176
8.
ELECTRICITY THROUGH GASES
viewed in a rotating mirror, striations will be observed moving
with considerable velocity.
In the case of argon, the striations
move
from anode to cathode with a velocity between 1,000 and
10,000 centimeters per second.
The potential gradient in a uni—
form positive column is proportional to the pressure and ranges
from 0.05 to 150 volts per centimeters for different gases at 1
millimeter pressure.
It is less, for a given tube, gas and pressure
when the current is greater.
Also, for a given gas, pressure and
current, it is less when the tube has a larger cross-section. The
number of positive ions is equal to the number of electrons at all
points in a uniform positive column; this is called a Naima. With
large currents, relative displacements of the ions and electrons in
the plasma have been found to produce electrical oscillations.12
When striati0ns exist, they are usually concave and diffuse
toward the anode.
Their separation increases as the pressure is
decreased or as the tube diameter is increased and is independent
of the current, provided this is large.
The potential rises and
falls along the positive column whenever striations are present;
the potential difference between striations amounting to only a
few volts.
The number of ions or of electrons per
cubic centimeter in the positive column ranges from one—tenth to one onehundredth of that in the negative glow; with a maximum of
approximately 1010.
If one of the electrodes is small and the other is comparatvely
large, a rectifying action takes place so that the current passes
in
only one direction. The large electrode acts as the cathode. This
phenomenon is due to the fact that a current much greater
than
the normal current density over the whole cathode cannot be
passed through the tube without applying much higher potentials.
Thus when the large electrode is negative, its large surface Will
allow a strong current to flow with normal potentials. However,
when the small electrode is negative, a relatively small current will
cause its surface to be completely covered with the negative glow.
Greater currents require abnormal cathode falls so that the maxi—
mum reverse potential applied is only suicient to cause a rela—
tively small current to ow.*
If the pressure is reduced until the cathode dark space
reaCh€S
* The direction of the current as discussed here is that of the electron stream
and hence is opposite to that used in practice.
8.
ELECTRICITY THROUGH GASES
viewed in a rotating mirror, striations will be observed moving
with considerable velocity.
In the case of argon, the striations
move
from anode to cathode with a velocity between 1,000 and
10,000 centimeters per second.
The potential gradient in a uni—
form positive column is proportional to the pressure and ranges
from 0.05 to 150 volts per centimeters for different gases at 1
millimeter pressure.
It is less, for a given tube, gas and pressure
when the current is greater.
Also, for a given gas, pressure and
current, it is less when the tube has a larger cross-section. The
number of positive ions is equal to the number of electrons at all
points in a uniform positive column; this is called a Naima. With
large currents, relative displacements of the ions and electrons in
the plasma have been found to produce electrical oscillations.12
When striati0ns exist, they are usually concave and diffuse
toward the anode.
Their separation increases as the pressure is
decreased or as the tube diameter is increased and is independent
of the current, provided this is large.
The potential rises and
falls along the positive column whenever striations are present;
the potential difference between striations amounting to only a
few volts.
The number of ions or of electrons per
cubic centimeter in the positive column ranges from one—tenth to one onehundredth of that in the negative glow; with a maximum of
approximately 1010.
If one of the electrodes is small and the other is comparatvely
large, a rectifying action takes place so that the current passes
in
only one direction. The large electrode acts as the cathode. This
phenomenon is due to the fact that a current much greater
than
the normal current density over the whole cathode cannot be
passed through the tube without applying much higher potentials.
Thus when the large electrode is negative, its large surface Will
allow a strong current to flow with normal potentials. However,
when the small electrode is negative, a relatively small current will
cause its surface to be completely covered with the negative glow.
Greater currents require abnormal cathode falls so that the maxi—
mum reverse potential applied is only suicient to cause a rela—
tively small current to ow.*
If the pressure is reduced until the cathode dark space
reaCh€S
* The direction of the current as discussed here is that of the electron stream
and hence is opposite to that used in practice.