Re: Magnetic field measurements

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

Original poster: "David Rieben" <drieben-at-midsouth.rr-dot-com> 

Harvey,

Thanks for your most informative explanation of this. I'm not too sure that
my "pea brain" comprehends much more  than I did before, but you're
explanation of the dif-
ferent aspects of magnetic field measurement was most
informative. It seems that magnetism is a farily complex phenomenon which is
not easily grasped in technical terms.

David Rieben

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Thursday, July 01, 2004 9:14 AM
Subject: Re: Magnetic field measurements


 > Original poster: Harvey Norris <harvich-at-yahoo-dot-com>
 >
 > 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
 >
 >
 >
 > =====
 > Tesla Research Group; Pioneering the Applications of Interphasal
Resonances
 > http://groups.yahoo-dot-com/group/teslafy/
 >
 >