Conduction of electricity by 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 10^7 electromagnetic units per gram. The charge of an electron is 1.5 X 10^-20 electromagnetic 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^-13 cms., 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 electro- scope is well insulated from the container, the electroscope will remain charged for a long time, showing that the air or what- ever 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 radio-active 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 radio active 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.
Structure of the Atom To explain the phenomena of the conductivity of gases, it is necessary first to make a brief statement concerning the structure of the atom. While our knowledge is far from complete, it is well established that the atom consists of a nucleus of positive electricity, about which revolve in closed orbits, electrons, in much the same way that the planets revolve about the sun, and that the relative dimensions of electrons, nucleus and orbits are about the same as in the solar system. The number of electrons present in a given atom has been estimated in various ways, and while the results are not entirely in agreement, it is probable that it is the same as the atomic number, that is, its number in the list of elements arranged in order of ascending atomic weights. The atomic number, except for the case of hydrogen, is approximately half its atomic weight. Since the atom as a whole is neutral, it is necessary that the positive nucleus should have a charge equal to ne, where e is the charge of the electron and n the number of electrons. The shape of the orbits, the law of force between nucleus and electron, and even the conditions of stability are problems which have not yet been solved, but are now being attacked from many angles. When external agencies such as X-rays, ultra violet light, radiations from radio active materials, etc., act upon a gas, it is found that the atomic structure is broken up. One or more electrons may be torn away from the system leaving it with an excess of positive electricity. We thus have present in the gas positive ions and negative electrons. The gas is then said to be ionized, and the means by which this condition is brought about is called the "ionizing agent." If two electrodes are introduced, and a difference of potential is maintained between them, the electrons move to the positive electrode, and, entering it, pass on through the external metallic circuit. The positive ions, on the other hand, move to the negative electrode and receive electrons from it, thus becoming again neutral molecules. Unless an ionizing agent acts continuously, the current through the circuit will persist only until the ions and electrons have been removed from the gas
The ionization Current Suppose now that an ionizing agent is acting continuously upon a gas in an ionization chamber as an arrangement such as that just described is called. At first it might be supposed that if the agent acts long enough all of the atoms would be ionized. This, however, is not the case; for, due to their undirected heat motion, ions and electrons collide, and recombine. When the rate of recombination is equal to that of ionization, a steady state is reached where only a definite fraction, usually a very small number, of the total number of molecules are in the ionized state. If the difference of potential between the plates is varied, and the current between them is measured and plotted as a function of voltage, it is found that the current increases with the voltage almost linearly at first, in accordance with Ohm's law; but for higher voltages, the curve is concave downward and when a certain voltage has been reached, no further increase in current can be obtained, unless the voltage is raised to very large values. The constancy of the current is due to the fact that all of the ions and electrons produced are swept out by the field. This current is spoken of as the "saturation current," from the similarity between the shape of this curve and the magnetization curve for iron. The voltage at which the horizontal part of the curve begins is called the "saturation voltage." If the distance between the electrodes is increased, it might, by analogy with metallic conductors, be thought that the saturation current would be reduced because of the increased path the ions and electrons must travel. It is found, however, that the cur- rent is actually increased. This is because there is a larger number of gas molecules subjected to the action of the ionizing agent, and hence more carriers are produced. Again, it is found that if the pressure of the gas is increased, the ionization current is increased. Both of these facts show that the saturation cur- rent through a gas is proportional to the mass of the gas between the electrodes.
Ionization by Collision. If the voltage between the plates of the ionization chamber is increased to sufficiently large values, the saturation current does not remain constant indefinitely, for fields may be reached at which the current again begins to rise, slowly at first and then very rapidly, finally resulting in a disruptive spark accompanied by the passage of a current of considerable magnitude. The field required for this increased current depends upon the distance between electrodes, their size and shape, and the nature and pressure of the gas. For air at atmospheric pressure and spherical electrodes of moderate dimensions, e.g., 1 cm. diameter, centimeter. It is the order of 10,000 Volts per centimeter It diminishes, however, as the pressure is reduced, and is most conveniently studied at pressures below 10 millimeters of mercury. This increase in current is due to the fact that ions are produced by collisions taking place between neutral molecules and ions as well as electrons already existing in the gas. The mechanism of this process is somewhat obscure, but it is clear that a definite amount of energy is required to disrupt a neutral atom. The kinetic energy of motion of the ions and electrons depends upon how far they have moved under the accelerating field before being stopped in the same way that the energy of motion of a freely falling body depends upon the distance through which it has fallen before being arrested. Thus, as the pressure of the gas is reduced, the average length of free travel is greater and the acquired energy available for ionizing purposes is increased. The conductivity of a gas therefore increases as the pressure is reduced. Since, however, the conductivity depends upon carriers which come originally from neutral molecules, the conductivity can not increase indefinitely with decrease of pressure, for the effect of the decreased available supply will eventually be felt. An optimum pressure therefore exists at which the increased range for acceleration is just balanced by the decreased supply of molecules. For air, this pressure is of the order of a few tenths of a millimeter of mercury. A further decrease in the pressure results in a rapid increase in the resistance of the gas. If a perfect vacuum could be obtained, the free space between electrodes would be a perfect insulator. While this is, of course, impossible, it is, nevertheless, easy with modern methods of evacuation to obtain pressures so low that no appreciable discharge can be detected with the highest fields available in the laboratory.
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