Light on Lightning

In Lincoln, Neb. Professor John C. Jensen of Nebraska Wesleyan University ponders lightning. Currently he has shed light thereon in the Physical Review.

Scientists do not understand thunder and they still argue about lightning. George Clarke Simpson's "breaking drop" theory has been most widely accepted. Experiments have shown that when falling water drops are made to break on a rising column of air, the drops take on a positive charge of electricity, the air and lighter spray a negative charge. Drops large enough to fall against a rising air current are likely to break up and take a positive charge. Reduced in size they are blown upward again, rising less rapidly than the negative air and spray. Their charge makes them coalesce again until they are large and heavy enough to hover for an instant, then begin to drop again. As the process continues, the raindrops become heavily loaded with positive electricity, while the rising air carries negative electricity to the top of the cloud. A lightning flash results.

The flash seems instantaneous, without beginning or end. But it must begin at one point, go to another, because the resistance of the air to the electric stress must break at one point. Dr. Simpson has shown that it always goes from a positively charged body to one negatively charged. The Earth's surface is negatively charged, the atmosphere positively. Whenever a lacy branching showed in photographs of the flash, Dr. Simpson has taken the direction the branches pointed as indicating the negative pole. But his theory has been that the top of the cloud is negative, the bottom positive.

On the other hand, Dr. Charles Thomson Rees Wilson (1927 Nobel Prizewinner with Arthur Holly Compton) has claimed that the top of the cloud is positive, the bottom negative, and Nebraska Wesleyan's Jensen last month backed him up in the Physical Review. Sitting at night in the window of a high campus building long-jawed, slow-spoken Professor Jensen has been taking photographs of lightning flashes for seven years using a large-size news camera with an extra large lens. For the past two years, with his son's help, he has also been using an insulated metal deck connected with a galvanometer and the ground to measure the nature of the Earth charge after a lightning flash. Jensen's findings have jolted Simpson's theory. He has showed that the lacy branching sometimes points away from the negative point. Offering no new theory, Jensen suggests that Simpson's theories must be revised. Lightning remains mysterious.

Lincoln, Neb. where Professor Jensen ponders the lightning, is a fair place for it. Better is Tampa, Fla. which holds the U. S. record with 94 thunderstorms a year. Best is Baliburg in Cameroon, Africa with 200 a year. San Jose, Calif, has the U. S. minimum with one a year. Most lightning goes from cloud to cloud. What goes from cloud to ground kills about 50 people per year in the U. S.

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