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Physics

Conduction of electricity in gases

Electrons: When a high tension discharge passes between electrodes sealed into a partially evacuated vessel, the gas becomes luminous showing a series of highly colored glows which are often very beautiful.  If the pressure is sufficiently reduced, a series of streams appears, proceeding in straight lines from the cathode.  These streams are known as “cathode rays,” and are found to be independent of the position of the anode, and often penetrate regions occupied by other glows in the tube.

The researches of modern physics have shown that these rays are streams of discreet particles of negative electricity, called “electrons.”  Their properties do not depend upon the material of the electrodes nor the nature or presence of the gas through which the discharge takes place.  They may be produced from all chemical substances, and consequently must play an important part in the structure of matter.

The velocities with which they move through the tube vary from one-thirtieth to one-third that of light.  The ratio of the charge of an electron to its mass is constant and is equal to 1.77 X 107electromagnetic units per gram. The charge of an electron is 1.5 X 10-20electromagnetic units and The mass is about 1/1800 that of the hydrogen atom.  The radius of an electron is estimated, at 1.9 X 10-13c.m.s, which is about 1/50 000 that of the atom.

For many years the mass has been regarded as purely electromagnetic in character; that is, while exhibiting inertia, it shows no gravitational attraction in the sense possessed  by ordinary matter.  Recently, however, certain experimental and theoretical evidence has been produced which makes it appear likely that this cannot be entirely the case.

Many attempts have been made to discover evidence of quantities of electricity smaller or larger than the electron, but none smaller have ever been found.  In fact, when quantities comparable to the electron have been isolated, they have always proved to be exact integral multiples of it.  The evidence points to the conclusion that electricity is atomic in structure and that the smallest possible element is the electron, which thus constitutes our natural unit of electricity.  Electric currents through conductors, as we know them in every day practice, are simply streams of electrons through or between the atoms and molecules making up the conducting body.

Conductivity of Gases: A gas in its normal state is one of the best insulators known.  This may be shown by mounting a gold leaf electroscope inside an enclosed space, and allowing only a small rod carrying a polished knob, for the purpose of charging, to project out.  If the support carrying the electroscope is well insulated from the container, the electroscope will remain charged for a long time, showing that the air or whatever gas surrounds the electroscope is a poor conductor of electricity.

If, however, X-rays are allowed to shine through the enclosure, or if a small quantity of some radioactive substance such as thorium or radium is placed inside it, or again if the products of combustion of a flame are drawn through it, it is then found that the gold leaves collapse quite rapidly, indicating that the gas has lost its insulating properties.

That the leakage has taken place through the air and not across the insulating support may be shown by using a second chamber connected with the electrometer enclosure by a glass tube, and introducing the X-rays, the radioactive substance or other agent into this, and then drawing the air thus acted upon into the first chamber.  The same effects are observed.

However, if glass wool is introduced in the connecting tube, or if the air is passed between two insulated plates connected to a battery before entering the electrometer chamber, it is found that its insulating properties are restored.   Experiments of this sort as well as many others of an entirely different nature have shown that the conduction of electricity through gases is due to carriers of electricity, and that the carriers are of two distinct types, positive and negative; the former are similar to the carriers of electricity through solutions and are called positive ions, while the latter are either negative ions or electrons.

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