16—14.
FLOW OF GASES AT LOW PRESSURE
373
.
kept on hand and melted whenever needed. The liquid may be
_
applied to warmed parts of a vacuum system witha brush-. Softer
waxes
are
formed by using a larger percentage of beeswax.
DeKotinsky, Everett, Apiezon and Picein waxes of low vapor
pressure may be purchased on the market. See, also, section16—1S.
16—14.
Flow of Gases at Low
experiments’at
low pressures, the rate of ow of the gas through various sizes of
tubing is often of great importance. In many experiments pure
gas is admitted continuously through a ne capillary
and pumped away by a mercury
An equilibrium
'
pressure is thus established in the experimental tube. If variou‘s
_
parts are connected together, the pressure may be very different
in the different
In calcu‘lating the rate of ow with a good
'
?
vapor pump, the pressure at the pump itself may be considered as
maintained at zero.
The ow through the various tubes may be
“
computed from the following formula of Knudsen which may be
-
derived kinetically. When the pressure is so low that thèfree
'
'
path is large compared with the radius of the tube,
.
,
_
'
_
_
_
_
Here 72 is the number of molecules of a gas
molecular weight M
:
passing per second through a tubeo_f length
L cm._and‘
radius R
‘
cm.
under a
—
bars at a temperature
of T°
If
ËÊ—_—IË
resistance ” of the
-_
tube, it is easy to*—Showïthat
a network
Of tubes
.
‘
connected
arid
__
-
,
‘for the combination…a£electncalreSIStanœsmS€€Sand Parallel-Ü
_
same reS1stanceastenmetersofï0cmtub1ng
"‘
‘!
—-P
“
‘