K9 Passage of electricity through gases
Gas discharges and ionisation processes
Ionisation of gases
Speaking quite generally, air insulates excellently: The gold leaves of an electroscope (Fig. 429) are caused to spread by a change and retain their positions almost unchanged for hours. Hence their charge does not move or only moves very slowly through the air to the earthed housing. However, you can greatly reduce the high insulation capacity of air: For example, if you place near the electroscope an active X-ray tube or a radium substance, the foils collapse quickly. As soon as you switch of the tube or remove the radium, the air recovers its former insulating capacity.
In order to explain how air becomes a conductor, we refer to an experiment by Wilson: If you cool down a large air column saturated with steam, the water precipitates, because the colder air cannot carry as much water vapour as before. In general, the excess of steam precipitates as fog, dust particles in the air induce condensation and become carriers of the water drops. If the air is free from dust and cooled down very rapidly, for example, by an adiabatic expansion, it can become saturated with water vapour. When it is expanded, the air then holds more vapour in gaseous form than corresponds to its temperature. This excess is in a labile sate; hence, if you insert just a trace of dust into the space, condensation in the form of drops occurs immediately. Strangely enough, the fog the also forms as X- or radium-rays act on the air. The fog is then so fine that is remains floating for a long time. It also differs from the fog, formed on dust particles, in that every droplet is charged electrically: If you let an electric field act on the fog (say, insert two parallel plates, connected to the poles of a battery into the space), you will see that the droplets migrate in equal numbers to the positive and negative poles. We recall here the electrolytic processes in which also very small charged particles - ions - carry the current. The processes in electrically conducting gases are indeed similar, however with one important difference. In the electrolyte, the ions already exist, in gases they must first be generated. The formation of ions in gases is called ionization, the gas in its state of conductivity is ionized, the equipment for the formation of ions is an ionizer. (Ionization by means of ultra-violet light, X-rays, radioactive rays, glowing metals, burning gas, electronic collision.)
The electroscope loses its charge by the action of X-rays: The X-ray splits on its way through the electroscope individual air molecules into positive and negative ions, some of which wander under the influence of the electric field to the gold foils, other to the equipment housing. For example, if the foils are charged positively, the negative ions wander there and lose their charge, so that the foils collapse.
Measurement of ionization flows
The rate at which the gold foils collapse during the experiment just described depends on the strength of the ionization. By counting the number of marks per minute on the scale of an ocular micrometer, you obtain a measure of the conductibility of the air and hence of the intensity of the radiation. The electroscope becomes thus an electrometer.
In order to investigate ionized gases in more detail, it is better to employ the plate condenser of Fig. 507. An earthed metal capsule K contains two plates P and P' a few centimetres apart, isolated at their supports I and I '. The plates are connected to the poles of an accumulator with about 100 Volt; one of the poles is also connected to the housing, that is, it is earthed. If you generate ions continuously in the air space between the two plates (say, by means of X-rays), +-ions will migrate without interruption to the negative plate P, --ions to the positive (earthed) plate P'. Hence electricity flows to both plates from the air space: However, it is continuously neutralized by input of opposite charge from the battery, whence the potential of the electrodes is continuously maintained by the battery. If the gas is sufficiently strongly ionized, so much charge flows per second to the plated from the battery that a sensitive galvanometer G between the plate P' and the positive pole of the battery continuously deflects. However, most of the time, the ionization currents are too weak for a galvanometer. You then employ a quadrant electrometer (Fig. 433), one pair of quadrants of which is connected to the plate P' and the other pair earthed. Then the charge flowing to the plate P' gradually charges fully the one quadrant pair, as is displayed by the slow and uniform motion of the needle of the electrometer. The rate of this motion is proportional to the flow through the air.
Recombination and velocity of ions
If you also earth in the ionization chamber of Fig. 507 the plate P, the number of ions in the chamber K increases, since the field which extracted them from the air space is absent. Does the ionization process last until all gas molecules have been split into ions or are there other processes to stop this? To start with, one expects that individual ions approach by themselves by diffusion the plates P, P' and the housing and thereby lose their charge. In fact, this is what happens, however, in general, the number of ions destroyed in this manner is very small compared with the direct reunion (recombination) of the ions in the gas space as a result of the forces of attraction, which are present between oppositely charged ions. This recombination apparently takes place the faster, the larger is the number of positive and negative ions in the space. Hence, if you focus your attention on a definite negative ion, the probability that it will disappear by recombination with a positive ion is proportional to the number N+ of the present ions; the same applies to the positive ions. The frequency of recombinations in a gas is therefore kN+N- or kN², since, as a rule, equal numbers of positive and negative ions are present. K is a factor which differs with the state of the gas. Hence the number of recombinations increases quadratically with the ion density. A strongly ionized gas de-ionizes on its own very quickly unless an external agent generates continually new ions. You can measure the recombination of ions in a strongly ionized gas: For example, Rutherford discovered that of 106 initially present ions half of them disappear after 0.7 sec and 90% after 6 sec. Also, under the action of a very strong ionization, the number of ions, which accumulate in a gas space (free of an electric field), remains very small compared with the number of present gas molecules.
In the experiment of Fig. 507 above, it has been assumed that the ions are transported instantly to the plates as the electric field is applied. In fact, this is not so! Their velocity is proportional to the strength of the electric field and amounts in air to a potential gradient of 1 Volt at 1 cm distance for the positive ions to 1.3, for the negative ions to 1.8 cm/sec, in contrast, hydrogen with less specific weight to 6.0 and 7.7 cm/sec, respectively. - The velocity of an ion in a gas is extraordinarily larger than the corresponding velocity in an electrolyte. H-ions move in pure water only at 1.08 cm/hr at a gradient of 1 Volt/cm, so that the H-ion in hydrogen moves 25000 times faster than in water.
Can a strong electric field remove the freshly formed ions from the gas so fast that no recombination whatsoever occurs? An experiment with the plate condenser (Fig. 507) gives the answer! Ionize the gas with a lastingly applied X-ray tube and increase, starting from zero, the field between P and P' by connecting to P at first one, then two, three, etc. accumulators. For each field value measure by means of G the current flowing through the gas. If you plot the strength of the ionization current against the field strength (Fig. 508), you obtain a curve, which at first rises very steeply almost linearly, then flattens out and levels out parallel to the abscissa. This behaviour is easily understood. In a weak field, the ions wander only slowly, whence they are in the gas space a relatively long time and therefore do not readily encounter opposite charged ions for recombination. As their velocity increases, the probability of a recombination drops. The number of ions, which reach the plate, and therefore also the current strength grow with increasing field strength to a value, at which the ions pass through the gas so fast, that recombination cannot occur in appreciable numbers. A further increase of the voltage cannot now cause an increase in the current, because all forming ions are transmitted to the plates P and P'. The maximum current is called saturation current, the corresponding Voltage saturation Voltage. - The saturation current measures directly the number of ions, generated in unit time, and hence also the strength of the ionizer. If there arise each second N pairs of ions in a gas space and each ion carries the load e, then I = eN, for the current is nothing else but the charge which flows each second through the conductor's cross-section.
Electric charge of ion. Elementary charge
The experiments described so far tell about the ratio of the charge to the mass of electrons as well as on electrolytic and gaseous ions; however, they do not allow a determination of the absolute value of the charge.
The first such method rests on the property of ions to be able to act as condensation nuclei for water vapour. In general, condensation succeeds the better, the faster the , with vapour saturated water vapour. In general, condensation succeeds the better, the faster the gas is cooled; this is done best of all by adiabatic expansion which, as we know, is connected with a reduction in temperature. If you expand the air volume to more than 1.3 fold, the positive as well as the negative ions acts as nuclei; below that ratio, condensation only occurs at the negative ions. The essence of an experiment, due to C.T.R Wilson and refined by J.J.Thomson, follows: You ionize a volume of air, saturated with steam by X-rays and then expand it t a little less than 1.3 fold, so that water droplets only form on the negative ions. You then count first the number N of droplets microscopically, then measure the total charge E of all droplets by driving the air through an electric field to a plate, connected to an electrometer. The ratio E/N yields the charge of a single ion. (In fact, you do not measure E and N in this way, but indirectly by a safer method,)
Such a method was published by Felix Ehrenhaft 1879-1052 1910 and developed into a measurement system by Millikan: He employed a tiny charged oil droplet and observed its rate of falling in Earth's gravity field as well as in an electric field, opposed to it. For the evaluation of the observations, an essential role has a problem in Hydrodynamics. If r denotes the radius of the droplet, d and m the density and viscosity of air, g Earth's gravitational acceleration, then the velocity of fall is given by:
v = [4/3r³pdg]/6rpm.
The expression in square brackets is the mass of the drop multiplied by the gravitational acceleration, that is, it is equal to the force of gravity acting on the drop. This force is reduced by eF, when the gravity field has superimposed on it an electric field of strength F, which attempts to raise the droplet with the charge e, because the force, exerted by the field on the charge is given by the product of this charge and the strength of the field. Hence you find for the velocity of the droplet during simultaneous action of both fields:
v' = [4/3r³pdg - eF]/6rpm.
If the gravitational force equals the electric force, the droplet remains in its place, if it is smaller, it rises.
If you determine experimentally the velocities v and v', you can eliminate from both equations the unknown and hard to measure radius of the droplet and obtain for the charge e an expression involving only known quantities. Millikan did not employ water droplets, because they evaporate too quickly, but tiny oil droplets, created by atomization, which in an ionized gas accept by accumulation of one ion a unit charge. Such a droplet is then allowed to fall slowly between two horizontal metal plates, which to start with are earthed. Prior to its reaching the lower plate, an electric field is created between the plates, which is strong enough to again raise the droplet. The drop itself is illuminated from the side and appears like a dust particle in the Sun's rays as a lighted dot with a dark background within the field of vision of the (horizontally lying) microscope. For the measurement of the velocity, the eyepiece of the microscope has two horizontal, very fine threads, which appear simultaneously with the droplet in the field of vision. You record the times, at which the falling droplet passes the two threads. Since it is readily determined, which real distance of fall corresponds to the distance between the threads, you find the velocity v in Earth's field as the ratio of the distance and duration of the fall. After application of the electric field, the droplet passes again the two threads, but in the opposite direction, so that you determine in the same manner the velocity of the rising droplet. Since the oil droplet evaporates only slowly, you can let it fall in the gravitational field as often as you like and again lift it in the electric field, thus determining very reliably the values of v and v'. Numerous and in many ways changed experiments yielded the elementary charge of 4.77·10-10 electrostatic units.??
Ionization by collision
We continue with the earlier described set-up for the measurement of the saturation current, but do not make the gas pressure in the ionization chamber equal to the atmospheric pressure, but equal to 1/100 of it. We ionize the gas again by X-rays and measure the ionization current at different field strengths. Since the ions at low gas pressure collide much less frequently than at high pressure, the number of recombinations is very small and saturation occurs already for very weak fields. Thus, the current attains at growing field strength very soon its saturation value and, according to our experience so far, should not change further irrespectively of a rise in the field strength. Indeed, you can raise the tension to the tenfold, even hundred-fold value of the saturation tension without any change in the current. One should expect that these conditions will only change at the onset of an independent discharge (spark). However, surprisingly the current rises already far below the tension, required for this, indeed, considerably so (Fig. 509). Thus, there occur in a gas at low pressures and large field strengths new ions, the generation of which cannot be reduced directly to the means of ionization.
In order to explain this phenomenon, we must return to the ionization process itself: By external action ( for example, by X-rays) one electron is separated from a gas molecule (or atom), and thereby it becomes a positively charged ion. Under the normal conditions, which held in Fig. 508, the separated electron will attach itself soon to a gas molecule, because free electrons can hardly exist in a dense gas. The molecule with the excess electrons form then a negative ion and remain so until it reaches the electrode or recombines with a positive ion, whereby both molecules return to the normal state.
However, at low gas pressures and in strong electric fields, the freshly separated electrons attain immediately large velocities and therefore cannot at collision with a neutral molecule attach themselves to them; in the contrary, they ionize it again. In this manner, a single electron can cause a large number of new electrons as a result of its velocity and every single one of the newly formed electrons acts in the same way and increases their total effect. These processes, which have been explained above all by Townsend 1900, are referred to as collision ionization. - Also the positive ions can be ionized by collision, however, due to their greater mass, it requires considerably stronger fields. Collision ionization can be employed for magnification of ionization currents by factors of one thousand, yes, even of hundred thousands, whenever they are too weak for measurements, thus making them accessible to measurements.
According to Joffé, collision ionization explains the disruption of good, solid insulators in the case of excessive tension. Isolators, which consist of alternating layers of better or worse conducting substances bear, provided the worse conducting layers are thinner than 5 - 10 m , much higher tensions than equally thick, homogeneous insulators made out of the worse conductor. Because the thickness of 5 - 10m is then not sufficient to let arise an ion avalanche and the better conduction layers impede their formation in a similar manner as the known, small stone walls in mountains stop the formation of snow avalanches. - Collision ionization explains the glowing cover (corona) which you can see at night around high tension lines. The field strength near the cables (only there!) is so large, that it ionizes the air there and activates glowing. Elmo's fire is also due to collision ionization, a glowing electric discharge, which readily arises at especially high atmospheric potential drops on sharply points objects on Earth's surface, like lightning rods, peaks of towers, ships' masts: At pointed objects, due to the high field strength, the velocity of the ions is so large that they ionize at collision air molecules. The current density with tufted Elmo's fire has been estimated at 0.1 - 0.2 miliampere/cm² (August Toepler 1836-1912).
The collision ionization, which occurs at high tension lines, is employed technically as electro-filter, in order to free waste gases of dragged along particles - be it to clean the gases (blast-furnace gas, ventilation air), be it to retain the particles (metal, carbon , cement). You generate in the space, through which the waste gas is fed, the required field between two specially shaped electrodes, with a high-tension direct current source (about 80000 Volt). The one electrode (ionizing electrode, mostly lattice shaped) ionizes the gas, with which it is in contact, and generates the charge carriers, which attach themselves to floating particles and charge them. The other (precipitation electrode, corrugated iron or a wire sieve) is earthed and attracts the charged particles, which immediately precipitate until they on their own or under the action of a shaking system drop out.
Ionization state of atmosphere
Earth's atmosphere is also an ionized gas. Its carriers of charges are the molecules of the air as well as droplets and dust particles. In the lower 10 km (troposphere), there are compared with the gas molecules only a very few ions which move with strong friction: In the electric field of 1 Volt/cm at about 1.5 cm/sec. (The Earth magnetic field has here no role whatsoever, but it is important in the upper atmosphere [100 km height]. The greatly diluted ionized and hence conducting upper air layers [80 - 100 km] are called the Heaviside layer after the discoverer Heaviside 1925 of their influence [as upper boundary] on electro-magnetic waves, which spread out over Earth's surface.)
The positive air ions wander in the direction of the normal electric field downwards as a conduction current. The conductivity of the atmosphere is largest when the weather is clear, smallest when it is hazy. Apart from the ion conducting current, there exists a convection current: Vertical air motion and precipitation like rain and snow carry electricity along with them.
Earth has negative surface tension , the atmosphere in the lowest layers positive charge, which is sufficient to compensate the surface charge. The conduction current - it transports downwards everywhere almost constantly 3x10-16 Amp/cm² - would have to destroy it very quickly. Nevertheless, its mean value remains constant, whence there must exist an almost equally strong, oppositely directed counter current. The question regarding what stabilizes overall the potential difference between Earth and the atmosphere in spite of unceasing electric flow is the basic problem of the research on the normal atmospheric electricity; it was not answered in 1935.
Also the causes of the ionization of the atmosphere have not yet been explained. As main ionisators of the lower layers of the atmosphere (troposphere) on has: The radiation of the radioactive substances in the top soil layers and their decay products, which enter as emanations the layer of air near Earth's surface as well as the penetrating radiation from above (Hess, Werner Heinrich Julius Kohlhörster 1887-1946), a radiation of unusually large penetration capability which infiltrates water layers of 50 m thickness. It increases fram the ground at first slowly, becomes stronger above 4000 m and attains 50 times the value at the ground at a height of 9000m (1.5 ions cm-3 sec-1). According to Kohlhörster, it is a gamma radiation and has so large an energy concentration (activity quantum) that not even the known radio-active substances can be sources of the radiation. Moreover, one has to assume for its appearance changes in energy as they might have to be expected during the formation of new atoms (Nernst). Most probably, it comes from the cosmos as light of the fixed stars; however, the Moon, Sun and planets prove to be ineffective,
One form of electric discharge in the lower layers of the atmosphere is lightning ( spark or line, also sheet and sphere lightning). It can only form when the electric field has become so strong that ionization by ion collision becomes possible. It is the potential equalization between two differently charged clouds or between a cloud and Earth in the form of several, consecutive, fast, partial discharges each with a duration of about 1/1000 sec, computed overall in tenths of seconds. The colour which is mostly off white, reddish or bluish, is due to the glowing of the gases in the track of the lightening (nitrogen, oxygen, hydrogen, rare gases). According to Julius Elster 1854-1920 and Hans Friedrich Geitel 1855-1923, the direction of the current during reddish ligthening is from Earth to clouds and inverse during bluish lightening. The maximal current strength is apparently (Friedrich Karl Pockels 1865-1913) between 9000 and 200,000 Ampere.
The lightening conductor brings the charge of the cloud in a harmless manner to Earth. Its projecting bar compresses the level surfaces of Earth's field over its top, whence the potential gradient becomes there largest and automatic discharge starts therefore effectively at this location. The discharge then follows the path of least electric resistance, that is, through the iron bar and the good conductor, connected to it, to Earth, a largish copper plate in a damp soil forms the end of the earthing line.
Forms of gas discharge at different pressures
In order to become overall acquainted with the phenomena during the passage of electricity through gases at lower pressure, we will employ a glass discharge tube about 4 cm wide and 20 cm long (Fig. 510) with two metal disks A and K as electrodes with wires melted airtight into the glass. You exhaust the air through the tube D to any desired rarefaction. You connect A to the positive, K to the negative pole of an electric machine. At atmospheric pressure, lightening like discharges occur then between A and K. If you reduce the pressure, the discharge becomes less violent; there arises a thread of light between the electrodes, which becomes stronger as the pressure is reduced (Fig. 511a). Gradually there arise distinguishable structures of light, which become the clearer the more the discharge at decreasing pressure occupies the entire cross-section of the tube. Fig. 511b shows the tube with air at 2 mm mercury pressure. Just ahead of the cathode lies a bluish disk of light, the negative glowing light; a rather light, reddish ring of light extends from the anode far into the tube. The light disk and ring of light are separated by Faraday's dark space.
When the pressure drops to a few tenths of millimetres, the negative glowing light increases and the positive light decomposes into layers, each of which is sharply bounded towards the cathode, indistinct towards the anode (Fig. 511c). Especially obvious is the bright light which the layered or unlayered, positive column displays. Its colour changes greatly with the kind of gas and the special conditions of discharge. Nitrogen glows bluish in narrow tubes, reddish to yellowish in wide tubes, helium deep yellow, hydrogen once purple-red, once whitish.
Incandescent lamps (neon: red, mercury vapour and argon: blue, in green glass blue-green) and advertizing tubes (nitrogen, carbonic acid) employ the brightness of the positive light, the neon glowing lamps (emergency lighting, signals) with a mixture of helium and neon that of the negative light.
When the pressure drops to a few hundredths of millimetres mercury, the glowing phenomena fade more and more. The positive column disappears, the negative glowing column lengthens and eventually fills the entire tube. In the process, it detaches itself from the cathode and is separated from it by a space with very little light (the dark room of Hittorf (Fig. 511d). During this type of discharge, emanate from the cathode bluish rays: Cathode rays (discovered by JPlücker in 1858). They propagate along straight lines until they encounter the anode or the glass wall, which will become greenish fluorescent.
As the pressure is reduced further, the cathode rays intensify; the green glowing expands over the entire glass wall, so that the weakly formed lights of the discharge inside the tube are no longer discernible. Simultaneously, there arise rays, named after their discoverer Röntgen (1895).
In order to produce X-rays (Röntgen rays) systematically, you employ a spherical discharge vessel out of glass with a 20 - 30 cm diameter* and let the cathode rays meet an electrode connected to the anode - anti-cathode - the inner end of which is a piece of heavy metal (mostly wolfram): This piece of metal becomes thereby a source of Röntgen rays. The higher the tension, with which the tube is operated, the faster are the cathode rays and the more penetrating (harder) are the X-rays. At 100000 Volt inductor tension, the electrons can reach half the velocity of light. At this velocity, they collide with the anti-cathode. A good tube can be operated all the time with 50000 Volt and 5 milli-Amp. The current is formed by positively charged gas atoms (ionized by separation of electrons) and negative electrons. The hollow mirror cathode compresses the cathode rays so that they generate on the anti-cathode, while not exactly a focal point, a burning spot of only a few mm². Only 1 - 2% of the cathode radiation energy become X-rays, the remainder becomes heat, so that the anti-cathode must be cooled artificially, in order to protect it against destruction. The X-rays propagate linearly from it in all directions and generate fluorescence wherever they meet the glass wall.
* We talk here about the X-ray tube of the medical doctor; the tube of the physicist is quite different. Fig. 513 shows one of its many forms (Siegbahn). It consists completely of metal with the exception of a porcelain insulator for the cable to the cathode. The anti-cathode, installed carefully and made airtight, can be exchanged. The radiation leaves by a window (at the bottom, right) made out of a minutely absorbing substance (aluminium foil, mica, goldbeater's skin).
This X-ray tube (ion tube), which depends on gas discharge, requires a certain amount of gas inside, whence it is damaged by two phenomena: The first raises the gas pressure, the second lowers it; the first arises because gases escape gradually from all components of the tube and worsen thereby the vacuum and the ability to penetrate of the radiation, the second is the ion collision against the cathode, which disperses the material of the cathode; the dispersed metal covers the glass wall, absorbs the gas and gradually increases the vacuum so that eventually no more discharge passes through the tube. These defects can be reduced within bounds (regeneration attachments), but incapacitate the tube earlier or later. All these difficulties are avoided by the tube of William David Coolidge 1873-1975 - electron tube .
Distribution of tension in layered discharge
The change of the light phenomena is accompanied by changes in the total tension between the anode and cathode as well as its distribution in the tube. In order to maintain the form of discharge in Fig. 511a, you require about 5000 Volt between the electrodes. If the gas pressure drops, the tension required for the maintenance of the current drops and reaches a minimum of several hundred Volt between 1 and 0.1 mm. If the pressure is further reduced, the tension in the tube rises quickly, possibly to 100000 Volt or more.
You can discover the distribution of the tension in the tube by airtight installation of about 1 cm diameter platinum wires - sondes - at right angle to the tube's axis and connecting them to a quadrant electrometer. In this manner, you obtain the potential at different locations of the gas* Fig. 514). The greatest drop in the tension lies close to the cathode - it is called cathode drop; in the dark space, the drop is small, it remains so in the positive column, where it oscillates in the layers correspondingly. An abrupt strong drop occurs only at the anode - anode drop. At lower pressures, the cathode drop increases extraordinarily, otherwise the tension distribution changes little. In a tube with a high vacuum, for example, in X-ray tubes, effectively the entire working tension is consumed in the cathode drop.
A presentation of the detailed quantitative conditions is here impossible; moreover, some aspects had not yet been clarified in 1935. However, the overall distribution of the gradient can be shown. The current of electrons and ions, which flows through the gas, influences the potential drop between the electrodes by space charges; the change of the gradient acts again on the ionization conditions and hence back to the current intensity. The ions and electrons, which transport the current, arise - and indeed in equal numbers - in the gas space, also partly at the electrodes, by electron- and ion-collision. All positive ions flow through the face of the cathode, all electrons and negative ions through the anode. In this way, there forms in front of the cathode a positive, in front of the anode a negative space charge. The (positive) ions move due to the magnitude of their mass more slowly than the (negative) electrons, whence the positive space charge in from of the cathode is large, the negative space charge in front of the anode small. The positive space charge shields the negative charge of the cathode against the internal space, similarly the negative space charge the positive charge of the anode; hence the potential gradient in the intermediate part of the discharge is considerably weaker than it would be if there were no current. Correspondingly, the gradient in the space near the electrodes has to be amplified, since the electrode tension is being maintained. The strong positive space charge near the cathode causes the abrupt cathode drop, the weak negative one near the anode the small anode drop. The cathode drop extends from the cathode to the first dark space up to the negative faint light fringe; the anode drop at the anode behaves similarly. The ionization conditions correspond to the distribution of the potential. The positive ions, mainly formed in the faint light fringe, encounter, accelerated by the cathode drop, the cathode and detach from it electrons, which, likewise accelerated by the cathode drop, decompose (at the expense of their kinetic energy) during collisions in the glowing light fringe molecules into positive ions and electrons. In the positive column, the gradient is just sufficient to accelerate the departing electrons so that they cause the gas to glow and make up for the loss of charge carriers (by diffusion to the wall and by reunion) by renewed formation by collision ionisation. The positive column is unimportant for the formation of discharges and can under certain conditions be suppressed. Since the anode drop, as has already be mentioned, is small compared with the cathode drop, the minimum tension for maintenance of the glowing charge is effectively equal to the cathode drop. Its great importance is due to this (first measurement by Warburg 1887). It depends on the type of gas and the metal of the electrodes; in rare gases, it is small, especially at eletro-positive electrode metal (important for the application in incandescent tubes filled with rare gas.
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