368
16.
VACUUM TECHNIQUE
damped down gives a measure of the pressure
of the gas.
The
instrument has a range
from 1.0 mm. down to 10”4 mm. but is
not satisfactory for lower pressures.
16—10.
Radiation Gauges.…—A type of Knudsen radiation
gauge is represented by (8) of gure 16—10.
This operates on the
principle of the repulsion between a hot and cold plate due to
molecular bombardment.
The electrically heated platinum strips
/Î and B act as the hot plate and the mica sheet C as the cold
plate. The latter, supported by a quartz ber, is rotated by the
unequal bombardment on the sides adjacent and away from the
hot plate, as observed by light reflected from the mirror D. The
amount of rotation is proportional to the pressure,
other conditions
remaining constant, and the distance between it and the hot
plate being small in comparison to the mean free path of the
molecules at the existing pressure.
When the gauge has been
properly designed, the pressure may
be calculated directly from
the rotation, spacing, temperature difference, etc., so that it serves
as an absolute manometer from
mm. to
mm. and possibly
lower.
This is the most accurate absolute instrument for work at
extremely low pressures but requires skill in construction and use.
16—11.
IÔnization Gauge‘s.—The ionization gauge (9) of gure
16—10, due to Buckley,5 has been found particularly useful, espe—
cially at low pressures.
Electrons from the hot filament F are
accelerated toward the grid G by the battery
Some of these
reach the grid and constitute the current [€ (0.5 to 20 milliamperes)
While others pass through into the region between the grid and
negatively charged plate P where they are repelled back to the
grid. The gas in this region is ionized by these electrons in pro—
portion to the number of particles present and hence in proportion
to the pressure.
The positive ions so formed are drawn over to
the plate giving a reading on the galvanometer of amount [p.
Calibration of the gauge by comparison with a standard gauge
at
pressures around 10“4 millimeters gives values of K in the equation
p = K],,.
(lé-6)
Under certain conditions K will be a constant.
It is usually
expressed in millimeters of mercury per ampere
of positive
ion
current.
16.
VACUUM TECHNIQUE
damped down gives a measure of the pressure
of the gas.
The
instrument has a range
from 1.0 mm. down to 10”4 mm. but is
not satisfactory for lower pressures.
16—10.
Radiation Gauges.…—A type of Knudsen radiation
gauge is represented by (8) of gure 16—10.
This operates on the
principle of the repulsion between a hot and cold plate due to
molecular bombardment.
The electrically heated platinum strips
/Î and B act as the hot plate and the mica sheet C as the cold
plate. The latter, supported by a quartz ber, is rotated by the
unequal bombardment on the sides adjacent and away from the
hot plate, as observed by light reflected from the mirror D. The
amount of rotation is proportional to the pressure,
other conditions
remaining constant, and the distance between it and the hot
plate being small in comparison to the mean free path of the
molecules at the existing pressure.
When the gauge has been
properly designed, the pressure may
be calculated directly from
the rotation, spacing, temperature difference, etc., so that it serves
as an absolute manometer from
mm. to
mm. and possibly
lower.
This is the most accurate absolute instrument for work at
extremely low pressures but requires skill in construction and use.
16—11.
IÔnization Gauge‘s.—The ionization gauge (9) of gure
16—10, due to Buckley,5 has been found particularly useful, espe—
cially at low pressures.
Electrons from the hot filament F are
accelerated toward the grid G by the battery
Some of these
reach the grid and constitute the current [€ (0.5 to 20 milliamperes)
While others pass through into the region between the grid and
negatively charged plate P where they are repelled back to the
grid. The gas in this region is ionized by these electrons in pro—
portion to the number of particles present and hence in proportion
to the pressure.
The positive ions so formed are drawn over to
the plate giving a reading on the galvanometer of amount [p.
Calibration of the gauge by comparison with a standard gauge
at
pressures around 10“4 millimeters gives values of K in the equation
p = K],,.
(lé-6)
Under certain conditions K will be a constant.
It is usually
expressed in millimeters of mercury per ampere
of positive
ion
current.