Light on Lightning

Occupants of Manhattan's Empire State Building, world's tallest structure, have quailed many times at a brilliant bolt of lightning accompanied by an immediate clap of thunder. Many of them have not realized that the building itself is often struck, that since the steel frame of a big building acts as a lightning rod, carrying off the charge, its occupants are well protected.

Because the Empire State Building is struck so frequently, researchers of General Electric Co., looking for light on lightning (to help them protect transmission lines) began to photograph it from another building. The research was under the direction of Karl Boyer McEachron, 48, G. E.'s ace lightning researcher, who has been designing experimental apparatus Pittsfield for 16 years, has produced artificial bolts of 10,000,000 volts, others of 250,000 amperes (amperage is the amount of current, voltage the pressure which drives it). Last week Engineer McEachron reported the results of three summers of Empire State Building study.

The G. E. men used a special fast camera of a type previously used by Dr. B. F. J. Schonland, ingenious lightning observer of the University of the Witwatersrand in South Africa. Some years ago Dr. Schonland found that in a typical lightning flash a "leader stroke" starts from a negatively charged cloud toward the positively charged earth. The leader comes down by steps, dying out after each step, diving about 200 ft. farther with the next. Often 30 or 40 steps may be necessary before the ground is reached, but the whole descent occurs in 1/100 sec. or less. When the stepped leader reaches the ground, the main stroke, more powerful than the leader, shoots upward to the cloud along the path created by the downward steps. In general the McEachron crew confirmed Schonland's findings, but they discovered that in some instances the leader stroke did not shoot downward from the cloud but upward from the building's top. They got pictures of branched lightning which forked upward.

They also found that often when the stroke comes from above, the presence of the charge in the air creates by induction an opposite charge in the target, and before the leader completes its course the target may emit upward streamers to meet the leader and guide it home.

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