Re: [Phys-l] Which one is positive?

[John Denker <jsd@av8n.com>]

On 06/17/2010 06:22 AM, ludwik kowalski wrote:
> Suppose you have a low voltage dc power supply whose terminals look
> alike. How to find out which one is positive? Here is one suggestion,
> made by chemist.
>
> "A good test for the proper connection is to use a silver wire anode and
> a platinum wire cathode in a sodium chloride solution. When current is
> passed, you should see an insoluble white precipitate of AgCl forming at
> the  silver anode.  If you don't see this, then your cell is connected
> backwards."
>
> I did not know this.

Is this a joke?  The electrochemical method sounds to me 
more impractical than measuring the height of a building 
with the proverbial barometer.
  http://www.rbs0.com/baromete.htm

If it's really a low voltage, the method won't even work,
because you're creating a small Ag/Pt battery, and if 
the voltage under test is small compared to the battery
potential, you'll get the wrong answer.

There are other problems, including abuse of the terms
"anode" and "cathode".
  http://www.av8n.com/physics/anode-cathode.htm

> From the keen-grasp-of-the-obvious department:  If you 
want to know which is positive, use a voltmeter.

If there is some heretofore-unmentioned constraint that
prevents you from using a voltmeter, you need to ask a
different question, being careful to say what the
constraints are.