Electric Pickle

Some specialty and novelty stores sell an electrical “appliance” that cooks hot dogs between two metal electrodes. A protective cover with a safety interlock closes over the device before electrical energy in the form of a standard AC current can be applied.

Suppose that instead of a hot dog one places a pickle between the electrodes. When the room lights are dimmed, the pickle glows impressively, predominantly at one end. What is the physics, and what might the glow look like?

Answer

Even though the electrical energy source is provided through an AC current, the pickle glows predominantly at one end with a yellowish color that is determined by the pickling solution and the pickle type. Reliably predicting which end will glow has not been achieved. There is no actual symmetry here in the shape or chemical composition of the pickle, so alternate glowing is less likely and never seen. The conjecture is that the pickle is now acting like an electrical diode, passing current in one direction only!

The authors listed below performed an experiment by taking a visible light spectrum of the glowing pickle, using a spectrometer with a diode array detector. A fiber-optic probe was used to channel the yellow glow to the spectrograph, and a calibration spectrum was taken of a sodium chloride flame test. The emission spectra of the two are nearly identical This pair of emission lines, at 589.00 nanometers (nm) and 589.59

nanometers, indicate a characteristic of sodium emission, called the sodium D line doublet. Josef Fraunhofer observed these lines in the emission spectrum of the Sun, in about 1817. We know now that these lines are due specifically to an electronic transition of sodium atoms in the gas phase.

The pickle conducts electricity due to the vinegar (acetic acid) and sodium chloride salt used to make it. Sodium ions in the pickle liquid attach electrons from the flowing current. These ions are neutralized electrically, forming excited sodium atoms in two different excited electronic states (hence the emission doublet). Because of the heat and sparks and general pandemonium around the electrodes stuck in the pickle, these sodium atoms are in the gas phase. They emit yellow light as they relax to the ground state.