MadSci Network: Chemistry

Re: Why does only one half of the electric pickle light up?

Date: Thu Aug 24 20:43:09 2000
Posted By: William Beaty, Electrical Engineer / Physics explainer / K-6 science textbook content provider
Area of science: Chemistry
ID: 962461521.Ch

Oh boy, pickle engineering analysis!

This effect is most probably caused by the different wattage intercepted by the nail which is suspended from the pickle and is arcing through space, versus the nail which is contacting the salty pickle directly. If the electric arc presents a large electrical resistance, while the direct contact between the nail and the pickel presents a small resistance, then a "flip-flop" situation exists, and the system is most stable when only one nail is arcing. If true, then AC or DC makes no difference, and the effect is caused by the instability of two electric arcs connected in series.

Let's try assigning some crude numbers to this. Suppose the pickel runs at about 50 watts, and suppose the electric arc has 100x more resistance than the direct metal/pickel contact. Knowing that W=V^2/R, we can calculate that 120 volts at 50W gives a total resistance of nearly 300 ohms, and if the resistance ratio is 100:1, then the electric arc has about 300 ohms while the direct metal/pickel contact has about 3 ohms.

What happens when we first plug the pickel into the wall? Well, if both nails make contact with the pickle tissue at 3 ohms resistance, then the total power is V^2/R, (120*120)/(3+3) or 2400 watts! Each nail would experience half the total, or 1200 watts. With such a high wattage concentrated at the surface of the nails, the water in contact with the nails would fairly explode! At this point a "random decision" would be made, because one nail would generate gas a bit faster than the other, and an electric arc would develop at one nail first. The resistance of the arcing nail would rise to the 300 ohms we calculated before, and the total wattage being used by the pickle would fall drastically. The total resistance of both nails in series goes from the 6 ohms of direct contact to the 303 ohms of one electric arc in series with one direct contact. The total energy goes back down to around 50W again, with almost all of it dissipated by the single electric arc, and the non-arcing nail loses the race. The direct contact gets about 0.5 watt, while the electric arc gets the remaining 49.5 watts, and as a result, the nail with the direct contact stays cool and does not trigger an arc, while the electric arc self-heats, maintaining its own existance. Once the arc appears at one end of the pickel, it tends to stay there.

Note that I'm picking these numbers out of the air, and making informed guesses as to what's occurring. I think the general idea is correct, but the actual values might be quite different than I've speculated above.

Why would the arc move from one end to the other? Well, have you noticed that the electric arc tends to carve a large hollow inside the pickel? In industrial settings this is called "electric discharge machining" and is used to carve odd-shaped holes in metal parts. The arc is extremely hot, and it vaporizes any pickle tissue that it touches. For this reason the arc tends to attack any bit of pickle tissue that protrudes, and when that part of the pickle is destroyed by the arc, the arc wanders to the next highest section of pickle-stuff. After a moment the nail would have no supports, and would fall down and touch the pickle directly. At this point the wattage would rise enormously again (because the two 3-ohm direct contacts in series present only 6 ohms load to the power supply). Again the 2400 watts would blast the pickle, the "random decision" would occur, and one of the nails would heat its way into an electric arc state.

As time went on, the nails would chew down through the pickel, randomly taking turns at supporting an electric arc, or at making direct contact with the pickle tissue. And while the arc was staying at one end, it would continuously move along the nail, creating a strange flickering/crawling light pattern.

I've always been meaning to make an "artificial pickle" that was transparent inside. Cast a block of unflavored Jello with lots of salt added, then use it as a "pickle." The clear surface would give us a window on the mysteries taking place inside. This might make a good student project, eh?

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