Thomas B. Jones
Professor of Electrical Engineering
University of Rochester

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The photograph was taken in a dark room and the camera shutter was held open for about 2 seconds, long enough to capture the entire ignition and combustion sequence. Because of this timed exposure, the flame looks much more extensive than it would if observed with slow-motion photography.

The demonstrator has brought the charged plate of the electrophorus close to the sparking ball outside the chamber on the left, while grounding the ball on the right by holding it. A spark has jumped to the ball from the electrophorus and, simultaneoously, across the gap inside the chamber where the flammable mixture is confined. Both sparks are plainly visible in the photo. The spark ignites the hydrocarbon/air mixture, resulting in a fireball that blows a paper diaphragm off the top. The diaphragm is held in place initially with a rubber band. Once ignition occurs, it is blown upward and apparently moves so fast as not to visible in the image. The flame is blue inside the tube because the mixture of the hydrocarbon and the oxygen in the air is very close to the stoichiometric value and initially the combustion is almost complete. At the same time, however, the pressure has built up very rapidly inside the chamber and blows the paper diaphragm off the top. Now, the burning mixture rapidly expands. The bright yellow-orange flame, almost entirely outside of the tube, is evidence that, during the expansion, the mixture cools and combustion becomes incomplete.


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