Arthur Rudolph Berthold Wehnelt was born in Rio de Janeiro, Brazil
on 4th of April 1871. His father came to Brazil to help in the local
shipbuilding technology. Arthur Wehnelt came back early to Germany
where he studied Physics first at the University of Berlin and then
at the University of Erlangen, where he received his doctorate. There
he remained until 1906 when he published his famous work entitled:
"On the emission of negative ions from glowing metal compounds
and related-phenomena", about the invention of the oxide-cotated
cathode. In 1906 he moved to the University of Berlin as a full professor
and later in 1926 he became director of the Physics Institute.
|Arthur Wehnelt the inventor of the a practical
electron emitter as know as Wehnelt cathode
Wehnelt was invloved into several fields of Physics. Among them
are: discharges in rarefied gases, cathode rays, X rays, photo-electric
and secondary emission as well as in the thermal conductivity of
In 1903 during an experiment about the emission of electrons from
hot bodies, he was using a hot platinum wire, when suddenly, as
a matter the cathode rays were emitted from certain small areas
of the wire he could observed a tell-tale brigh blue glow.
In the beguinning he assumed that impurities had lodged on those
small areas, and thought them to be metal oxides from the vacuum
piping system used for rarfying testing tube.
At this point in this historical experiment, as a deeper thinker,
patiente and practical experimenter Wehnelt made detailed follow
up tests with many substances and finally he determined that the
oxides of the alkaline earth metals in particular gave the best
electron emission. Thus, it gave birth what became the barium-strontium/calcium
oxide-cotated cathode, a practical and copious electron emitter,
as known as Wehnelt Cathode, which over 75 years was worldwide used
for the manufacturing of the thermionic valve.
Arthur Wehnelt died in Berlin on 15th of February 1944. His famous
invention, the Wehnelt cathode was one of the most important innovations
in the evolution of thermionics.
|* Carioca (karioka)
is a term originated from a native Brazilian Indian language
“Tupi” whose tribes was located in the Rio de Janeiro
area in the beginning of the Portuguese colonization back in
1500’. Literally it means house of the Whiteman, or anything
related with Rio de Janeiro. As a matter of crossing languages
lexical (Portugese/native Indian), later on this Indian term
was used to indicate the one born in the city of Rio de Janeiro.
As an avid valve collector and deeply interested in the valve
technology certainly as Brazilian, author is very proud Mr.
Wehnelt can be considered a true famous “Carioca”
AN OVERVIEW ON THE CATHODE DESIGN
The cathode is an essential part
of the thermionic valve, because it supplies the electrons mandatory
for its operation. When heat energy is applied to the cathode it
starts to emit electrons. The method of heating the cathode may
be used to identify its differents its shapes. For instance a directly
heated cathode, or filament cathode, is a wire heated by the passage
of an electric current; an indirectly heated cathode, or heater
cathode generally is heated by radiation or conduction and it consists
of a filament enclosed by an insulated from a closely fitting metal
sleeve, which is coated with a suitable electron-emitting material.
Pure drawn Tungsten or Tantalum
- Good stability in the electron emisssion
- Pure metal avoids contamination of other part of the valve.
- Low electron emission efficiency.
- Higher operation temperature – >1700ºC –
dazziling white – involves some difficulty from the
standpoint of mechanical support.
- The oldest of form o hot cathode.
- It requires minimum current drain and it is used for battery
- See Fig. 1
- It has emisssion efficiency between that of
pure tungsten and coated cathode.
- It liberates electrons at medium temperature - > 1700ºC
– bright yellow.
|- It is a possible source of contamination, as
the Thorium may, under certain condition, tend to activate other
parts of the valve as the grid.
- The electron emisssion is dependent of the perfection of the
- The filament material is activated by the
introduction of Thorium oxide into the base metal, which is
later, reduced to pure Thorium and forms a monomolecular base
layer on the surface of the filament.
- See Fig. 2.
Pure Nickel or Nickel alloys
- Low operating temperature - 700- 750ºC
- light red.
- High electron emisssion efficiency in the vacuum, as it
can reach 100 mA/W.
- it tends to generate gas.
- It tends to contaminate other parts of the valve.
- In the indirectly heated cathode it comprises
a thin metal sleeve coated with an active material similar
to that used in the coated-type filaments and Tungsten or
Tungesten-Molybdnium heater contained within an insulated
from the sleeve. See fig 3
-Originally Platinum was used as the base metal.
- The metal base is with one or more carbonetes as Barium,
Stroncium and some time Calcium, which are reduced during
theexhaust to a combination of oxide and the pure metal foming
a thick layer over the base metal.
- See Fig.4.
|Fig 1 - Illustration of highly developed form of pure Tungsten
multi-phase cathode for high-voltage transmitting valve.
||Fig 2 - Close up of a typical tungsten-thoriated cathode for
low voltage transmitting valve
|Fig. 3 - An indirectly heated cathode. Oxide-coated
gauze strips surround the heater spiral, and radiation losses
are reduced by double metal screen.
||Fig. 4 -Filamentary oxide coated cathode with
single shield. The heat-conservating shield allows increasing
the efficiency of the electron emission.
5 - The first valve made with oxide-coated cathode manufactured
in Germany circa 1912. It was used for telephone relay purpose
developed by Robert von Lieben, Eugene Reisz e Sigmund Strauss
giving birth to its name as LRS repeater. Later on the companies:
AEG, Siemens & Halske, Felton & Guilleaume Carlswerk,
A. G. and Telefunken founded a laboratory for the valve manufacturing
known as “Lieben Konsortium”. In the enclosed
photograph on the left the LSR repeater. On the right
Courtesy of "Siemens Corporate