Re: Mysterious electric current ! (fwd)

[Mark Sylvester <msylvest@spin.it>]

I tried out Yehoshua's curious effect in the lab this evening, and think I
found the answer. I used two copper calorimeters, croc clips and leads and a
lightspot galvanometer that gives a good 20mm deflection per microampere.

Sure enough, I was getting up to 2 microamps pouring tap water from one
vessel to the other, the current direction initially changing with the
direction of pouring. The conventional current seemed to go with the water
flow. Then it stopped working in one direction. Dumping the water quickly
produced a bigger current than trickling it slowly. The more I investigated,
the more confusing effects accumulated. 

Then while switching the leads I momentarily had both croc clips on the same
vessel, and there was a good microamp flowing, and the answer was clear
before my eyes: thermocouple. My hot hand was setting up a sufficient
temperature gradient for the chrome plated croc clips on the copper to
generate the current. 

Dumping the cold water back and forth while holding the copper vessels in my
hands was producing a variety of temperature gradients, depending on just
where the leads were attached and so on. Using solutions of different
conductivity would certainly change the current, as reported originally, but
I cannot say why the mercury had no effect without more information.

A nice one.

Mark.



> Date: Wed, 19 Feb 1997 16:27:03 -0500 (EST)
> From: Joseph Bellina <jbellina@saintmarys.edu>
> To: Teaching Physics <phys-l@atlantis.cc.uwf.edu>
> Subject: Mysterious electric current ! (fwd)
> Message-ID: <Pine.HPP.3.91.970219162601.19859E-100000@jade.saintmarys.edu>
>
> 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
Mark Sylvester
UWCAd, Duino, Trieste, Italy.