How to Better Share the Current between Electrodes

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mrjeffy321

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I have an electrolytic cell with 10 (graphite) anodes and 1 (steel) cathode. The steel cathode in placed in the center and the 10 carbon-rod anodes are placed in two concentric rings around the cathode (4 rods on the inner “ring”, 6 rods on the outer ring).
I have electrically connected all of the carbon rods together using Copper wire. I first connected all the electrodes together as a ring, and then I connected the two rings together in 3 separate places. Ever point on any of the carbon rods are electrically connected to every other point on the carbon rods.

When I fill the cell with electrolyte and connect up the wires I find that the carbon rod that I connect the + wire to seems to get a disproportionally large chunk of the total current. Almost all of the current is going through that rod and a much smaller fraction is sent to the other 9 rods.
I figure that all those rods are, essentially, connected in parallel with the one cathode. Although small, the difference in the resistance between the electricity flowing straight down into the cell versus first traveling through the network of wires and then down a separate rod is keeping the current from being shared equally between the rods.

Ideally, I would like (roughly), the same current through each rod…have the total current into the cell split fairly equally between the 10 rods.
What modifications might I make to make this happen?
 

marcusl

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Make sure the wires you use are large enough. Small wires can have a significant voltage drop (especially if the current is large) that will prevent all rods from being at the same potential. You can measure the potentials at each rod, in fact, with a digital voltmeter.
 

mrjeffy321

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You suggestion did help to reduce the problem.
I was surprised to measure such a large difference in potential between the rods…as much as 3.5 volts in some cases.
What I did was to install a whole new network of wires (the same system as before) using a much thicker gauge wire…all in addition to the wire set up I already had. The large gauge wire add-on brought the difference in potentials down to less than 2 volts (usually less than 1.5 v). So the problem is better, but still could improve.

Design wise; is there something better I could do? Such as, perhaps, creating a central connecting point (which is not a rod), and then from this connect all the rods using equal length wire?
Right now, the top of the cell looks like a spider’s web of wire connections (which were not fun to install and the hot glue I used to insulate the connections and hold the wires in place easily melts from the heat dissipated by the current).
 

Gokul43201

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1. What gauge wires are you using?
2. How are the wires connected to the graphite rods?

Make sure the gauge is enough that the wire resistance is small compared to the electrode+electrlyte resistance.Check the various resistances with a multimeter. Also important is the contact resistance (you can check this too with the ohmmeter). For the current to flow from the n'th rod to the n+2'th rod, does it have to go through some part of the n+1'th rod (or does it only go through copper wire)? The latter option would be preferable to prevent drops from poor electrical contact.
 

marcusl

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Definitely look into Gokul's suggestions. They could solve your problem. Something you can do to clean up the "spider web" is to mount your carbon rods to a solid copper plate. Drill 10 holes in the plate to mount the rod clamps (or whatever attachment means you are using). A single hole, maybe for a 1/4-20 nut, bolt and washer, can be used to clamp down your current feed wire (or, even better, ring lug). How much current are you supplying?
 

mrjeffy321

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1. The thicker wire is 18 gauge. I do not know how thick the thinner wire is, but it is much smaller.
2. The wires are wrapped tightly around the rods and then hot-glued in place so they don’t slip and come loose.

Gokul did bring up something I had not thought about...connecting all the wires together directly (Copper wire to Copper wire) and not through a rod (Copper to Carbon to Copper). In some places, I was doing the latter just because that was easier to install. I think that probably there is a big loss in the poor electrical connections between the wires and the carbon rods.

I will try to figure out a better way of installing the rods and connecting the wires to the rods after I take a trip the hardware store to see what kind of stuff they have which I might be able to use. Maybe I'll get some metal clams which will fit around the rod and squeeze the wires against the rod.

The current through the cell is usually between about 10 and 11 amps.
 

Danger

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Since I have no idea how big your electrodes are, I don't know if this is sensible or not. For connecting them to the copper plate, if you're following marcusl's suggestion, how about automotive battery clamps? You could use over-length bolts through the wire clamp part and screw them into the plate for anchorage. Then just tighten the terminal clamp part around the electrode.
Or would the lead mess up the chemical reaction?
 
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Just asking isn't the use of steel for the cathode a bad Idea due to the possible production of Hexavalent chromium when it breaks down?
 

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