Magnetic Field Measurements
Q)
June 30,2004. Original poster: DRIEBEN@midsouth.rr.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
interrelated? 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-Oersteds, and Ampere-turns and I'm having trouble
comprehending exactly what aspect of magnetism that these other units of measure
are designed to gauge. Thanks, David R.
1) Original poster:
"Laurence Davis"
<meknar@hotmail.com>
>
> David, I struggled with this as well. This link from surrey is quite
helpful... (cruise around the links on this
> page, its rather informative)
http://www.ee.surrey.ac.uk/Workshop/advice/coils/terms.html basic
transformer theory is in there too, or check here...
http://sound.westhost.com/xfmr.htm hope it helps. Larry.
2) Original poster: Ed
Phillips
<evp@pacbell.net>
>
>> The devil himself seems to have inspired the bewildering sets of units for
magnetism and electromagnetism.
Amen to that!
> I've found that using the gauss and oersted seem to work just fine for me, and
almost all of the magnetic material
> catalogs I have seen seem to use them still.
Agreed. And some are using a product of Gauss and Oersteds or Mega-Gauss-Oersteds.
Seems that since we're having to go to "mega" with this measurement we could
just use "Tesla-Oersteds" for this unit ;^)) I suppose it's like the Farad,
which is way too impractically large of a unit for everyday capacitance
measurement as we usually prefix it with "micro" (x10e-6), "nano" (x10e-9), or
even "pico" (x10e-12).
> The Tesla is too large, while the gauss is of the same order as the earth's
field. Any CONSISTENT system is of course OK. For electrical units the choice
isn't quite as large, although there are the electrostatic system, the
electromagnetic system, and now the SI system.
>
> It's of interest to note that almost all of the units are named for some
famous contributor to the field, Gilbert being the first and Gauss and Oersted a
couple of centuries behind. Maxwell is a bit later and
> Tesla later still, so you pays your money and takes your choice. Ed.
3) July 1, 2004.
Original poster: "Dr. Resonance"
<resonance@jvlnet.com>
Physics I & II by Halliday & Resnick covers the basic in good form. It's
commonly used as a college level physics textbook. Usually available thru
interlibrary-loan program. Or, a used copy thru Barnes & Nobles used book
division. Dr. Resonance.
4) July 1, 2004.
Original poster: "john cooper"
<tesla@tesla-coil.com>
Don't know if they have a website but this book may be of help:
The Magnetic Measurements Handbook by Jack M. Janicke
Magnetic Research Press
A Division of Magnetic Research, Inc.
122 Bellevue Ave.
Butler, NJ 07405
I think that I bought mine directly from them 3 or 4 years ago when I was
building a magnetometer. John.
5) July 1, 2004.
Extracted from; B vs H: Sears /Zemansky References; meaning of the Tesla
http://groups.yahoo.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 distinction 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;
Magnetomotive 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.