Thanks for the added details and picture from the procedure. The snap connector on the 9V battery has two leads. The red one is positive and the black one is negative, so it is the negative terminal of the battery that is connected to the right side copper wire because and the other wire of the conductivity sensor leads to the negative terminal on the multi-meter.
So, there is DC current flowing through the solution from one copper wire to the other and this can cause electrolysis of the solution. I still cannot tell you why the right copper wire turned brownish after you hooked up the circuit, but it is most likely a deposit of some metal, maybe iron, or else a reaction of the copper with something in the liquid.
Did you try cleaning off the copper wire? Did the deposit come off or did it seem 'plated' onto the copper? I would clean the tarnished copper wire or else replace it with new wire and try the conductivity bridge again with salt water and see if you get the same brown deposit--I'm betting that you won't.
Let me know what happens.
Sybee
Electrolytes - Endurance Athletes
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