16—9.
MOLECULAR GAUGES
367
from a hot lament sealed in the vacuum system, and vice versa.
Maintaining constant current through the lament, its temperature
and also its resistance change with the pressure of the gas and may
be measured with a Wheatstone Bridge. The greater the pressure,
the lower the resistance.
The gauge is calibrated by comparison
with a McLeod gauge, a liquid air trap being placed between the
two to freeze out the mercury vapor. It has been found “better to
,
maintain the lament at constant temperature, observing the
'
-
voltages V necessary to do this as the pressure changes. The
greater the pressure, the greater the reading of the voltmeter.
If Vo corresponds to the lowest pressure attainable and 7 to a
higher pressure, it is found that (V2 — 1702)/V02 plotted against
,
the pressure gives a straight line below 0.05 mm. which is the same
for
gases containing no hydrogen. However, separate curves
,
for each gas
must be made.
Pressures as
low as 10”5 mm. can be measured. 'Readings may be obtained
quickly, especially by the rst method described,
if the gauge is
placed in a constant temperature bath. Recording metèrs may
be used to obtain permanent records.
In practice, a tungsten
lament lamp (10—40 watts) with rigid lament may
be used for
this gauge.
Using the constant temperature method, the bridge
is balanced at the start so that “the lament« is around 100° C.,
i.e., below red heat.
,
’
‘
—
A convenient thermocouple gaugïg4 for USE in the pressure range
from 0.1 to 10*3 millimeter Of mercury is described in5reference-4._ -
16—9. - Molecular GaugeS.—Themolecular gaugêS (@ and_ (7)
of
operate on the principle of
In
-
'
disc  is rotated by the magnet Bturnëd1by the rotating'magnetlc
eld C'
.
As a‘result,thelight
quartz ber,
’
is _deected by the
intervenlngi"gaS;_
The
_
distance between
the
mean
path“î£of the moleculôsat the €X18t1ng pressure
The
’
MOLECULAR GAUGES
367
from a hot lament sealed in the vacuum system, and vice versa.
Maintaining constant current through the lament, its temperature
and also its resistance change with the pressure of the gas and may
be measured with a Wheatstone Bridge. The greater the pressure,
the lower the resistance.
The gauge is calibrated by comparison
with a McLeod gauge, a liquid air trap being placed between the
two to freeze out the mercury vapor. It has been found “better to
,
maintain the lament at constant temperature, observing the
'
-
voltages V necessary to do this as the pressure changes. The
greater the pressure, the greater the reading of the voltmeter.
If Vo corresponds to the lowest pressure attainable and 7 to a
higher pressure, it is found that (V2 — 1702)/V02 plotted against
,
the pressure gives a straight line below 0.05 mm. which is the same
for
gases containing no hydrogen. However, separate curves
,
for each gas
must be made.
Pressures as
low as 10”5 mm. can be measured. 'Readings may be obtained
quickly, especially by the rst method described,
if the gauge is
placed in a constant temperature bath. Recording metèrs may
be used to obtain permanent records.
In practice, a tungsten
lament lamp (10—40 watts) with rigid lament may
be used for
this gauge.
Using the constant temperature method, the bridge
is balanced at the start so that “the lament« is around 100° C.,
i.e., below red heat.
,
’
‘
—
A convenient thermocouple gaugïg4 for USE in the pressure range
from 0.1 to 10*3 millimeter Of mercury is described in5reference-4._ -
16—9. - Molecular GaugeS.—Themolecular gaugêS (@ and_ (7)
of
operate on the principle of
In
-
'
disc  is rotated by the magnet Bturnëd1by the rotating'magnetlc
eld C'
.
As a‘result,thelight
quartz ber,
’
is _deected by the
intervenlngi"gaS;_
The
_
distance between
the
mean
path“î£of the moleculôsat the €X18t1ng pressure
The
’