Re: Magnetic N and S poles

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

Brian McInnes asked:

> (2) What were the introductory texts of the 30s and 40s that
> preceded Sears, and Hallidy/Resnick and all the clones and
> derivitives since then?  Are any of these remembered? Are
> any available for perusal in libraries or library repositories?

I have "A Textbook of Physics For Students of Science and
Engineering" by C. A. Culver (1936). The author was a professor
of physics at Carleton College. Electrostatics and currents are
more or less as in today’s textbooks. But the chapter on magnetism
begins with Coulomb’s law for magnetic poles: F=m1*m2/r^2.
The magnetic field H is defined in terms of force per unit pole.
There are many examples in which students are asked to calculate
H at difference locations around a bar magnet according to this
magnetostatic definition. Magnetic field created by a wire is said
to be measured by using a magnetic dipole (compass needle). No
attempt to redefine magnetic field in terms of q*v or in terms of
I*dl can be found in the later sections of the text.

The first time I saw B defined in terms of q*dv was in Sears and
Zemansky. When was it first published? Probably before 1963.
Look at the "first physics" books you have and share the title
of the oldest one in which B is defined by F=q*(v cross B).

> I suspect that no INTRODUCTORY textbook published before
> 1945 defined B in terms of  Lorentz formula. But I may be wrong.
Ludwik Kowalski
P.S.   ********************************

"First Course in Physics" by Millikan and Gale, copyright 1906.
No electric field lines in illustrations of Static Electriciy. The
only field lines are magnetic, not electric. The first is a circular
H line about a straight wire. The magnetic needle "always sets
itself tangent to the circumference of a circle whose center is
the wire and whose plane is perpendicular to the wire." The
right hand rule is then introduced as a method of correlating
the direction of I with the directions of magnetic lines.

Lines of force seem to be defined loosely as patterns displayed
by iron filings. On the next page the unit Ampere is "DEFINED
AS A CURRENT WHICH, WHEN FLOWING THROUGH
A CIRCULAR COIL OF 10 TURNS AND 10 CM RADIUS,
WILL PRODUCE AT ITS CENTER A MAGNETIC FIELD
OF STRENGTH EQUAL TO 2*PI DYNES." I suppose that
"the unit pole" was implied in this definition.

The authors show that a wire loop behaves like a magnetic
dipole but nowhere do I see an attempt to define magnetic field
in terms of q*v or in terms of I*dl. It is the other way around;
the current is defined in terms of its magnetic effect, for example
on the compass needle or on the N pole of a long bar magnet.
The effect of the magnetic field on a wire is described and an
electric motor is explained.

The later version of the textbook Copyrighted in 1928 (with the
third author, C.W. Edwards) does not seem to be very different
in that respect. Another highschool textbook "First Principles of
Physics" by R.W. Fuller, R.B. Brownlee and D.L. Baker
(copyright 1932) is very similar to Millikan’s book.