_
16—11.
IONIZATI_ON GAUGES
369
In order that the positive ion current shall be directly propor—
tional to the pressure of the gas, the following conditions must be
fullled:
@
-
-
.
1. The pressure must be lower than that critical value for Which
,
_
the mean—free—path of the electrons in the gas is less than the
distance between the grid arid plate. If the pressure is greater, an
electron may suffer more than one collision with gas molecules,
destroying the desired one—to—one relationship. This is avoided
by proper spacing of the electrodes and restriction of the range of
pressure used, to below approximately 10“3 millinieter of mercury.
There are commercial tubes especially designed for this purpose,
“
such as the FP62.
But certain tubes, such as the 101D, 102D,
205D, 216A and 45, may also be
2. The number of ions produced by each electron must not
change appreciably with variation of
energy of the electron.
’
The latter is usually eXpressed in terms of the accelerating voltage
'
between the grid and the lament. The probability of ionization
for electrons of different speedS is zero up
to a small accele‘rating
_
voltage, called the minimum ionizing potential,
rises to a
broad maximum somewhere between 100 and 500 volts. For
still higher voltages, the chance that an electron will ionize
‘
atom decrease slowly. Best operationof the gauge is, therefore,
obtained in the region of the broad maximum where‘ the probability
‘
is nearly constant.. This maximum“ o_CCur-s
-at dieret'_voltages_
-
and is of different magnitude :for different gases,ÿ necesS1tàtmg
a
separate calibration for”
coibina'ti‘0n- of gaSeS.—
For
_
air, a maximum exists between 2550
-Th‘_ÏS>
,
ever, is not critical and
in
.
_
actual operation. , '.
,
…
'
'.
.
_
'
3. The
,
by the grid currentmustbekeptœn3tant
.
'
brought ab01—it“bySlmplÿmammlnmgCOHStant
the
—
.
’
tube, ‘sinç‘èf thètemperatuœandhenœtheemlssmn0f
_
varies with‘thëpressureduet@thecoglln€eñeCt0fheat
.
mehr; Whenlgl“lsesabovethe‘i€Slf€dC‘”“—°‘tantvalue>ltsmClease
‘
.
'
is used toreducethefam€ntcum°>ntWhl°h“…“““"‘duces the. ‘
L
16—11.
IONIZATI_ON GAUGES
369
In order that the positive ion current shall be directly propor—
tional to the pressure of the gas, the following conditions must be
fullled:
@
-
-
.
1. The pressure must be lower than that critical value for Which
,
_
the mean—free—path of the electrons in the gas is less than the
distance between the grid arid plate. If the pressure is greater, an
electron may suffer more than one collision with gas molecules,
destroying the desired one—to—one relationship. This is avoided
by proper spacing of the electrodes and restriction of the range of
pressure used, to below approximately 10“3 millinieter of mercury.
There are commercial tubes especially designed for this purpose,
“
such as the FP62.
But certain tubes, such as the 101D, 102D,
205D, 216A and 45, may also be
2. The number of ions produced by each electron must not
change appreciably with variation of
energy of the electron.
’
The latter is usually eXpressed in terms of the accelerating voltage
'
between the grid and the lament. The probability of ionization
for electrons of different speedS is zero up
to a small accele‘rating
_
voltage, called the minimum ionizing potential,
rises to a
broad maximum somewhere between 100 and 500 volts. For
still higher voltages, the chance that an electron will ionize
‘
atom decrease slowly. Best operationof the gauge is, therefore,
obtained in the region of the broad maximum where‘ the probability
‘
is nearly constant.. This maximum“ o_CCur-s
-at dieret'_voltages_
-
and is of different magnitude :for different gases,ÿ necesS1tàtmg
a
separate calibration for”
coibina'ti‘0n- of gaSeS.—
For
_
air, a maximum exists between 2550
-Th‘_ÏS>
,
ever, is not critical and
in
.
_
actual operation. , '.
,
…
'
'.
.
_
'
3. The
,
by the grid currentmustbekeptœn3tant
.
'
brought ab01—it“bySlmplÿmammlnmgCOHStant
the
—
.
’
tube, ‘sinç‘èf thètemperatuœandhenœtheemlssmn0f
_
varies with‘thëpressureduet@thecoglln€eñeCt0fheat
.
mehr; Whenlgl“lsesabovethe‘i€Slf€dC‘”“—°‘tantvalue>ltsmClease
‘
.
'
is used toreducethefam€ntcum°>ntWhl°h“…“““"‘duces the. ‘
L