Re: Magnetic field measurements

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Harvey Norris <harvich-at-yahoo-dot-com> 

--- Tesla list <tesla-at-pupman-dot-com> wrote:
 > Original poster: DRIEBEN-at-midsouth.rr-dot-com
 >
 > Hi all,
 >
 > I was wondering if someone could direct me to a
 > website that
 > thoroughly describes the different units of magnetic
 > field
 > measurement and how they are interrealted? It seems
 > that there
 > are numerous aspects of magnetic field measurement.
 > I know
 > that the Gauss and Tesla are the basic measures of
 > magnetic
 > lines of flux per cm2 of surface area of the
 > magnet or magnetic surface and that 10 kGauss = 1
 > Tesla.
 > However, there are Webers, Gilberts, Oersteds,
 > MegaGauss-Oer-
 > steds, and Ampere-turns and I'm having trouble
 > comprehending
 > exactly what aspect of magnetism that these other
 > units of
 > measure are designed to guage.
 >
 >
 > Thanks,
 > David Rieben
Extracted from;
  B vs H: Sears /Zemansky References; meaning of the
tesla
http://groups.yahoo-dot-com/group/teslafy/message/225
Units of a B vs H curve found in Sears Zemansky
University Physics indicate B expressed as flux
density as Webers/sq Meter, but the horizontal values
of H are noted in units as ampere/meter. I found that
H definition very bothersome, and went back to
ascertain the unit of Induction. Most know that in the
fifties a honorary unit was named the tesla after
Nikola T. by the IEEE, where in fact Tesla had been
vice president from 1892-1894. In fact the
standardization process of electrical terms have taken
quite some evolvement since Tesla. The American
Standard Definitions of Electrical Terms was not
published until 1941, preceeded by efforts from 1928
to establish standards. The first edition of the IEEE
Standard Dictionary of Electrical and Electronic Terms
did not appear until 1972.

The mks unit of magnetic induction B,(which is a
vector quantity) is one tesla= one newton per
ampere-meter, hence the unit of magnetic flux is one
newton-meter per ampere. The unit of 1 NM/A is named
after Wilhelm Weber(1804-1890). The magnetic
induction equals the flux per unit area across an area
at right angles to the magnetic field. Since the unit
of flux is 1 weber, the unit of induction,1 tesla, is
equal to 1 weber /square meter. The magnetic
induction B is often referred to as the flux density.
(Univ. Phys.-pg 428)
This is a subject that initially I thought easy to
make a distintion between B and H by the mere units
made to formulate them. However further inspection
reveals this is not an easy digression at all. What
seems to be the defining point is how we initially
equate a force with an amperage. In the mks system the
ampere is defined as follows from pg 453;
One ampere is that unvarying current which, if
present in each of two parallel conductors of infinite
length and one meter apart in empty space, causes each
conductor to experience a force of exactly 2*
10^(-7)newton per meter of length.
To decipher the meaning of the ampere-meter, this
seems to sum it up on pg.427;
The magnitude of the B vector at any point can be
defined by the equation F=qvB sin (phi),where q is the
magnitude of a moving charge at the point, v is the
magnitude of its velocity, and phi is the angle
between v and the direction of the field. The mks
unit of B is therefore one newton per (coulomb meter
per second). But one coulomb per second equals one
ampere, so the unit can be expressed as one newton per
ampere meter. This unit is called one tesla

Lastly I have the scribbles from a 1975 physics course
at Kent State from the old standard tattered physics
text, Sears & Zemansky, University Physics;

Magnemotive force (mmf) = total force that produces
magnetic flux. B is expressed as the magnetic
induction, or flux density. If we know the total flux,
the density must be that value divided by the interior
area or A. Thus this first method gives the magnetic
interaction with those first two dimensions to
determine the density. The English unit of mmf is the
Ampere-turn, the equivalent cgs (centimeter/gram
second) unit is named the Gilbert, where the
conversion ratio is shown by 1 Gilbert=.796 Amp turns,
and conversely 1 amp turn= 1.25 Gilbert.
The field intensity or H is the force per unit length
of flux path. We are simply now applying the
definition of B for 3 dimensional space, where in the
English system this is made as amp-turns/inch. The cgs
equivalent is the Gilbert/cm, named the Oersted. 1
Oersted= 2.02 amp-turns/inch. These definitions may be
simplistic as they were made for future reference back
then.

Around the early 80's I also attended Akron State Univ
after dropping out, but the different text from that
same Elementary Classical Physics course does not seem
to deal with H at all, as the other text did. In the
early 90's I purchased another Physics text,(Physics
for Scientists and Engineers) in which the following
is noted on pg 654;

We have named B the magnetic field and H the magnetic
intensity. These names are not universal. Sometimes B
is called the magnetic flux density and H is called
the magnetic field. Admittedly, the terminology is
confusing, and universal adoption of a single set of
terminology is unlikely in the near future.
Fortunately, the usage of the symbols B and H as we
have defined them is nearly universal. Thus the
calculation of a magnetic force on a moving charge or
a current nearly always involves B; similarly H is the
appropriate field in Ampere's Law.

It has cost me a bit of time to try and understand
that thing with Amperes law, as I did not pay
attention then, and integrals need that concept of
summation. I think it can be summed up by guessing
that a linear relationship is made between the amount
of magnetic field B obtained at a certain distance r
away from a conductor of i current. This becomes a
ratio, where a constant is derived. That constant is
known as the permeability of free space,mu(0) or k
determined by the equation (B)(2*pi*r)=k*i

What it seems to be is that B/H= the permeability
constant k, which of course also changes with core
material. To end this long post this is from pg 491
concerning ferromagnetics;
Iron,nickel,cobalt and gadolinium are the only
ferromagnetic materials at room temperature. Because
of the complicated relation between the flux density B
and the magnetic intensity H in a ferromagnetic
material, it is not possible to express B as an
analytical function of H.{Note; I assume the
analytical equation with k=1 then does always apply
with a non saturable air core inductor} Instead the
relation between these quantites is represented by a
graph of B vs H, called the magnetization curve of a
material. The permeability, equal to the B/H ratio {is
not constant for that material.}

HDN



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