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Updated: June 27, 2025
These gates have no regular guard, but are covered by sentries, who patrol the avenue that encircles the city just within the walls as our metropolitan police patrol their beats. The walls of Zodanga are seventy-five feet in height and fifty feet thick. They are built of enormous blocks of carborundum, and the task of entering the city seemed, to my escort of green warriors, an impossibility.
What I have done is to wind single loops of plain wire on each side of that room down there, as well as to wind around the room a few turns of concealed copper wire. These collectors are fitted to a crystal of carborundum and a telephone receiver." We had each tried the thing and could hear a distinct buzzing in the receiver.
I do not know why the name "carborundum" has been given to it, unless there is something in the process of its manufacture which justifies this selection. Through the kindness of the inventor, I obtained a short while ago some samples which I desired to test in regard to their qualities of phosphorescence and capability of withstanding high degrees of heat.
Carborundum, a silicide of carbon, is a valuable substance produced by the action of the heat of an electric furnace on an intimate mixture of carbon and sand. It has an extensive use as an abrasive for grinding and polishing. Artificial graphite is another product produced by the long-continued action of the heat of the electric furnace on carbon under certain conditions.
Still, the tests with the powder are not conclusive, because powdered carborundum probably does not behave like a phosphorescent sulphide, for example, which could be finely powdered without impairing the phosphorescence, but rather like powdered ruby or diamond, and therefore it would be necessary, in order to make a decisive test, to obtain it in a large lump and polish up the surface.
A carborundum coating seems to be more durable than other coatings, not only because the carborundum can withstand high degrees of heat, but also because it seems to unite with the carbon better than any other material I have tried. A coating of zirconia or any other oxide, for instance, is far more quickly destroyed.
If the carborundum proves useful in connection with these and similar experiments, its chief value will be found in the production of coatings, thin conductors, buttons, or other electrodes capable of withstanding extremely high degrees of heat. The production of a small electrode capable of withstanding enormous temperatures I regard as of the greatest importance in the manufacture of light.
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