Re: Magnetic N and S poles

[Ludwik Kowalski <kowalskiL@MAIL.MONTCLAIR.EDU>]

Robert Cohen wrote:

> ... Unfortunately, I don't know what "unit pole" means.
>  Consequently, it never seemed like an option.  If "they"
> used to do this, I'd like to know how.  Maybe someone
> can define "unit pole" for me?

According to "A Textbook of Physics For Students of Science
and Engineering" by C. A. Culver (1936) "A POLE OF UNIT
STRENGTH IS ONE WHICH WHEN PLACED AT A
DISTANCE OF ONE CENTIMETER IN A VACUUM FROM
ANOTHER POLE OF LIKE STRENGTH EXPERIENCES
A MECHANICAL FORCE OF ONE DYNE." Like poles give
rise to a force of repulsion and unlike poles give rise a force
of attraction. The law of attraction and repulsion was established
by Coulomb from experiments with long magnetized needles.

The unit of H, Oersted, is defined as dyne/pole, in the context
of this model. "IF A MAGNETIC POLE [OF STRENGTH M]
BE PLACED AT SOME POINT IN A MAGNETIC FIELD
THAT POLE WILL EXPERIENCE A MECHANICAL
FORCE ... [F=M*H] WHERE H IS THE FIELD INTENSITY
IN OERSTEDS. ...

IN A DISCUSSION WHICH FOLLOWS IT WILL BE USEFUL
TO REMEMBER THAT THE TOTAL MAGNETIC FLUX
(LINES OF FORCE) EMANATING FROM A POLE IS 4*PI*M....
IN THE CORRESPONDING ELECTROSTATIC CASE, THE
LINES ABOVE REFERRED TO HAVE NO PHYSICAL
EXISTENCE, BUT CONSTITUTE A CONVENIENT AND
USEFUL CONVENTION."

B is introduced later via B=mu*H, where mu is a dimensionless
material constant called magnetic permeability. It implies that the
dimension of B (gauss) is the same as that of H (oersted). In fact,
mu is practically equal to unity in most materials. B is called flux
density.

"MAGNETIC FLUX DENSITY IS OFTEN EXPRESSED IN
GAUSSES, A GAUSS BEING EQUIVALENT TO ONE
MAXWELL (ONE LINE PER CM^2). FLUX DENSITY (B)
IS OFTEN REFERRED TO AS THE MAGNETIC INDUCTION,
MEANING BY THIS THAT THE INTRODUCTION OF A
MAGNETIC MATERIAL AS A CORE IN A SOLENOID
MAKES IT POSSIBLE TO ESTABLISH BY INDUCTION
A LARGE NUMBER OF LINES PER SQUARE
CENTIMETER AND HENCE A LARGE TOTAL FLUX."

By the way, this magnetostatic model was still very common
in Europe when I was a student. If my memory can be trusted,
the Lorentz formula was introduced as "an experimental fact"
and not as a basis of definition of B, and of its unit. That kind
of approach was no longer common in 1960's. But which
INTRODUCTORY physics textbook was the first to define
B from the force on a moving charge?
Ludwik Kowalski