Arthur Wehnelt the inventor of the a practical electron emitter as know as Wehnelt cathode
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.
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 metals.
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”


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.

Filament cathode

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 operaaed valves.
- See Fig. 1

Filament cathode


- 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 Thorium layer.

- 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.

Coated-filament cathode

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.

Fig. 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 Archives, Munich".