Re: [Phys-l] para/dia-magnetism

[John Denker <jsd@av8n.com>]

On 04/19/2011 09:59 AM, Carl Mungan wrote:
> In particular, what connection (if any) is there to the terms 
> "dielectric" (for which the induced electric dipole moment p is 
> parallel to the external electric field E) and "diathermal" (which is 
> a material which allows the flow of heat readily, as opposed to 
> "adiathermal").
>
> Seems like contradictory uses of the word "di" here, but maybe I'm 
> mixing up "di" and "dia" or maybe I'm trying to make a connection 
> when there isn't any.

There is no doubt that di (as in dielectric) and dia (as in
diathermal) have the same root and the same meaning.  You can
easily check this for yourself.  I find the Century Dictionary
to be convenient, because the whole thing is online
  http://www.leoyan.com/century-dictionary.com/

And professional lexicographers hold it in high regard.  It is
of course a snapshot of the language as of 110 years ago ...
but ancient roots remain the same.

I reckon the idea of dielectric refers to the displacement
current.  AC flows through a dielectric, and the more dielectric
it is, the more current there is (per unit dV/dt, in a given
geometry).

So that part makes sense.

===================

As for diamagnetic, I agree that sounds backwards.

Taking a wild guess here, it /might/ be explainable in terms of
the issue of whether B or H should be considered "the" magnetic
field.  I am not a historian and I hate to throw around rumors,
but I /think/ I heard somebody say that as late as 1960 it was
somewhat controversial for Feynman to call B "the" magnetic
field (demoting H to second-class status).  Please don't quote
me on this, but take it as a hypothesis to be checked.

I think the old terminology was
  H = magnetic field                       [1]
  B = magnetic induction

This [1] looks crazy to me ... but I was exposed to Feynman at 
an impressionable age.  The counterargument is that if you are 
building iron-core electromagnets and transformers and suchlike, 
you are more interested in H than B, so maybe [1] is not quite
as crazy as it looks.

If you reverse the roles of B and H in the equation, you reverse
the meaning of diamagnetism.  Sort of.  Maybe.  Please check.