364
16.
VACUUM TECHNIQUE
tube, it will condense on a small area, but in a poorer vacuum, it
Will spread all over the inside. The getter Will take up gases
'
evolved during the life of the tube. The effectiveness of getters
is approximately in the order of their chemical
Nitrogen, oxygen
and hydrogen slowly disappear from a low
pressure
tube in which a tungsten lament is maintained at high
temperature.
Pressures of the order of 10“6 mm. can be obtained
by the use of such incandescem‘ laments when the preliminary
—
pressure was only about 0.1 mm.
Despite the great care in manufacture,
and kenotron rectifying tubes have a slight
tendency to soften.
But, if used from time to time, the slight
traces of gas are removed and the tube remains hard.
PRESSURÉ GAUGES
16-6.
Pressure Units.—The pressure
in a vacuum system may
be measured in millimeters of mercury (mm.), microns (y) or bars.
The micron is one one—thousandth of a millimeter of mercury.
The bar * is the pressure
exer’ted by a force of one dyne uniformly
'
distributed over one square centimeter.
1 mm. Hg. = 1000”. = 1325 bars.
(16—2)
“.
The pressure in a chamber may be measured with the usual
(l) closed and (2) open manometers shown in gure 16—9, if it
'
is not too low.
Increased sensitivity may be had by mounting
the manometers off the vertical.
For pressures where a Geissler
discharge passes through the tube, the length of the cathode dark
Space may be used as a rough measure of the degree of
.
16—7.
Gauge.—Figure 16—9 (3) shows a McLeod
-whiçhthe low pressure gas is compressed toa pressure
'
ea51lyObserved._ The large bulb With capillary tube on top hasa
VolumeV, determined, at “thetime of construction. The gas,
lowpressurep13 tof bemeasured,enters this, bulb freely through
the mercury reservon‘B
“Pt®aP@mtCawhel‘eltoœuresa Small 'V61ùme @ àt Thenff@mB@eslaw
’
'
'
PV=Pv 1 :
(lé—3)
asj denedabove,
as°nemïcmbaf)
16.
VACUUM TECHNIQUE
tube, it will condense on a small area, but in a poorer vacuum, it
Will spread all over the inside. The getter Will take up gases
'
evolved during the life of the tube. The effectiveness of getters
is approximately in the order of their chemical
Nitrogen, oxygen
and hydrogen slowly disappear from a low
pressure
tube in which a tungsten lament is maintained at high
temperature.
Pressures of the order of 10“6 mm. can be obtained
by the use of such incandescem‘ laments when the preliminary
—
pressure was only about 0.1 mm.
Despite the great care in manufacture,
and kenotron rectifying tubes have a slight
tendency to soften.
But, if used from time to time, the slight
traces of gas are removed and the tube remains hard.
PRESSURÉ GAUGES
16-6.
Pressure Units.—The pressure
in a vacuum system may
be measured in millimeters of mercury (mm.), microns (y) or bars.
The micron is one one—thousandth of a millimeter of mercury.
The bar * is the pressure
exer’ted by a force of one dyne uniformly
'
distributed over one square centimeter.
1 mm. Hg. = 1000”. = 1325 bars.
(16—2)
“.
The pressure in a chamber may be measured with the usual
(l) closed and (2) open manometers shown in gure 16—9, if it
'
is not too low.
Increased sensitivity may be had by mounting
the manometers off the vertical.
For pressures where a Geissler
discharge passes through the tube, the length of the cathode dark
Space may be used as a rough measure of the degree of
.
16—7.
Gauge.—Figure 16—9 (3) shows a McLeod
-whiçhthe low pressure gas is compressed toa pressure
'
ea51lyObserved._ The large bulb With capillary tube on top hasa
VolumeV, determined, at “thetime of construction. The gas,
lowpressurep13 tof bemeasured,enters this, bulb freely through
the mercury reservon‘B
“Pt®aP@mtCawhel‘eltoœuresa Small 'V61ùme @ àt Thenff@mB@eslaw
’
'
'
PV=Pv 1 :
(lé—3)
asj denedabove,
as°nemïcmbaf)