110
5.
THERMIONIC EMISSION
»
9
laments are usually operated near the temperature, 1900° K, at
which temperature
the diffusion is so slow that only patches of the
-
surface are covered with thorium. The advantage of thoriated over ‘
-
pure tungsten laments is that the work function is lowered from
4.52 to about 2.6 volts.
Wehnelt found that a drop of sealing wax on a platinum la—
ment increased the electron emitting power, so that larger emission
currents could be obtained even when operating at lower temperatures, with attendant increase in life.
There are, today, a number
of similar eoated laments which have low work functions. For
'example, a mixture of three parts of barium carbonate (B aCOg) and
two parts of strontium carbonate (SrC03), formed into sticks With
'
a binder such as beeswax or parafn, may be used. This is rubbed
on a warm (100—200° C.) platinum lament and “ashed” in air,
at or near 1000°
It is then sealed into the tube and the evacua—
tion process carried out. These are of the soæalled combined type
'
of coated laments, since, probably, a layer of barium and stron— Ï
tium platinides exists under a monatomic layer of barium and stron—
'
tium atoms.
In the uneombined type, a fty-fty mixture of the -
carbonates, supported by cellulose nitrate, is sprayed at 100—200° C.
onto a supporting metal; or one may clip the metal into the coating
mixture.
The “ashing” is not carried out until the lament is
sealed into the tube and the evacuation well under way. The
ium_and str0ntium does not then combine to form molecules W1th
the platinum or other supporting metal. Rather, it is probable that
a layer of their oxides are formed. These are referred to as “oxide"
coated laments. Since there is no chemical combination with the
supporting material, this base metal may consist of platinum,
nickel, or alloys such as konel metal or platinum—niCkel mixtures
The base metal is chosen to have the idesired tensile strength and
,
-
electrical resistance at the operating temperature rather thari for
_
-
its electron emitting ability. Cesium has also been used on m<îta1
*
surfaces
in order to lower the work function. '
‘
_
indication of the improved efciency of these “dull
lcmitter”.flamentsOver the pure metals, it may be stated thata
square
of tungster‘r lament operated at 23 700 KW1H
give 3milliamperes o_fë electron current per watt of heat1ngenel‘gÿ,
"lamth at…19009—K. will give about 100 mllhamper68
5.
THERMIONIC EMISSION
»
9
laments are usually operated near the temperature, 1900° K, at
which temperature
the diffusion is so slow that only patches of the
-
surface are covered with thorium. The advantage of thoriated over ‘
-
pure tungsten laments is that the work function is lowered from
4.52 to about 2.6 volts.
Wehnelt found that a drop of sealing wax on a platinum la—
ment increased the electron emitting power, so that larger emission
currents could be obtained even when operating at lower temperatures, with attendant increase in life.
There are, today, a number
of similar eoated laments which have low work functions. For
'example, a mixture of three parts of barium carbonate (B aCOg) and
two parts of strontium carbonate (SrC03), formed into sticks With
'
a binder such as beeswax or parafn, may be used. This is rubbed
on a warm (100—200° C.) platinum lament and “ashed” in air,
at or near 1000°
It is then sealed into the tube and the evacua—
tion process carried out. These are of the soæalled combined type
'
of coated laments, since, probably, a layer of barium and stron— Ï
tium platinides exists under a monatomic layer of barium and stron—
'
tium atoms.
In the uneombined type, a fty-fty mixture of the -
carbonates, supported by cellulose nitrate, is sprayed at 100—200° C.
onto a supporting metal; or one may clip the metal into the coating
mixture.
The “ashing” is not carried out until the lament is
sealed into the tube and the evacuation well under way. The
ium_and str0ntium does not then combine to form molecules W1th
the platinum or other supporting metal. Rather, it is probable that
a layer of their oxides are formed. These are referred to as “oxide"
coated laments. Since there is no chemical combination with the
supporting material, this base metal may consist of platinum,
nickel, or alloys such as konel metal or platinum—niCkel mixtures
The base metal is chosen to have the idesired tensile strength and
,
-
electrical resistance at the operating temperature rather thari for
_
-
its electron emitting ability. Cesium has also been used on m<îta1
*
surfaces
in order to lower the work function. '
‘
_
indication of the improved efciency of these “dull
lcmitter”.flamentsOver the pure metals, it may be stated thata
square
of tungster‘r lament operated at 23 700 KW1H
give 3milliamperes o_fë electron current per watt of heat1ngenel‘gÿ,
"lamth at…19009—K. will give about 100 mllhamper68