CHAPTER II ELECTRIC CONDUCTION. GAS AND VAPOR CONDUCTORS Gas, Vapor and Vacuum Conduction 18. As further, and last class may be considered vapor, gas and vacuum conduction. Typical of this is, that the volt-ampere characteristic is dropping, that is, the voltage decreases with in- crease of current, and that luminescence accompanies the con- duction, that is, conversion of electric energy into light. Thus, gas and vapor conductors are unstable on constant- potential supply, but stable on constant current. On constant potential they require a series resistance or reactance, to produce stability. Such conduction may be divided into three distinct types: spark conduction, arc conduction, and true electronic conduction. In spark conduction, the gas or vapor which fills the space be- tween the electrodes is the conductor. The light given by the gaseous conductor thus shows the spectrum of the gas or vapor which fills the space, but the material of the electrodes is imma- terial, that is, affects neither the light nor the electric behavior of the gaseous conductor, except indirectly, in so far as the section of the conductor at the terminals depends upon the terminal sur- face. In arc conduction, the conductor is a vapor stream issuing from the negative terminal or cathode, and moving toward the anode at high velocity. The light of the arc thus shows the spectnun of the negative terminal material, but not that of the gas in the surrounding space, nor that of the positive terminal, except indi- rectly, by heat luminescence of material entering the arc con- ductor from the anode or from surrounding space. In true electronic conduction, electrons existing in the space, or produced at the terminals (hot cathode), are the conductors. Such conduction thus exists also in a perfect vacuum, and may be accompanied by practically no luminescence. 28 ELECTRIC CONDUCTION . 29 Disruptive Conduction 19. Spark conduction at atmospheric pressure is the disruptive spark, streamers, and corona. In a partial vacuum, it is the Geissler discharge or glow discharge. Spark conduction is dis- continuous, that is, up to a certain voltage, the "disruptive voltage," no conduction exists, except perhaps the extremely small true electronic conduction. At this voltage conduction begins and continues as long as the voltage persists, or, if the source of power is capable of maintaining considerable current, the spark conduction changes to arc conduction, by the heat de- veloped at the negative terminal supplying the conducting arc vapor stream. The current usually is small and the voltage high. Especially at atmospheric pressure, the drop of the volt- ampere characteristic is extremely steep, so that it is practically impossible to secure stability by series resistance, but the con- duction changes to arc conduction, if sufficient current is avail- able, as from power generators, or the conduction ceases by the voltage drop of the supply source, and then starts again by the recovery of voltage, as with an electrostatic machine. Thus spark conduction also is called disruptive conduction and discon- tinuous conduction. Apparently continuous — though still interipittent — spark con- duction is produced at atmospheric pressure by capacity in series to the gaseous conductor, on an alternating-voltage supply, as corona, and as Geissler tube conduction at a partial vacuum, by an alternating-supply voltage with considerable reactance or resistance in series, or from a direct-current source of very high voltage and very Umited current, as an electrostatic machine. In the Geissler tube or vacuum tube, on alternating-voltage supply, the effective voltage consumed by the tube, at constant temperature and constant gas pressure, is approximately con- stant and independent of the effective current, that is, the volt- ampere characteristic a straight horizontal line. The Geissler tube thus requires constant current or a steadying resistance or reactance for its operation. The voltage consumed by the Geiss- ler tube consists of a potential drop at the terminals, the "termi- nal drop, " and a voltage consumed in the luminous stream, the *' stream voltage.'* Both greatly depend on the gas pressure, and vary, with changing gas pressure, in opposite directions : the terminal drop decreases and the stream voltage increases with increasing gas pressure, and the total voltage consumed by the 30 ELECTRIC CIRCUITS tube thus gives a minimum at some definite gas pressure. This pressure of minimum voltage depends on the length of the tube, „ »,. IHQ PRE! SURE p . \ / \ ?" VO. TAGS y / 0., ""' ... Hf. / \ f^ ,o >f P % .»«• -y IR / / ' .1 4Mf. ^ ~~- S.0 8 LOG p J n /, mrr ■«« UR6, p \ /' \ T.G^ /- / ,0.0 «.P '*^ / e \ -? r s \ 4 > X \ Mi RCU. I,. 0» / Xo lAM ■^ ; \\ s \ m ^ — ■ \ ■~- P r / \ ^• ^, -^ ,' ui- ._ L- \ > ■--^ i' / ■—- ~^ ._^ ^ ,' '' 7^ — , U' ^ ^ ^y ^ / < ^' -' / ^ / ^- -' ' y ^ ^ ,. ■'■ ' 11- ^ ' teristic I: -p- The resistance, r, required to give the stability limit at current, i, thus is found by the condition "-% '»> Substituting equation (4) into (6) gives . c( i + 8) (7) 38 ELECTRIC CIRCUITS as the minimum resistance to produce stability, hence, „• = £(i+«) = 0.5 ci (8) where ei = arc stream voltage, and E = e + ri = a + 1.5 ^SL+J) (9) is the mmimum voltage required by arc and series resistance, to just reach stability. (9) is plotted as curve IV in Fig. 20, and is called the stability curve of the arc. It is of the same form as the arc characteristic I, and derived therefrom by adding 50 per cent, of the voltage, ei, consumed by the arc stream. The stability limit of an arc, on constant potential, thus lies at^ar^xcess of the supply voltage over the arc voltage e = a + Ci, by 50 per cent, of the voltage, ci, consumed in the arc stream. In general, to get reasonable steadiness and absence of drifting of current, a somewhat higher supply voltage and larger series resistance, than given by the stability curve IV, is desirable. 24. The preceding applies only to those arcs in which the gas pressure an the space surroimding the arc, and thereby the arc vapor pressure and temperature, are constant and independent of the current, as is the case with arcs in air, at "atmospheric pressure." With arcs in which the vapor pressure and temperature vary with the current, as in vacuum arcs like the mercury arc, different considerations apply. Thus, in a mercury arc in a glass tube, if the current is sufficiently large to fill the entire tube, but not so large that condensation of the mercury vapor can not freely occur in a condensing chamber, the power dissipated by radiation, etc., may be assumed as proportional to the length of the tube, and to the current p = eii = di thus, ei = d (10) that is, the stream voltage of the tube, or voltage consumed by the arc streana (exclusive terminal drop) is independent of the ELECTRIC CONDUCTION 39 current. Adding hereto the terminal drop, a, gives as the total voltage consumed by the mercury tube e - o + c! (11) for a mercury arc in a vacuum, it is approximately 1.4 (12) where d= diameter of the tube, since the diameter of the tube is proportional to the surface and therefore to the radiation coefficient. Thus, \Al = 13 + - (13) At high currents, the vapor pressure rises abnormally, due to incomplete condensation, and the voltage therefore rises, and n 1 1 1 i 1 1 1 1 1 1 1 VOLT-AMPERE CHARACTERISTIC OF VACUUM MERCURY ARC L-40 CM. = 2.2 CM. APPROX. e = i.lf .21 -5.8 i \ . — - ^ L- -— " ~- — ■ t Fia. 21. at low currents the voltage rises again, due to the arc not filling the entire tube. Such a volt-ampere characteristic is given in Fig. 21. 26. Herefrom then follows, that the voltage gradient in the mercury arc, for a tube diameter of 2 cm., is about ^ volts per centimeter or about one-twentieth of what it is in the Geissler tube, and the specific resistance of the stream, at 4 amp., is 40 ELECTRIC CIRCUITS about 0.2 ohms per cm.*, or of the magnitude of one one- thousandth of what it is in the Geissler tube. At higher currents, the mercury arc in a vacumn gives a rising volt-ampere characteristic. Nevertheless it is not stable on constant-potential supply, as the rising characteristic applies only to stationary conditions; the instantaneous characteristic is drop- ping. That is, if the current is suddenly increased, the voltage drops, regardless of the current value, and then gradually, with the increasing temperature and vapor pressure, increases again, to the permanent value, a lower value or a higher value, which- ever may be given by the permanent volt-ampere characteristic. In an arc at atmospheric pressure, as the magnetite arc, the voltage gradient depends on the current, by equation (1), and at 4 amp. is about 15 to 18 volts per centimeter. The specijfic re- sistance of the arc stream is of the magnitude of 1 ohm per cm.', and less with larger current arcs, thus of the same magnitude as in vacuum arcs. Electronic Conduction 26. Conduction occurs at moderate voltages between terminals in a partial vacuum as well as in a perfect vacuum, if the terminals are incandescent. If only one terminal is incandescent, the con- duction is unidirectional, that is, can occur only in that direction, which makes the incandescent terminal the cathode, or negative. Such a vacuum tube then rectifies an alternating voltage and may be used as rectifier. If a perfect vacuum exists in the conducting space between the electrodes of such a hot cathode tube, the con- duction is considered as true electronic conduction. The voltage consumed by the tube is depending on the high temperature of the cathode, and is of the magnitude of arc voltages, hence very much lower than in the Geissler tube, and the current of the mag- nitude of arc currents, hence much higher than in the Geissler tube. 27. The complete volt-ampere characteristic of gas and vapor conduction thus would give a curve of the shape in Fig. 22. It consists of three branches separated by ranges of instability or discontinuity. The branch a, at very low current, electronic con- duction; the branch 6, discontinuous or Geissler tube conduction; and the branch c, arc conduction. The change from a to 6 oc- curs suddenly and abruptly, accompanied by a big rise of current, as soon as the disruptive voltage is reached. The change 6 to c ELECTRIC CONDUCTION 41 occurs suddenly and abruptly, by the formation of a cathode apot, anywhere in a wide range of current, and is accompanied by a sudden drop of voltage. To show the entire range, as abscissse are used -^/i and as ordinates %/«• Mill e APPROXIMATE VOLT AMPERE CHARACTERISTIC OF GASEOUS CONDUCTION - ™ '■ \ s \ s > ~ lOOD t inrn ^ ~ *• , .wo \ . w , - I. »£ "~' — -£. lOD ~W s i ^1 ".1 -.1 3 3 Review 38. The various classes of conduction: metallic conduction, electrolytic conduction, pyroelectric conduction, insulation, gas vapor and electronic conduction, are only characteristic types, but numerous intermediaries exist, and transitions from one type to another by change of electrical conditions, of temperature, etc. As regards to the magnitude of the specific resistance or resiat- ivity,the different types of conductors are characterized about as foUows: 42 ELECTRIC CIRCUITS The resistivity of metallic conductors is measured in microhm- centimeters. The resistivity of electrolytic conductors is measured in ohm- centimeters. The resistivity of insulators is measured in megohm-centimeters and millions of megohm-centimeters. The resistivity of typical pyroelectric conductors is of the mag- nitude of that of electrolytes, ohm-centimeters, but extends from this down toward the resistivities of metallic conductors, and up toward that of insulators. The resistivity of g£ts and vapor conduction is of the magnitude of electrolytic conduction: arc conduction of the magnitude of lower resistance electrolytes, Geissler tube conduction and corona conduction of the magnitude of higher-resistance electrolytes. Electronic conduction at atmospheric temperature is of the magnitude of that of insulators; with incandescent terminals, it reaches the magnitude of electrolytic conduction. While the resistivities of pyroelectric conductors extend over the entire range, from those of metals to those of insulators, typical are those pyroelectric conductors having a resistivity of electrolytic conductors. In those with lower resistivity, the drop of the volt-ampere characteristic decreases and the insta- bility characteristic becomes less pronounced; in those of higher resistivity, the negative slope becomes steeper, the instability in- creases, and streak conduction or finally disruptive conduction appears. The streak conduction, described on the pyroelectric conductor, probably is the same phenomenon as the disruptive conduction or breakdown of insulators. Just as streak conduc- tion appears most under sudden application of voltage, but less under gradual voltage rise and thus gradual heating, so insulators of high disruptive strength, when of low resistivity by absorbed moisture, etc., may stand indefinitely voltages applied intermit- tently — so as to allow time for temperature equalization — while quickly breaking down under very much lower sustained voltage.