Re: a physics textbook error, a late reaction

[Brian McInnes <bmcinnes@PNC.COM.AU>]

Dear Willem,
Thank you for your detailed historical tour through the
publishing of researches associated with Ohms "law".
However, after reading it, I am left with a concern on how
much of the original result was looking at whether various
substances conducted electricity and how well they did it
and whether any of it directly impinged on the "law".  As
you write

> Around 1825 Ohm used a
> Voltaic pile
> and a primitive galvanometer to establish the conductivity
of wires
> made out
> of different metal. He found the following conductivity "scale"
> copper, gold,
> silver, zinc, brass, iron, platinum, tin, lead, copper being ten
> times
> "better" than lead at conducting electricity. His scale was
flawed,
> among
> other things because of a large impurity content in the silver
> wires.
> Nevertheless this  was the very first recorded measurement of
> conductivity


Now what Ohm's Rule of Thumb (Yes, I agree most strongly
with you, Chuck) is that there is a linear relation between
potential drop and current.  Did Ohm explicitly come to that
conclusions from his research?

Again, you write

> his conclusions were published in 1827 in his
> book "Die
> galvanische Kette" (The galvanic circuit). In this book the notion
> of
> "potential drop" is first introduced and what we now know
as "Ohm's
> law" is
> for the first time enunciated in a more or less recognizable form.


I'd very much like to see what this "more or less
recognizable" form was.  What worries me is that the range
of the proportionality is so restricted that I wonder how
Ohm derived it from his readings.

His early work you refer to seems only to be with the
conductivity of different metals.


You write about later work
,
> In later experiments Ohm used a
> bismuth-copper
> thermocouple at a temperature difference of 100° C as a current
> source, a
> suggestion made by Poggendorf. He conducted an extensive series of
> measurements


Did his formulation of his law become more recognizable in
this work?

Brian McInnes