Switch of purpose by purpose

I noticed that Fessenden’s patents for aiding plant growth with electric charge are classified by the patent office as:

A01M21/046 – Apparatus for destruction by steam, chemicals, burning, or electricity by electricity

He was trying to grow plants with electricity by electricity. He wasn’t trying to destroy plants with electricity by electricity.

Why is this the only category? I googled ‘patent agriculture high voltage electricity’ and found the answer. All modern patents, from 1975 to now, use electricity by electricity to kill weeds, not to grow plants. This 1975 patent by Allis Chalmers covers a gadget resembling Fessenden’s charge dispenser, meant to be pulled behind a tractor. It zaps all plants above a certain height, since faster-growing plants tend to be weeds.

Technology no longer cultivates. It’s all weapons of war now.

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More soberly, it appears that the experiments with electroculture had dubious results. This 1925 USDA pamphlet summarizes their own research and a wide variety of research in US and Europe. The US work mostly showed no improvement, while the Euro work consistently found 20-30% improvement in yield with charge applied. Fessenden notes this problem in his patent. His main goal was to recommend his 5-second pulsation, which worked better than DC in his experiments. Far as I can tell, nobody else pursued the pulsation. He discussed the possibility of resonance:

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With most plants there are two or three critical frequencies; for example, one revolution in four seconds, one in two and a half seconds, and one in one-half second, but the relation of these critical frequencies to each other has not so far been determined. The critical frequency, or frequencies, depend upon the kind of plant and upon its age and possibly upon the distance apart of the branches.

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The branch spacing definitely suggests a resonant pattern, perhaps determined by the internal transmission of charge between branches and main stem.

Speculating on Euro vs US. The cited experiments were in England and Germany, which are considerably wetter and less sunny than the farming areas of US. The earth is negative wrt the atmosphere, so positive ions in the atmosphere drift downward and get picked up by leaves. When the air is damp the potential difference is partly ‘shorted out’ by the moisture. Euro plants might be hungry for positive ions, which the added charge supplies? Or, given Shepardson’s idea that the ions are a substitute for sunlight, the ions simply did the same job as added light?