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Re: Water resistors electrodes

Original poster: Jim Lux <jimlux-at-earthlink-dot-net> 

At 08:00 AM 6/2/2004 -0600, you wrote:
>Original poster: Marco.Denicolai-at-tellabs-dot-com
>Hi all,
>This may be an old question but...
>I want to make a water resistor using copper sulphate solution and
>copper electrodes. In the range of 10 Mohms.
>Will material from the copper anode be transferred to the cathode and
>change the resistor characteristics?
>Will I get some precipitate (Cu) with AC or not?
>Is there a better material for the electrodes?
>Best Regards

Yes, some material will be transferred, but not a whole lot.  You can 
calculate how much by figuring out how many coulombs (amps * seconds) of 
charge will transfer, then convert coulombs to moles  (about 1E5 
coulombs/mol, as I recall). A mol of Cu is 63-64 grams (probably about 10 cc?)

Since you're probably not running a continuous current of amps...

Say 1 mA (10kV across the resistor) for an hour (3600 sec)... 3.6 coulombs 
-> 2 mg of Cu (I might be off by a factor of 10, but it's a small amount 
either way)

The challenge is always sealing the electrodes to the tubing or pipe, 
especially if you want a totally sealed resistor that is orientation 
insensitive.  Copper works great. You can get copper hardware at places 
selling to the boating/marine market. Brass would probably also work, and 
is more available.

by the way, 10 Meg won't take much copper sulfate, or it will take a long 
skinny tube.

from http://home.earthlink-dot-net/~jimlux/hv/rwater.htm

"As a practical starting point for your calculations: A 1 meter length of 
1/4 inch id tubing filled with 0.1 Molar Copper Sulfate (1.6 g in 100 cc 
water) has a measured resistance of 39.8 Kohms. "