126
6.
HIGH VACUUM, LOW-VOLTAGE TUBES
the plate voltage increases. Over this region the tube is said to
have a negative resistance.
This feature has been made use Of in
*
the so-called dynatron oscillators and tube voltmeters.
When the
_
tube is used as an amplier in the usual manner, the plate voltage
'
is made greater than that of the screen. The interpretation of the
curves in gure 6—14 will be given in the next section.
\
'
’
6—8.
Secondary Emission.—The plate current, [,, in gure
6—14 of the preceding section, does not increase continuously over
the region ÂB as the plate voltage is increased because secondary
electrons are ejected from the plate by the primary electrons from
the lament.
The secondary electrons are attracted to the screen
grid Which is more positive than the plate. That this is true is
shown by the increase in .the screen grid current at C. Since
electrons are travelling both toward and away from the plate,
curve I,, represents the net electron current to the plate. As the
plate voltage is increased, more secondary, electrons are ejected
and pass
to thescreen. It is even possible that each primary electron may eject more than one secondary electron, in which case
:
the plate current is reversed; more electrons are owing from it to
-
the screen than approach. Under these conditions, the curve ÂB
crosses the voltage axis at a point where the number of secondary
electrons is equal to the number of primary electrons. When the
'
plate has a suiciently high potential to draw the secondary elec—
.
_trons back to it, (as at Esg in gure 6—14), the plate current rise_s in
the usual manner to its saturation
the screen not only
,fails to attract secondary electrons but also“ fails to capture as
many primary electrons as before.
'
,
Let S
the number of secondary electrons produced by each
"
primary electron. It has been found6 that
_“
1. As the velocity of the primary electrons is increased from
Zero to a few hundred volts, S increases from a low value,
_
‘
j has zéro, to a maximum S…. With still further 1ncrease1n
energy of the primary electrons, S slowly decreases.
—.
2Thecurve of S for different voltages accelerating the
_
primaryeleCtrons(l, above) is not smooth but has
Â. 3Smlsab©ut10 {to .
for outgassed ordinary metalS,*
and maybe as great as 8 mm
foralkalimetal lms on an oxidized metal plate.
.
'
6.
HIGH VACUUM, LOW-VOLTAGE TUBES
the plate voltage increases. Over this region the tube is said to
have a negative resistance.
This feature has been made use Of in
*
the so-called dynatron oscillators and tube voltmeters.
When the
_
tube is used as an amplier in the usual manner, the plate voltage
'
is made greater than that of the screen. The interpretation of the
curves in gure 6—14 will be given in the next section.
\
'
’
6—8.
Secondary Emission.—The plate current, [,, in gure
6—14 of the preceding section, does not increase continuously over
the region ÂB as the plate voltage is increased because secondary
electrons are ejected from the plate by the primary electrons from
the lament.
The secondary electrons are attracted to the screen
grid Which is more positive than the plate. That this is true is
shown by the increase in .the screen grid current at C. Since
electrons are travelling both toward and away from the plate,
curve I,, represents the net electron current to the plate. As the
plate voltage is increased, more secondary, electrons are ejected
and pass
to thescreen. It is even possible that each primary electron may eject more than one secondary electron, in which case
:
the plate current is reversed; more electrons are owing from it to
-
the screen than approach. Under these conditions, the curve ÂB
crosses the voltage axis at a point where the number of secondary
electrons is equal to the number of primary electrons. When the
'
plate has a suiciently high potential to draw the secondary elec—
.
_trons back to it, (as at Esg in gure 6—14), the plate current rise_s in
the usual manner to its saturation
the screen not only
,fails to attract secondary electrons but also“ fails to capture as
many primary electrons as before.
'
,
Let S
the number of secondary electrons produced by each
"
primary electron. It has been found6 that
_“
1. As the velocity of the primary electrons is increased from
Zero to a few hundred volts, S increases from a low value,
_
‘
j has zéro, to a maximum S…. With still further 1ncrease1n
energy of the primary electrons, S slowly decreases.
—.
2Thecurve of S for different voltages accelerating the
_
primaryeleCtrons(l, above) is not smooth but has
Â. 3Smlsab©ut10 {to .
for outgassed ordinary metalS,*
and maybe as great as 8 mm
foralkalimetal lms on an oxidized metal plate.
.
'