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INVENTIONS OF NIKOLA TESLA.

tion is affected. In a resonator, the density is small, but thefrequency is extreme, and may introduce a considerable error.It appears certain, at any rate, that the periods of vibration of acharged body in a gaseous and in a continuous medium, suchas oil, are different, on account of the action of the former, asexplained.

Another fact recognized, which is of some consequence, is,that in similar investigations the general considerations of staticscreening are not applicable when a gaseous medium is present.This is evident from the following experiment:—A short andwide glass tube is taken and co vered with a substantial coating ofbronze powder, barely allowing the light to shine a little through.The tube is highly exhausted and suspended on a metallic claspfrom the end of a wire. When the wire is connected with oneof the terminals of the coil, the gas inside of the tube is lightedin spite of the metal coating. Here the metal evidently doesnot screen the gas inside as it ought to, even if it be very thinand poorly conducting. Yet, in a condition of rest the metalcoating, however thin, screens the inside perfectly.

One of the most interesting results arrived at in pursuing theseexperiments, is the demonstration of the fact that a gaseous me-dium, upon which vibration is impressed by rapid changes ofelectrostatic potential, is rigid. In illustration of this result anexperiment made by Mr. Tesla may by cited:—A glass tube aboutone inch in diameter and three feet long, with outside condensercoatings on the ends, was exhausted to a certain point, when, thetube being suspended freely from a wire connecting the upper coat-ing to one of the terminals of the coil, the discharge appeared inthe form of a luminous thread passing through the axis of the tube.Usually the thread was sharply defined in the Tipper part of thetube and lost itself in the lower part. When a magnet or thefinger was quickly passed near the upper part of the luminousthread, it was brought out of position by magnetic or electro-static influence, and a transversal vibration like that of a sus-pended cord, with one or more distinct nodes, was set up, whichlasted for a few minutes and gradually died out. By suspendingfrom the lower condenser coating metal plates of different sizes,the speed of the vibration was varied. This vibration wouldseem to show beyond doubt that the thread possessed rigidity,at least to transversal displacements.

Many experiments were tried to demonstrate this property in