Subject: Re: paramagnetic dusts

From: David Menne <[email-masked]>
Date: Wed, 4 Dec 2002 11:03:48 +0800

----- Original Message -----
From: "Dale Wilson" <[email-masked]>
To: <[email-masked]>
Sent: Wednesday, December 04, 2002 6:17 AM
Subject: Re: [SANET-MG] paramagnetic dusts

> David and Alan,
>
> Could it be that the amount of paramagnetism in rock dust is mostly a
> reflection of the iron content?  There are not very many stably
> paramagnetic elements, and the most common is iron.  Maybe finely
> ground rock dust high in iron produces plant growth responses under
> some conditions because it supplies iron.

Magnetizable minerals include the ferromagnetic minerals (strongly
magnetizable) and any of the paramagnetic (moderately magnetizable) minerals
and other substances.

The former include magnetite, hematite, iron titanium oxides, pyrrhotite,
maghemite, greigite and goethite, minerals capable of acquiring remnant
magnetization and useful for paleomagnetic studies.

The latter include a broad array of substances all of which contain Fe2+,
Fe3+, or Mn2+ ions.  These paramagnetic minerals may include clay minerals
(chlorite, smectite and glauconite), iron and manganese carbonates
(siderite, rhodochrosite), ferromagnesian silicates (olivine, amphiboles,
pyroxenes, etc.), as well as a variety of ferric-oxyhydroxide mineraloids.

Regarding paramagnetism, I cannot see that ferromagnetism should not offer
very similar effects.

It is only in the magnitude and linearity of the magnetic permeability that
they differ.

A simple definition of paramagnetism is that the atoms or molecules of a
paramagnetic substance have a net magnetic spin such that the spins are
capable of being temporarily aligned in the direction of an applied
electromagnetic field when they are placed in that field. This produces an
internal magnetic field (magnetic moment). They differ from magnetic
substances (such as iron, nickel, & cobalt) where such spins remain aligned
even when they are out of the applied field, e.g. are permanent.

Magnetic susceptibility is measured, according to the physics handbook, in
millionths of a CGS unit (Centimeters Grams Second), 1 x 10[- 6] CGS, or
microCGS.

Ferromagnetic effects should be far greater [effective].

The fact that coarser rock crystals demonstrate better performance is
interestingly supported by the theory that coarse paramagnetic crystals
should draw in magnetic fields slightly, depleting the surrounding soil :
effectively acting as magnetic lenses.

Possibly at suitably oriented sharp points, the local magnetic intensity is
raised [But very marginally !!! Almost undetectably small - the
magnetisation vector of paramagnetic materials is about 4 orders of
magnitude lower than the
local magnetic field strength of the earth].

One should be able to achieve as much, and more, with tiny quantities of
ferromagnetics : 1 atom of ferromagnetic will effect the same change as
1000000000 atoms of a paramagnetic.

This makes me think it is not the paramagnetism which causes the effects, it
is possibly just fortituous that another favourable property is directly
related to paramagnetism. My mind [conditioned by conventional scientific
training] can only conceive of an electric phenomenon as alluded to in your
explanation.

A description of what and how the Ptotovale metal detectors work sheds a but
moe light on the above :

The Pulse Induction technique used by all of Protovale's metal locators
should respond well to metallic good conductors in bulk, weakly to poor
conductors, and not at all to purely magnetic non-conductors. Nevertheless,
a small number of situations exist where a significant signal is received
from certain magnetic minerals.

To understand the origin of these occasional spurious signals, it is
necessary to consider both the P.I. technique and the mechanism of magnetic
phenomena.

In the P.I. method, pulses of current are repeatedly sent through a coil in
the search head, and produce a pulsed magnetic field which propagates to the
target. At the instant of switch-off of this primary field, a large
transient back-emf voltage is induced in the coil which however only lasts
for a few microseconds after which there is no voltage across the coil until
the switch-on of the next pulse.

The switch-off will also induce eddy-currents to flow in any conductive
target present; since there is no source of energy to maintain them, they
will decay away but nevertheless persist for a time of several to a few
hundred microseconds, ie for longer than the primary switch-off transient.
These eddy-currents generate a secondary magnetic field which propagates
back to the search head and induces a voltage in the same coil that
generated the primary field pulse. The electronics of the receiver circuit
samples the coil voltage after a delay which is long enough to miss the
primary back-emf spike, but short enough to include the eddy-current signal
(if present).

Poorly-conducting materials such as thin foil and alloys such as stainless
steel produce a signal with a very rapid decay time comparable to the
sampling delay in the receiver circuit: careful selection of this delay time
(and of other related pulse-widths) enables an instrument to be set to
either detect or ignore such objects. The ionic conductivity of salt or
brackish water is so low (by comparison) that any signals generated decay
away completely within the delay time, and so generate no response.

A purely magnetic non-conductor, such as ferrite and most magnetic minerals,
will become magnetised by the primary field and will de-magnetise
immediately on the removal of the primary field, and so will not induce any
signal in the coil during the delayed sample. A conductor which is also
magnetic (ferrous) will produce a signal in exactly the same way as a
non-magnetic conductor, but the strength of the response will be magnified
by the effective permeability of the target (which depends on shape and
orientation as well as on the absolute value of the relative permeability of
the material).

The above list of examples should account for all target types, but there
remains one further phenomenon: magnetic viscosity. Here the material is
non-conductive, so eddy-currents are not generated. The material exhibits a
magnetic permeability (or susceptibility), but this should not generate any
signal during sampling. However the term "viscous" refers to the fact that
the induced magnetisation does not vary instantaneously with applied field,
but changes sluggishly, so that at a finite time after the primary
switch-off the magnetisation is still present and reducing; this produces a
signal of much the same time-characteristics as an eddy-current signal and
so does generate a response in the detector.

David Menne : http://www.plantsfood.com

"We can't solve problems by using the same kind of thinking we used when we
created them" - Albert Einstein (1879-1955)

"Only the small secrets need to be protected. The big ones are kept secret
by public incredulity" - Marshall McLuhan (1911-1980)

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