Text: Conduction of Electricity Through Vacuum and Gases With Applications to Design of Radio Apparatus BY E. M. SARGENT Radio News April, 1921 AN investigation of the subject of electrical conduction in gases and in a vacuum discloses many facts that are of peculiar interest to the radio experimenter, particularly if the investigation is conducted from a non-radio point of view. Some of the outstanding characteristics of electrical currents when passing through gases are outlined below, after which follows a more practical discussion of their importance when applied to the design of radio telegraph apparatus. Gas in its normal state is non-conducting for most practical purposes. For example, when a telegraph line is operated, no account is taken of the leakage current that flows through the air from the wire to the ground This leakage current is so small as to be entirely negligible when compared with the leakage through insulators, etc. Nevertheless, it is there. Where high voltages are used, particularly voltages high enough to cause sparking, gas currents become of importance. Currents are conducted through a gas by small carriers called ions. These ions are of two kinds, one having a positive charge and the other a negative one. Negative ions usually consist of single electrons. A normal gas contains equal numbers of positive and negative ions. Even when undisturbed physically, as when a gas is kept in a covered glass jar, the small particles that make up the gas are in constant motion, and collisions are continually taking place between the molecules and ions. When collisions take place between molecules and ions, some of the molecules are disrupted and new ions are formed, while when collisions take place between ions of unlike sign the charges neutralize and new molecules are formed When the gas has stood for some time undisturbed it reaches a steady state at which new ions are being formed at the same rate at which they are recombining to form molecules, and the total number of ions in the gas is then constant. When this state is reached, each particle has the same amount of kinetic energy as the others. Ions are produced in gases in any of the following ways: I. By heating. "Heating" physically means increasing the amount of kinetic energy in each particle of the gas. When the kinetic energy is increased, collisions will be more frequent and greater forces will take part in them. These greater forces will break up a greater number of molecules, thus forming more ions. 2, By the electric spark. This is a special case of heating, although the initial ionization takes place in a somewhat different manner, and will be discussed later. 3. By the electric arc. This is also a case of heating. 4. By radio activity. Radio active substances have the power of dissociating the molecules of a gas in the vicinity of the radio active substance. Here the ionizing is done not by the interbombardment of the gas particles but by the bombardment of the gas by the particles that are ejected from the radio active substance. These particles travel with extremely high velocities, some approaching the velocity of light and are capable of producing very high ionization if allowed to act for some time on a gas in a confined space. 5. Light, particularly the sun's rays and illuminating sources strong in ultra violet light produce ionization to some extent. REMOVAL OF IONS FROM A GA8. When either a positive or negative ion comes in contact with a cold metal surface it gives up its charge to the metal. Recombination to form molecules then takes place near the metal surface. Ions may be removed from a gas by passing the gas through small diameter metal tubing, or through metallic wool. In general, negative ions are smaller than positive ones. All particles in a gas in the steady state have the same amount of kinetic energy (1/2 mv2). Therefore since the positive ions have many times the mass of the negative ones, the negative particles have much greater velocities. This can be experimentally proved by blowing an electrically neutral gas through a metal tube having a small diameter. The gas may emerge from the tube with a positive charge. This is due to the fact that in the short time the gas was passing through the tube, more negative ions than positive reached the walls of the tube, due to their higher velocities. Thus more negative ions than positive were discharged and the gas emerged with-a positive charge. The charge on an ion is about 3.5 X 10-10 electro static units. Expressed in practical units, each ion contains about 1.17 X 10-19 coulombs of electricity. When a current of one ampere is flowing in a circuit, one coulomb of electricity per second is flowing past each point in the circuit, or .855 X 10-19 ions pass each point in the circuit per ampere of current. When two plates are immersed in a gas, the saturation current that will flow depends only on the ionization between the plates. For a gas of uniform ionization the saturation current will then be inversely proportional to the distance between the plates. The current through a gas with constant distance between electrodes follows Ohm's law in that the current is proportional to the voltage for very small e.m.f.s. Only and quickly reaches a saturation point at which increase of voltage has very small effect on the current. The reason for this is that there are only a limited number of ions available and that when the voltage gets large nearly all the ions are carrying current and a further increase in voltage cannot cause an increase in current until the voltage gets large enough to create more ions. This case will be discussed under spark gaps. The statement that the current does not follow Ohm's law made above should be strictly interpreted to mean that there is not a straight line variation between current and voltage. There is no known current that does not exactly follow Ohm's law if the law is properly applied. Currents through the electric arc, electric spark, plate to filament current in a vacuum tube, and the current described above through a gas are currents ordinarily referred to as "not following Ohm's law." In all four of these cases the effective resistance or impedance of the medium is a variable that is a function of the voltage and when the proper value of resistance for a particular voltage is substituted in the equation the calculated current will be correct. The same is true of alternating currents where the effective resistance is a function not of the voltage but of the frequency, and the "resistance" must be broadly interpreted as the total impedance to current flow. For all alternating current cases I = E/Z. Statements in this paper that a current doesn't follow Ohm's law then must be taken to mean as outlined above. IONIZATION OF A GAS BY INCANDESCENT SOLIDS. If a negatively electrified body be brought near an incandescent pure metal it will be discharged, but if brought near an incandescent oxide will be unaffected. A positively charged body will be affected in the opposite way. The metal will not discharge but the oxide will. This is an important fact to be considered when selecting material for vacuum tube filaments. The electrification produced by incandescent solids depends on four factors: 1. The temperature of the wire. The higher the temperature the greater the speed with which the particles are shot off from the wire. They have more kinetic energy at high temperature and therefore are capable of producing greater ionization. 2. Pressure of the gas. 3. Nature of the gas. 4. The nature of the incandescent wire. Even in a vacuum electrification is produced on bodies near incandescent solids. This electrification is due to particles leaving the solid. Edison noticed in connection with experiments on electric lights that negative electricity escaped from glowing carbon in a high vacuum. This is called the Edison effect, after the discoverer. Emission of negative particles however is not confined to solids. The saturation current between an incandescent wire and a cold cylinder or plate can be expressed in the form I = ao1/2 c- b/o where 0 is the absolute temperature of the wire and a and b are constants. When the hot wire is surrounded by gas instead of being in a vacuum, the electrons ionize the gas at high temperatures and thus produce positive as well as negative ions. The source of ionization is at the surface of the incandescent metal. The same number of ions will then be produced no matter what the distance between the electrodes is. Therefore the saturation current will be the same for a given incandescence regardless of the distance between plates as the saturation current depends only on the number of ions available to carry current. The voltage required to produce this saturation current however varies greatly with the distance between the plates. If ionization is confined to the surface of the incandescent metal, the current between the metal and a cold p!ate will be carried by ions of one sign only, even though ions of both signs are present at the metal surface. If two platinum electrodes are immersed in a vessel containing gas and heated to a bright red, current from a battery will pass between them. If now a cold metal plate be placed between the two hot ones, the current will be completely stopped and will not recommence until the middle plate reaches the ionizing temperature. The reason for this is that the cold plate discharges the ions that were carrying the current and continues to do so until it gets hot enough to form ions itself.

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