_
172
‘
8.
ELECTRICITY THROUGH GASES
slope (S) of the
—
V curve and the surface area (Â) of the probe;_
'
Thus, solving the equations above and evaluating the constants,
’
we nd
.
,
_
S
n = 3.32 >< 1011[—,
(8—13) :
where 8 is measured in practical units (amperes2/volts) and  is
in square centimeters.
Densities as high as 1011 electrons per
cubic centimeter have been observed.
'
An accurate value of the space potential (Vs) can be deter.
.
minedas follows.
The straight portion of the
—
V curve is ,
_
extrapolated to the voltage axis, as indicated at
gure 8—12.
Then z' = 0 and equation 8—12 becomes
’
0 = a + SVG.
(8—14)
‘
.
The constants a and 8 are evaluated from equations 8—10 and 8—11
—
and the relation Vee_= 3kT/2 is introduced. This leads to the
equation —
.
Vs = Vo +
(8—15)
‘
where Ve is given by equation 8—7.
_
‘
_
'
_
8—13.
'The Glow Discharge.—As stated before, at pressures
of a few centimeters, a wavy streamer of light is seen in a tube
_
.
-
through which electricity is passing. This broadens as the pres—
sure is reduced until it lls the cross—section of the tube.
Closer …
'
examination will reveal regions along the
tube where the intensity
_
.
of the light is not so great as in other regions. Figure 8—13 sh0ws
_
:
:
CÀ
_
GL0W
'57R/AT/0N;
_
,
_
…
Pas/WW:
COLUMN
MODE
--*
'
_
-
'
8—13. ;Aglow discharge at approximately—0.1 millimeter pressure.
'
_
the
& glow or Geisslef discharge when the pressure .,
'
;
.
'
'ab0'11t 051 millimeter, together.withthe names which have been
_
,
grventp
the variousregi0ns, While gure “8—14 shows qualitatively
-
,
the. various characteristics of a glow discharge."
‘