178
8.
ELECTRICITY THROUGH GASES
-
:
are formed around the grid wires.
If the grid is suiciently nega.—
tive, the sheaths will be large enough to overlap an'd prevent the
.
current ow to the plate. If, now, the grid is made progressively
less negative, the sheaths become thinner until, at a critical value
of the grid potential, they no longer close the largest hole in
the grid. The electric eld of the plate is then able to attract
electrons in the region between the cathode and the grid and a
current ows through the tube. When the current has once
started, the sheaths all become much thinner and, although sub-.
ject to small changes if the grid is made more negative, cannot
again increase in size to the point of stopping the discharge until
the current is practically cut OH by‘a
A
a
8600 Ep
reduction of the plate voltage to zero.
{? \‘200 \\
\
0
The critical or striking potential,
-
ao°‘x\ \\ ao\°\ \ΰ
of the grid depends on the potential, E10
\ \
\
‘,
of the plate, as shown in gure 8—15.
In this gure, the curves show the
0
‘\
+
value of the grid potential, for a given
EC GRID STRIKING POTENTIAL plate potential, WhÎCh Will 'jUSt start
Fra. 8_15_
Striking curves of
the current through the …tube. Curve
gas-lled triodes.
 is for the more usual
“
negativelv
'
“
controlled” tubes; B is“ for a
“
posr—
tively controlled
”
tube.
In the latter case, negative space
,_
charges on the glass walls add their effects to that of the gr1d
itself.
Tubes lled with a rare gas such as argon are used for
the lower voltages and are not subject to temperature effects
»
For tubes operated at the higher voltages (above a few hundred
Volts) mercury vapor is used. The pressure of the vapor increases
With temperature and the striking potential becomes more nega—
—
tive,as indicated by the dotted lines in gure 8—15.
,
'
ï
«
vTh@. ratio of the plate voltage, Ep, to the grid striking potential
Ec,iS knoWn as the grid control ratio p… Thus,
Th1sis
given tube over the straight portion ofthe
Str1k1ngcurve:
.
,
_,{gæéÿwlnpnç
grid—glo'w tubes, a cylmdcal
Ï
_
The anode cons1stsofaWlre
8.
ELECTRICITY THROUGH GASES
-
:
are formed around the grid wires.
If the grid is suiciently nega.—
tive, the sheaths will be large enough to overlap an'd prevent the
.
current ow to the plate. If, now, the grid is made progressively
less negative, the sheaths become thinner until, at a critical value
of the grid potential, they no longer close the largest hole in
the grid. The electric eld of the plate is then able to attract
electrons in the region between the cathode and the grid and a
current ows through the tube. When the current has once
started, the sheaths all become much thinner and, although sub-.
ject to small changes if the grid is made more negative, cannot
again increase in size to the point of stopping the discharge until
the current is practically cut OH by‘a
A
a
8600 Ep
reduction of the plate voltage to zero.
{? \‘200 \\
\
0
The critical or striking potential,
-
ao°‘x\ \\ ao\°\ \ΰ
of the grid depends on the potential, E10
\ \
\
‘,
of the plate, as shown in gure 8—15.
In this gure, the curves show the
0
‘\
+
value of the grid potential, for a given
EC GRID STRIKING POTENTIAL plate potential, WhÎCh Will 'jUSt start
Fra. 8_15_
Striking curves of
the current through the …tube. Curve
gas-lled triodes.
 is for the more usual
“
negativelv
'
“
controlled” tubes; B is“ for a
“
posr—
tively controlled
”
tube.
In the latter case, negative space
,_
charges on the glass walls add their effects to that of the gr1d
itself.
Tubes lled with a rare gas such as argon are used for
the lower voltages and are not subject to temperature effects
»
For tubes operated at the higher voltages (above a few hundred
Volts) mercury vapor is used. The pressure of the vapor increases
With temperature and the striking potential becomes more nega—
—
tive,as indicated by the dotted lines in gure 8—15.
,
'
ï
«
vTh@. ratio of the plate voltage, Ep, to the grid striking potential
Ec,iS knoWn as the grid control ratio p… Thus,
Th1sis
given tube over the straight portion ofthe
Str1k1ngcurve:
.
,
_,{gæéÿwlnpnç
grid—glo'w tubes, a cylmdcal
Ï
_
The anode cons1stsofaWlre