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, 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 25,000
times faster than in water.
Saturation
current
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 Wilson and refined by 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
80,000 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 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,
Lightning
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. 511 a). 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. 511 b 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.
511 c). 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 Plücker 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* (Fig. 512) 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 100,000 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 50,000 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 100,000 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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