16—3.
PUMPS REQUIRING A FORE VACUUM 357
is considerably greater than the diffusion rate of the gas molecules
around it, the chance of diffusion of the gas molecules in the oppo—
site direction is very slight (computed as 1 in 1020). Further, the
mercury, being condensed immediately below the jet, exerts no
greater vapor pressure in H than that which it has at room temperature.
This is around 10”8 mm. and, for lower pressures, the
"
l
—
«--C
_“
;
13
_
-
D
“.
_
.
G
.
'_
'
;.
A
-:'-…—z* -.
'A
..
..
..._
,
_
E
_
305
_
.
-
FIG. 16—4. 'Mercury
'
-
_
liquid air trap ] is used to prevent mercury vapor from entering
the remainder of the vacuum
'
.
,
-
In practice, it"is found that the speed, of these pumps depends
on the size and shape of thejet. : Pumps of th€type shownat
the
right in
16—4 do not usually “take hold
’
unt11
the fore
_
vacuum is lower than that created
or mechan_