Page:Popular Science Monthly Volume 75.djvu/297

Rh The insulatory resistance of the bare human skin varies from 1,000 to 6,000 ohms. In many animals the insulation is increased by non-conducting hair, wool, fur, etc. And naked man finds it expedient to reinforce his own insulation by clothing of silk, satin, hair, wool, flannel and other non-conducting materials. We are exhilarated by a dry atmosphere: depressed by a damp one, because the moist air, being a conductor, carries off some of our electricity to the earth, while dry air is a more complete insulator and prevents this leakage.

Besides contact with the air, the feet of animals are in actual contact with the earth itself, and accordingly ought to be endowed with a specially good insulation.

Finding no data on this point, I submitted to the U. S. Bureau of Standards some specimens of a horse's hoof, to have their insulation tested. The director, Professor S. W. Stratton, wrote me that the resistance of the first specimen, when dry, was 4,700 million ohms. This was a part of the "frog" of the foot. A second specimen, taken from the peripheral margin of the hoof was tested, of which the bureau reported that "by the direct-deflection method, using 120 volts and a very sensitive galvanometer, the deflection was so small that it could not be read." "The resistance was equal to or greater than 22 billion ohms. This corresponds to a specific resistance of about $$16.5 \times 10^{10}$$ ohms per centimeter cube." Professor Stratton adds: "Of course the actual resistance may be much higher, as it was too high to determine with any accuracy by this means."

Subsequently, Professor Chas. W. Mortimer, of the George Washington University, by using his Wheatstone Bridge apparatus, kindly tested for me, altogether, 67 specimens of animal and some vegetable structures, as to the insulating power of their external coverings. The specimens included the feet, claws and bills of sheep, rabbits and chickens; the fresh human umbilical cord, foetal membranes and placenta; the shell of an egg; the external coverings of fruits (oranges, apples, nuts, etc.) and of vegetables (turnips, onions, etc.).

In no instance did the external covering fail to exhibit a relatively greater resistance than the internal structure. In most of the specimens the resistance hovered about 10,000,000 ohms, some more, some less. In one instance, that of a green pea pod, the resistance of the unbroken pod was 500,000 ohms, while the external surface of the green pea itself was 10,000,000 ohms.

I did not test any cereal grains, but Mr. Lyman J. Briggs, of the Bureau of Plant Industry, U. S. Department of Agriculture, has recently ascertained that the resistance of wheat grains, varying with