reversed dry cell

[Mark Sylvester <msylvest@spin.it>]

> From: "Cooper, John N" <jcooper@bucknell.edu>
>
>   Keith C. Tipton wrote:
>
> > How do batteries' performance vary with time on the shelf?
>
> Keith,
>      Most commercial batteries (dry cells) are composed of solids whose
> activities are nearly constant, hence their potentials don't diminsh MUCH
> as the charge goes down....


I am reminded to ask about a curious thing I noticed some weeks ago.

We were doing introductory labs on currents and p.d.'s and the class had to
make I/V measurements on elements inside black boxes. To save the meters I
give them lots of dry cells and trays which take four at a time, so that the
maximum current they can put through anything is limited.

One group "complained" that there was something wrong with one of their
batteries: it decreased the voltage instead of increasing it. I went to have
a look, and said, somewhat facetiously, we'd first check that the battery
had the correct polarity. Imagine my surprise when I found that it did not:
the  + end was negative and the - end was positive. This was an alkaline
type 1.5V battery of the kind you normally use in a tape recorder. It was
from a batch we'd bought last year, and so had probably been subjected to
unusual treatment in the course of student experiments. It gave something
under 1V with the wrong polarity, but could not supply much of a current. It
was quite consistent - we kept it as a curiosity for a couple of weeks, and
it maintained its negative emf throughout.

Has anyone seen this before?

Mark

Mark Sylvester
UWCAd, Duino, Trieste, Italy.