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Mysterious electric current ! (fwd)



Anyone care to comment?

---------- Forwarded message ----------
Date: Thu, 19 Feb 1998 19:50:17 -0800
From: Yehoshua Sivan <yoshusi@talmon.openu.ac.il>
To: chemed-l@atlantis.cc.uwf.edu
Subject: Mysterious electric current !

Today I saw a phenomenon which left me very inquisitive. Maybe someone
can either explain it or refer me to a source.

Two glass beakers (250 ml; I imagine with smaller ones the effect would
be less) were used, with a roughly made lining of aluminum foil; to the
lining in each beaker a crocodile clip was attached, and the two wires
were joined to a micro-ampermeter. I imagine that it would be much
simpler to use two aluminum beakers (such as are often used for
calorimetry experiments), or copper ones.

First of all pure ("distilled") water was placed in one beaker, and then
poured into the second, so that for a couple of seconds the stream of
water became a contact between the two sections of aluminum foil. No
current registered.

The procedure was repeated with tap water (ours is very hard, with
plenty of ions). THIS TIME A SMALL CURRENT (under 5 micro-A) DID
REGI=FDSTER. It seemed to me that the current was in the same direction
when the water was poured back, but I did not have time to check this
properly.=20

Later I repeated the experiment with a saturated NaCl solution, and the
current was much more significant (about 10-15 micro-A), so that I was
able to see a small current in the reverse direction as the pouring drew
near to the end. Maybe because my set up was so crude (I just had a few
minutes to try this out), I didn't come to a definite conclusion as to
whether the direction of current depended on which direction the pouring
was done.

Using mercury, there was no current at all.

I ruled out static electricity as the cause, as the distilled water gave
no current. I am currently teaching electro-chemistry, and thought that
the Helmholtz double layer effect (slight ionization of the aluminum,
leading to a slight negative charge on the foil) might help me, thinking
that where there was little water (the lower beaker at the beginning of
the pouring) there would be less charge than on the upper beaker. But
then I thought that this would make no difference, because it is the
charge concentration which is significant, and this would be the same
for both.

My physics teacher colleague who showed me the effect had no explanation
for it. He thought that since an electric current could be defined as a
flow of charge in one direction, moving past a stationary opposite
charge, this would be the cause of the current on pouring. This idea
doesn't "hold water" to my way of thinking, because both the positive
and negative ions are flowing together here, in the same direction. And
if this idea was correct, then perhaps the mercury should have given the
effect, which it didn't.

Looking forward to hearing either further observations (there are
several variables: volume of water, rate of pouring, type of metal
container, concentration of salt solution, etc.) or an explanation.

Yehoshua Sivan
Menachem Begin High School
Safed
Israel
yoshusi@talmon.openu.ac.il