164
8.
ELECTRICITY THROUGH GASES
_
8—6.
Conductivity at Reduced Pressures.—At atmospheric
pressure
a gas does not conduct electricity unless very high poten‘
tials are used or the gas is ionized by some agency such as X-ray3,
radioactive rays, or ultra-violet light. However, as the pressure
is lowered it is found that a gas becomes conductive, reaching a
maximum and then, at very low pressures again becoming an
extremely poor
conductor.
To study the passage
of electricity through a gas while the
pressure
is being lowered, connect a vacuum pump
and gauge
(see chapter 16) to a long glass tube containing two metal plates
sealed in With wires.
The current from the secondary of an induction coil or transformer passes
between the metal plates causing,
among other things, the emission of light by the particles of gas
left in the tube.
When the pressure
of the gas has been reduced
to a few centimeters of mercury, a wavy
streamer of lightpasses
from one electrode to the other.
As the pressure
is still further
reduced, the streamer broadens until it lls the entire cross—section
.
of the tube With a glow of light. The color of the light emitted
_
depends on the gas in the tube, having_a greenish color with mercury vapor, red with hydrogen, brilliant red—orange with neon,
yellow with helium, and bright red with nitrogen. At
mately one—half millirheter pressure, the conductivity is very good,
as may be shown by a spark gap in parallel with the tube. Unless
the gap is Very short, the electricity chooses the longer path
‘
through the discharge tube.
'
8—7.
and Stopping Conditions—The starting condi—
tions for a glow discharge tube may be crudely described as fol—.
'
lows. It is assumed that there are always a few electrically
particles in the tube at the start. - These are accelerated
"
'
when a potential difference of a few hundred volts is applied
between theelectrodes and acquire sufcient energy
to excite and
to ionizethe neutral gas atoms or molecules.
In turn, the
‘ _
created by collision, and the photo-electrons produced by the light
…
atoms, create additional electncalcharges
,
‘of ionizationS, recomb1nat1ons,
etC,1s established and a measurable
ows
‘
…
.
_
,
’
There is“ ai certain minimum of voltage, called the sfm‘king palm
be appliedacross the tube befbrethe
8.
ELECTRICITY THROUGH GASES
_
8—6.
Conductivity at Reduced Pressures.—At atmospheric
pressure
a gas does not conduct electricity unless very high poten‘
tials are used or the gas is ionized by some agency such as X-ray3,
radioactive rays, or ultra-violet light. However, as the pressure
is lowered it is found that a gas becomes conductive, reaching a
maximum and then, at very low pressures again becoming an
extremely poor
conductor.
To study the passage
of electricity through a gas while the
pressure
is being lowered, connect a vacuum pump
and gauge
(see chapter 16) to a long glass tube containing two metal plates
sealed in With wires.
The current from the secondary of an induction coil or transformer passes
between the metal plates causing,
among other things, the emission of light by the particles of gas
left in the tube.
When the pressure
of the gas has been reduced
to a few centimeters of mercury, a wavy
streamer of lightpasses
from one electrode to the other.
As the pressure
is still further
reduced, the streamer broadens until it lls the entire cross—section
.
of the tube With a glow of light. The color of the light emitted
_
depends on the gas in the tube, having_a greenish color with mercury vapor, red with hydrogen, brilliant red—orange with neon,
yellow with helium, and bright red with nitrogen. At
mately one—half millirheter pressure, the conductivity is very good,
as may be shown by a spark gap in parallel with the tube. Unless
the gap is Very short, the electricity chooses the longer path
‘
through the discharge tube.
'
8—7.
and Stopping Conditions—The starting condi—
tions for a glow discharge tube may be crudely described as fol—.
'
lows. It is assumed that there are always a few electrically
particles in the tube at the start. - These are accelerated
"
'
when a potential difference of a few hundred volts is applied
between theelectrodes and acquire sufcient energy
to excite and
to ionizethe neutral gas atoms or molecules.
In turn, the
‘ _
created by collision, and the photo-electrons produced by the light
…
atoms, create additional electncalcharges
,
‘of ionizationS, recomb1nat1ons,
etC,1s established and a measurable
ows
‘
…
.
_
,
’
There is“ ai certain minimum of voltage, called the sfm‘king palm
be appliedacross the tube befbrethe