In reference to the patent: US4756116
How was this patent issued?
It is certainly not original, and appears to be little-more than a trivial re-hash of Dr Horner's "Galvanic Protector" published in "The Magazine of Domestic Economy", Volume 6 1841 and "Gardeners Chronicle", February 20, 1841.
I quote Gardener's Chronicle, February 20, 1841:
If a snail or slug be placed on a plate of zinc, to which a narrow plate or strip of copper is fixed, it creeps unmolested on its surface; but as soon as it touches the copper, it receives a galvanic shock (its moist soft body acting as the moistened cloth above mentioned, and thus forming the galvanic circle complete), and immediately recoils, twisting itself back, and rarely venturing a second time to touch the copper, to receive another shock. This (to us) amusing experiment, I have tried again and again, and of course always with the same results. To protect a seedling crop, then, in a border, or in frames, & etc., I have zinc plates of one, two, or three feet in length, and four or five inches in breadth; with a strip of copper plate, one inch broad, placed on the upper part, and secured close with two or three rivets of zinc, as in the figure. These plates are fixed in the ground to the depth of a couple of inches, around the plants to be protected. As just explained, the snail creeps up the zinc, but receives a galvanic shock as soon as its horns or head touch the copper, causing it to recoil and turn back; an insurmountable fence can thus, in a moment, be formed around whatever we wish to save from these marauders; and if made in a circular form, or in short lengths, the plates may be contrived to meet every possible exigency. I have myself used this protector in all cases with complete success, and a scientific gentleman of this place, to whom I acknowledge myself indebted for the suggestion of the principle, last year saved his dahlias (which on other occasions had always been nearly all devoured by snails as soon as planted out), by cylindrical hoops of the zinc and copper plate placed around each plant. If the lip of the tongue be placed midway on the zinc and copper at the same time, an unpleasant metallic or saltish taste is perceived: this is the sensation which proves so unpalatable and detrimental to the slug, and in which the protective power consists.
Hull. F. R. Horner, M.D.
P.S. The cost of the plates complete is about sixpence per foot: they will of course last for an indefinite period.