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I brewed a CuSO4 resistor in a glass jar. I got the idea from here. This was done by creating a solution of Copper Sulphate in distilled water. The lid on the jar was provided with a BNC socket into which copper leads were soldered that are immersed in the solution described above. For the sake of providing a mechanical support & ground, I attached a couple of Aluminium plates on either side of the mounting lid threads. At the moment this is only a proof of concept for me without any value defined.

After putting it together, I attempted to determine the value of the brewed device using a digital multimeter. The device appears to behave like a capacitor.Upon connecting the leads the initial resistance displayed was 148 ohms. This decayed to 136, and lower. The longer the DMM leads are connected, the lower the resistance value, and the longer the amount of time required for the rendered value to decrease. A classic decaying exponential curve.

Why does this brewed CuSO4 resistor behave like a capacitor? What can I do to have a static value rather than a decaying value?

chicks
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VU2NHW
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    this sounds like a battery cell more than a resistor? In fact, I'm pretty sure you've built a battery. – Marcus Müller Dec 23 '22 at 10:13
  • If the two metals touching your CuSO₄ solution are really just copper and Aluminium (pure, clean, both), then I'd roughly expect a potential of around 1.7 V between the two terminals of your device, with nothing but a voltmeter connected. – Marcus Müller Dec 23 '22 at 10:20
  • The only metal touching the CuSO4 solution is Copper. A couple of small aluminium pieces - no more than an inch in diameter - provide mechanical strength to the BNC Socket at it's mount point. – VU2NHW Dec 23 '22 at 12:14
  • so, both electrodes are copper? – Marcus Müller Dec 23 '22 at 12:41
  • The link you posted gives me a 404-File Not Found error :( (maybe a photo would help drive the concept home?) – Marcus Müller Dec 23 '22 at 13:00
  • (matter of fact, even in the archive.org archival of the page from 2017, the image links on the page are dead. That looks a bit like a broken website :( ) – Marcus Müller Dec 23 '22 at 13:07
  • That's odd ... It rendered beautifully for me yesterday. Also, both electrodes are copper. Anyhow, here is [another link](https://www.pulsedpower.net/Applets/pulsedpower/coppersulfateresistor/coppersulfateresistor.html) – VU2NHW Dec 23 '22 at 13:53
  • Thanks for the new link. Sadly, I don't understand the drawing. Where is your BNC connector in there? what connects to what part of the connector? What are the (two?) copper electrodes? I honestly think we'll need a photo of your setup, or at least a technically precise drawing :) – Marcus Müller Dec 23 '22 at 18:06
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    The resistance is dropping? That's not a capacitor. A capacitor's resistance on a multimeter increases as the capacitor charges up. I would suggest measuring it with bigger voltages, say a 10 or 20 volt supply and your meter measuring current. Also, such a chemical and fluid system is never going to be very stable, so I'd say lower your expectations. – electrogas Dec 23 '22 at 18:17
  • @electrogas: Upon flipping the electrodes, the resistance increases for a while - then starts to decrease again. – VU2NHW Dec 24 '22 at 06:26
  • @MarcusMüller: My contraption is quite different. I use a glass jar. Therefore the BNC connector is mounted on the lid of the jar. Both elecrodes are connected to the centre-pin, and ground-tab of the connector itself with a separation under 1cm. – VU2NHW Dec 24 '22 at 06:32
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    if you have a different thing than what is depicted, please add a picture of your own; I'd love to understand this. – Marcus Müller Dec 24 '22 at 09:21
  • It's not a battery, because both electrodes are made from identical material. – Mike Waters Dec 30 '22 at 22:31
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    It just occurred to me that this is off-topic. eeSE or chemSE might be a better place for it. – Mike Waters Dec 30 '22 at 22:39
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    We can migrate this to another site. Please ask permission to migrate in the main chat of your target site, and let us know by pinging me (@mikewaters) in a new comment here. – Mike Waters Dec 30 '22 at 22:46

1 Answers1

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I'm not sure of all the chemistry but I would assume that something is changing in the electrodes and/or the solution as you drive current though it. Perhaps the copper wire is being eroded in patches where it's clean, slightly increasing its surface area. Or there are impurities that are plating out in the surfaces.

So it is behaving a bit like a battery, not just a resistor.

Try measuring the cell for residual voltage once you've "charged" it?

What resistance do you see if you keep swapping the probe polarity fairly quickly? For a slow-moving reaction like this, swapping probes is a bit like measuring its AC impedance.

tomnexus
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    Upon flipping the probes, the displayed resistance changes. I flipped probe poles when it was decreasing from 100 through 96. The displayed resistance then started to go up. It went up as far as 156, then started to decrease again. It appears to behave as an electrolytic capacitor - I think. – VU2NHW Dec 23 '22 at 13:50
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    The voltage across the terminals of the BNC reads 0V. – VU2NHW Dec 23 '22 at 15:01
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    This is emulating a lead-acid battery -- initially both plates are pure lead (or copper, here), but they become coated with sulfide and sulfate as the battery charges. Over time, the resistance should begin to rise again as the "battery" charges up to near its maximum (probably close to 2V) or the test voltage, whichever is less. – Zeiss Ikon Dec 28 '22 at 12:14
  • @ZeissIkon Both plates in a lead-acid battery are a grid-like structure filled with a lead compound. On charging, the compound in one grid changes to another one. Not sure what happens to two pure copper electrodes. – Mike Waters Dec 30 '22 at 22:35
  • @MikeWaters Modern "dry charge" batteries are manufactured with pure lead and lead sulfide coated plates, so they can be filled with acid and be "already charged" -- but it's perfectly possible to make a working lead-acid battery from pure lead for both electrodes and establish the sulfide-sulfate polarity in either direction by charging. Copper sulfate is soluble, however (lead sulfate is not), hence the odd behavior of this "battery". – Zeiss Ikon Dec 31 '22 at 21:08