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Copied from Ref listed at end of page  with some recent photos added to the drawings.

CONDUCTION OF ELECTRICITY THROUGH GASES 
Advance Laboratory Practice In Electricity and Magnetism By Earle Melvin Terry McGraw Hill Book Co. 1^st edition 1922     

149. 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.   

150. 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.

  151. 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.  

152. 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.

    153. 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.  ________________________________________________

A circuit diagram below showing an experimental set up to observe the visible  electric and magnetic characteristics of gases brought to high temperature by electric current passing through the gas a low pressures.

  

• Above  is a drawing of a discharge tube running on a steady direct current (d.c.)showing the  curious bright and dark band and its cathode dark spot..
   

• If the tube is run on 60 cycles per second alternating current the polarities of the electrodes change from + to - 60 times a second giving two discharge columns which over lap each other thus blending. out to some extent the dark cathode feature. Since this  change goes back and forth  60 times a second  the eye sees both at the same time, i.e. they're blend together.
   

• Using a magnet with its magnetic field perpendicular to the arc current flow separates the two columns so that one can see  each arc column with its cathode dark space. ( Why does a magnetic field do this?)

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155. Phenomena of the Discharge Tube Running on Direct Current—If electrodes are mounted at the ends of a tube such as shown in Fig. 112, containing  air at ordinary pressures and a sufficiently high voltage is impressed between them, the phenomenon first observed is the ordinary spark similar to that between the electrodes of a static machine.  If, however, air is gradually removed, the sparks become less violent, and fine streamers of bluish color are observed.  As the pressure is further reduced, these streamers broaden out and fill the entire tube, and a pink color appears at the anode.  With further exhaustion, the pink color extends some distance from the anode and dark spaces appear in the region of the cathode.  When the pressure has been reduced to 

Direct Current above

 

 about half a millimeter of mercury, the discharge assumes a very characteristic appearance.  Closely surrounding, but not quite touching the cathode, is a thin layer of luminosity known as the cathode glow.  Next to this is a region, from which no light is observed, called the Crooke's dark space, and beyond this is a rather broad region of luminosity known as the negative glow. Following this is another non-luminous region, called the Faraday dark space.  Between this dark space and the positive electrode is a region called the positive column, which may be seen as a continuous band of light or, under certain conditions of current and voltage, as a series of light and dark striae.  The positive column seems to be definitely associated with the anode, for if the tube is increased in length or bent into a curve, the positive column increases or bends with it, while the other parts of the discharge remain fixed and are thus shown to be associated with the cathode.  These luminous regions are indicated in Fig. 112.    If the pressure is still further reduced, the striae of the positive column become fewer in number and wider in extent and finally .disappear.  The regions associated with the cathode also become larger and, with the disappearance of the positive column, the dark spaces fill nearly the entire tube.  With sufficient exhaustion,  the Crooke's dark space completely fills the tube, and the voltage required for a passage of current becomes very high.  At this stage, the walls of the tube fluoresce brilliantly with colors depending upon its chemical composition, being bluish for soda, and bright green for German glass.  If the exhaustion is carried far enough, the tube becomes a non- conductor of electricity. 

   156. Theory of the Discharge.—Since no external ionizing agent is acting, it is obvious that the discharge is maintained by ions produced by collision, and the varied distributions of the luminous regions indicate that the electric fields and the velocities of the carriers can not be uniform throughout the tube.  It has not yet been definitely determined whether luminescence arises from ionization of neutral molecules or whether it accompanies the recombination of an ion and an electron to form a neutral molecule.  At the present time, the evidence seems to favor the latter hypothesis.  Another widely accepted view is that when a molecule has been shaken up by collision with an ion or electron to such an extent that its electronic orbits are badly distorted, but not disrupted, light emission accompanies its return to the equilibrium state.  On the latter theory, luminous regions do not necessarily coincide with regions of ionization.  Some of the more important phenomena characterizing the several regions enumerated above are the following.    1.  Cathode Glow.—The field strength in this region is large and often the greater part of the entire potential difference occurs in this limited space.  The magnitude of the fall in potential depends upon the nature of the gas and the material of the electrode, ranging from 470 volts for water vapor to 170 volts for argon with platinum electrodes.  If metals such as magnesium, sodium, or potassium are used, much smaller values are obtained because of the greater ease with which these substances emit electrons. The large potential gradient here is caused by the accumulation of positive ions in front of the cathode.  Because of the greater mobility of electrons, they rapidly move away from this region thus leaving a preponderance of positive ions.  The ionization is caused by collision of the positive ions either with gas molecules or the cathode itself.   2. Crooke's  Dark Space.—It was pointed out above that a certain amount of energy is required to produce ionization.  The electrons from the cathode glow must move through a certain difference of potential before they possess the requisite kinetic energy for this purpose.  The Crooke's dark space represents this distance for it is here that electrons, liberated in the cathode glow, are acquiring the necessary energy of motion to produce the ionization of the negative glow.  It is, in general, a rough measure of the mean free path of the electrons.  No ionization occurs in this region and the current is carried almost exclusively by the electrons.   3. Negative Glow.—The luminosity of this region is due to ionization by electrons from the Crooke's dark space.  The positive ions produced here move slowly out of the negative glow into the Crooke's dark space and their presence reduces the potential gradient to such an extent that electrons, originating in the negative glow, do not gain sufficient speed to produce ionization; and hence, after those entering from the Crooke's dark space have been stopped by the ionization process, no further ionization occurs.    4. Faraday  Dark Space.—The current in this region is due largely to electrons which enter it from the negative glow. Because of the accumulation of electrons in the negative glow, the potential gradient through the Faraday dark space and even up to the anode is quite large.  The electrons are accordingly accelerated through this dark space and when they have attained velocities sufficient for ionization, the positive column commences.    5. Positive  Column.—The  potential  gradient  is  practically constant throughout this region and ionization by collision may take place all the way, resulting in a uniform column of light. Ordinarily, however, there are local accumulations of positive ions, which result in a decrease in the potential gradient with a consequent reduction in the acceleration of the electrons.  There are then regions in which the velocities are too small to produce ionization and the striae commonly observed, result.  Under these circumstances, the positive column is, to a certain extent, a repetition of the phenomena of the Crooke's dark space, and the negative glow.