could find no
reference on the web.
I keep coming back to the "hand held digitial gaussmeter shown
at
http:www.ascscientific/gauss.html
and it seems to me that this or something similar would
work??? (Does anyone know why this web address does NOT
highlight and the one below DOES highlight???
Peter: The only item that I know of that shows it is
specifically to measure the "magnetic susceptibility" of
something is a rather pricey item (I think US$2000.00 or so)
made in Australia and found at
http://www.geoinstruments.com.au/main.html. I also know
TerraPlus and some other firms have them but I think price
even greater than that shown above.
At the risk (or probably certainty) of sounding really
stupid - is a "magnetic suceptibility meter" the same as a
"gaussmeter"???
George: Your method sounds like the most exciting of the
bunch, but considering the knowledge base that I am starting
from and the fact that I am the only man in the world who has
frittered away six months and haven't even selected my
toroidal form yet, if I got started on your project, I'd be
six feet (1.82880 metres) under before I finished. I am not
depreciating or denigrating your greatly appreciated answer -
only trying to interject a bit of humor into MY own personal
failings. Or possibly "LMF".
Again thanks and very best regards to all,
Dale
PS I regret boring you with these things you all know so well
but I just can't seem to learn it from books. In fact for
whatever reason I cannot even read a full page in a technical
book anymore without my mind wandering so bad that I start
the same page five times before I give up.
----- Original Message -----
From: "George Davidson"
To: "The Proton Mag Forum"
Sent: Thursday, June 15, 2000 11:40 AM
Subject: Re: tests for magnetic materials
> The Proton Mag Forum
>
> Mag forum,
>
> Another angle:
>
> Am late on this discussion as I have been maggin in
Mocambique
> and have returned without malaria, cholera or yellow fever.
>
> I have wound a number of coils mostly solenoids and found a
> simple test for magnetic buildup which may be useful, as
follows:
>
> In the field, a magnetized solenoid will show a deviation if
swung
> from E-W to W-E, at worst up to 5 nT . Couldnt find any
effect
> of ferrous contaminated Cu wire and assumed it would cancel
> itself out within the wire**. Did get deviations when
mag probes
> were rubbing against anchor chain in the bilges of the boat
or
> dragged across a warehouse floor or deck of a steel survey
vessel.
>
> The test system I used was a hollow solenoid into which a
bottle
> could be placed for testing the various fluids and over
which a
> PVC housing with various plastics could be tested . Ran it
> overnight at 2 A from 24v DC to get some serious
polarization and
> tested it in the field on top of a 3 Metre wooden pole
which could
> be rotated and the trace observed. Using a notebook and a
> PICOSCOPE oscilloscope a number of measurements of various
> type s could be obtained
> . After that one can use the least offensive of the
various
> materials from the same stocks to build the final toroid
etc.
>
> I like the idea of bifilar windings but always think of the
field
> situation like what cable to use and where the earth will
be .
> Cable is always a problem and stray voltages on a boat can
realy
> become a character -building experience. In addition
,cables can
> be microphonic and change capacitance (pF) with wave
action ...
>
> George
>
> **PS Quiz: a ferrous particle in the very centre of a
current
> carrying copper wire will become polarized in which
direction?
>
>
>
__________________________________________________ ____________
________