Salt water capacitor question?

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The salt water is only used to make one of the plates. A bottle full of salt water, with say foil on the outside. It is easier to fill with conductive water than to try to stick foil on the inside. These devices are used to get a high voltage capacitor, with the relatively thick glass of the bottle providing insulation. I am sure the salt water has some effect on the capacitance and the series AC and DC resistance, but most would just ignore these effects. The capacitance per bottle is quite small, something like 800pF (one pF is one millionth of one microfarad). Doesn't seem much point in using this with low voltages like 200V, where an 800pF x 400V or 600V capacitor costs peanuts and is smaller than a postage stamp roughly. If it is a DC biased application, electrolytic capacitors are much smaller for a capacitance of tens to hundreds of micro-farads, and far less messy. If it is AC application there are capacitors used for motor starting, the capacitance is a few micro-farads. Still much smaller than a bottle, and it would take about 12,500 bottles to equal 10 micro-farads.

I have seen a picture with about 50 bottles in a box, made from beer bottles. I guess the attraction is more to do with getting the empty bottles than reality, unless it is a true high voltage application, many thousands of volts at significant RF current like with Tesla coils. They make sense for the traditionalist, and a rugged capacitor is needed for the spark gap type.

If you roll up 2 sheets of foil and thin sheets of plastic insulation (search for capacitor construction or see second link below) you will get a smaller device. Maybe plastic tape would do, though there can be issues with pin holes. Also alternate glass or plastic sheets with foil will work. All the even foils are one plate, and the odd foils the other plate. The insulation only needs to be thin for 200V. Paper has been used in the past, usually oil impregnated and sealed with wax. Moisture is the enemy of these things. Put them in a bottle of oil by all means....Show more

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How did you make the capacitor? And what was the salt water, a plate, the dielectric? Capacitor charge and then block DC so there wouldn't be any current flowing through the capacitor....Show more

Hi Steve h,

I am surprised to hear that salt water capacitors even exist. The salt water would electrolyse as soon as a potential difference is applied and hydrogen would be generated....very volatile!!!

Where did you see salt water capacitors? A link would be useful....Show more