This is an unusual experiment which is very simple and is detailed here. Basically two aluminium electrodes in a sodium bicarbonate solution will glow and sparkle with sufficient applied voltage. One electrode here is aluminium foil and the other is an aluminium bar.
“Continue reading” for details and photos.
The top photo shows a sparkling of both electrodes with 150 VAC applied. The right photo shows a diffuse glow seen with dark adapted eyes, which starts at about 60 VAC associated with a little sparkling of the solid electrode.
Here’s my take on what is happening. If you insert an aluminium electrode in a solution then it will form an oxide layer and when the metal is positive will block current flow. This, as I understand, is the basis for electrolytic capacitors and the reason they are polarized. Here we have two aluminium electrodes which is the equivalent of back to back electrolytics which are then non-polarized.
Initially there is a lot of current flow e.g. 10V at 1A but this rapidly drops as the oxide layer builds up. After a few minutes there is little flow even at 100 VAC. Exceed this however and things start to happen fast. At 150 VAC the current rises to 5 A (750 W) and widespread sparking is seen particularly at the electrolyte/air/electrode junction. I interpret this as breakdown of the oxide layer with voltage. There is a diffuse glow starting at about 60 VAC but the sparkling becomes more prominent with increasing voltage.
Note that the alkali solution boils rapidly with the heat. Eye protection is very important here.
You know, I didn’t even measure the capacitance. This is a “must redo” experiment with more data and better photos and videos in close up of the sparking changes on the electrodes… and measuring capacitance.
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Photo Date: 2006